--- title: "Lot Sizing for CNC Shops: Methods, Formulas, and Batch Size Rules" description: "Lot sizing methods (L4L, EOQ, EPQ, POQ) explained for CNC shops: when to run short batches or long runs, using real setup and cycle time data." image: https://www.jitbase.com/hubfs/lot-sizing-cnc-shops.webp --- [+1 438 942-8501](tel:+1%20438%20942-8501) [Log In](https://app.jitbase.com/) - [English](https://www.jitbase.com/blog/lot-sizing-cnc-shops) - [Français](https://www.jitbase.com/fr/blog/taille-de-lot-atelier-cnc) [![JITbase black logo](https://www.jitbase.com/hs-fs/hubfs/Logos/jitbase-logo-black-horiz.webp?width=175&height=88&name=jitbase-logo-black-horiz.webp)](https://www.jitbase.com) - Solutions Back ##### [Machine Monitoring](https://www.jitbase.com/machine-monitoring) - ##### [Machine Monitoring](https://www.jitbase.com/machine-monitoring) Back - [Live Dashboard](https://www.jitbase.com/machine-monitoring) - [Track OEE](https://www.jitbase.com/machine-monitoring#Track-OEE) - [Track Downtime Reasons](https://jitbase.com/machine-monitoring#Track-Downtime-Reasons) - 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[Français](https://www.jitbase.com/fr/blog/taille-de-lot-atelier-cnc)   [Book a demo](https://www.jitbase.com/demo) [Connect Machines for Free](https://sso.jitbase.com/realms/webapp/protocol/openid-connect/registrations?client_id=website-registration&response_type=code&scope=openid&redirect_uri=https://admin.jitbase.com) ![CNC operator staging a batch of machined parts next to a machining center: lot sizing with JITbase](https://www.jitbase.com/hubfs/lot-sizing-cnc-shops.webp) Blog [Production planning and scheduling](https://www.jitbase.com/blog/tag/production-planning-and-scheduling) # Lot Sizing for CNC Shops: Methods, Formulas, and Batch Size Rules [*written by* Judicael Deguenon on October 5, 2026](https://www.jitbase.com/blog/author/judicael-deguenon) Lot sizing decides how many parts you run between two setups. Make lots too small and the machine spends its day changing over. Make them too big and finished parts pile up on shelves while urgent jobs wait. The formulas taught in MRP and operations courses are a useful starting point, but they rest on assumptions that rarely hold on a high-mix CNC floor. This guide explains the main lot sizing methods, shows where they break in a machine shop, and gives practical batch size rules built on real setup and cycle time data. **Key takeaways:** - The classic lot sizing methods (lot-for-lot, fixed quantity, EOQ, EPQ, POQ) all sit on the same trade-off: setups on one side, inventory and lead time on the other. - In a CNC shop, the right lot size depends on where the machine sits: longer runs protect a bottleneck, shorter batches cut work in progress everywhere else. - Every formula is only as good as its inputs. Measured setup and cycle times matter more than the choice of method. ## What Lot Sizing Means in a CNC Shop Lot sizing is the decision of how many units to produce in one run before the machine is set up for something else. Larger lots spread the setup over more parts, but they tie up cash in inventory, take up floor space and lengthen the time other jobs wait. Smaller lots keep work in progress (WIP) low and the shop responsive, but they multiply setups. Two quantities are often confused: - **Process batch:** the number of parts run on a machine between two setups. - **Transfer batch:** the number of parts moved together to the next operation. They do not need to be equal. A machine can run 400 parts in one setup while sending them downstream in groups of 50, so the next operation starts hours earlier instead of waiting for the whole lot. In CNC machining, the setup is rarely just a fixture change. It usually includes loading and measuring tools, setting work offsets, proving out the program and waiting for [first article inspection](https://www.jitbase.com/blog/first-article-inspection-cnc-shops). That full sequence, from the last good part of one job to the first good part of the next, is the number that lot sizing needs. ## The Main Lot Sizing Methods Most MRP and ERP systems offer some combination of the following methods. | Method | Rule | Best fit | Main limitation | | --- | --- | --- | --- | | Lot-for-lot (L4L) | Produce exactly the net requirement of each period | Make-to-order work, expensive parts, quick changeovers | Maximum number of setups | | Fixed order quantity (FOQ) | Run the same quantity every time | Physical limits such as bar stock length, fixture capacity or packaging | Ignores changes in demand | | Economic order quantity (EOQ) | Q = √(2DS / H) | Stable demand, purchased items | Assumes the whole lot arrives at once and demand is constant | | Economic production quantity (EPQ), also called economic batch quantity | Q = √(2DS / (H × (1 − d/p))) | Parts made in-house at a known production rate | Same stable-demand assumptions as EOQ | | Periodic order quantity (POQ) | Cover a fixed number of periods, usually EOQ divided by average demand per period | Lumpy demand planned by period in MRP | Still depends on EOQ inputs | The formulas look academic, but a spreadsheet solves them in two minutes. Here is what each letter means in shop terms: | Letter | What it means | Where to find it | | --- | --- | --- | | D | How many parts you ship per year | Order history or ERP | | S | What one setup costs | Setup hours × hourly rate, plus inspection | | H | What it costs to keep one part in stock for a year | A percentage of unit cost covering cash, space and risk | | d | How fast customers use the part | Demand per day or per week | | p | How fast the machine makes the part | Parts per hour from the real cycle time | Just keep d and p in the same time units, for example both per day. ### EOQ vs Economic Batch Quantity EOQ was designed for purchased items that arrive in one delivery. When a shop makes the part itself, finished parts come off the machine gradually while some are already being consumed, so average inventory is lower. The economic batch quantity (EPQ) corrects for this with the (1 − d/p) term, which gives a slightly larger lot than EOQ for the same costs. For in-house CNC work, EPQ is the more accurate of the two. Some MRP and ERP systems also offer dynamic methods for demand that changes from period to period. These belong to long-term planning: the article on [how planning and scheduling tools complement MRP and MES](https://www.jitbase.com/blog/how-do-jitbases-planning-scheduling-tools-complement-mrp-and-mes-systems) explains where that layer stops and daily shop floor scheduling begins. ## Why Textbook Formulas Break in High-Mix CNC Shops The formulas are sound. The problem is that a typical job shop violates most of their assumptions at once. **Setup is time, not only cost.** On a bottleneck machine, every hour of setup is an hour of output the whole shop loses. On a machine with spare capacity, the same hour mainly costs operator labor. Using a single setup cost for every machine produces lot sizes that are too small where capacity is scarce and too large where it is not. **The inputs are usually estimates.** Setup times often come from quotes or routings that were never checked against the floor. Because the economic lot size grows with the square root of the setup cost, a setup estimated at half its real value produces a lot about 29% too small. Cycle times matter too: they set the production rate p and, on a bottleneck, the output lost during each setup. **Capacity is finite.** Each formula optimizes one part number in isolation. Add up the economic lots of every part sharing the same machine and the total can exceed the hours available. **Demand is lumpy and due dates matter.** A customer order due Thursday does not care about the annual average. Lot sizes have to fit the schedule, not only the cost curve. **Quality and tooling set their own limits.** Each new setup needs a first article check, tool wear can cap a long run, and some customers require each lot to trace back to a single material heat. **Size your lots on real setup and cycle times, not routing estimates.** JITbase measures actual cycle times, setups and changeovers on every connected CNC machine. [See how production monitoring works](https://www.jitbase.com/production-monitoring) ## Short Batches vs Long Runs: Decision Criteria When the formula and the floor disagree, these criteria help decide which way to lean. | Criterion | Favors longer runs | Favors shorter batches | | --- | --- | --- | | Role of the machine | Bottleneck, capacity is the constraint | Spare capacity available | | Setup compared with run time | Long, complex setup | Quick changeover, shared fixtures | | Demand pattern | Stable repeat part | One-off or volatile demand | | Quality risk at restart | High, difficult first article | Low, stable process | | Downstream operations | Downstream has its own buffer | Next operation is waiting for parts | | Due dates | Few urgent orders on this resource | Several urgent orders share the machine | | Cash and floor space | Storage available, low unit cost | Expensive parts, limited space | Setup reduction changes the whole balance: every hour removed from a changeover lowers the economic lot size and makes shorter batches affordable. The guides on [reducing setup time with better scheduling](https://www.jitbase.com/blog/reducing-setup-time-with-better-scheduling) and on [kaizen experiments to cut setup and cycle time](https://www.jitbase.com/blog/5-kaizen-experiments-cut-setup-cycle-time) cover the practical levers. ## Lot Sizing in Practice: 5 Steps No formula needed to get started: 1. **Find the bottleneck.** Look for the machine with the longest queue and the highest utilization. 2. **Measure real setups.** Track a few weeks of actual setup times per part family, inspection wait included. 3. **Start from weekly demand.** Express each lot as weeks of demand covered, which is easier to discuss than a raw quantity. 4. **Run longer lots at the bottleneck, shorter ones elsewhere.** Group part families that share fixtures and tools on the bottleneck. 5. **Review every quarter.** Recheck sooner if demand, setup times or the bottleneck change. ## The Data You Need: Real Setup and Cycle Times Before running any formula, collect: - **Actual setup durations** per part family and machine, based on the full setup sequence described above. - **Real cycle times**, comparing the estimate from the program with measured time on the machine. The guide to [extracting cycle times from G-code](https://www.jitbase.com/blog/extract-cycle-times-from-g-code-in-7-steps) explains how to get a reliable baseline. - **Demand history** per part number over 6 to 12 months, including how often the part repeats. - **Holding cost inputs:** unit cost, storage constraints and the risk that parts become obsolete after an engineering change. - **The current bottleneck**, identified from machine utilization and queue length, not from habit. Machine data supplies most of this automatically. With JITbase, the machine signal records production, setup and changeover states, and operators enter stop reasons and confirm tasks themselves on the screen at the machine. Setup durations then come from the floor rather than from the quote. ## Worked Example: Same Part, Two Machines The figures below are illustrative. Consider a part with the following data: - Annual demand (D): 4,800 parts, or about 92 per week. - Unit cost: $40, with a holding cost (H) of 20% per year, so $8 per part per year. - Setup: 2 hours, plus $60 for first article inspection. - Cycle time: 6 minutes, or 10 parts per hour. With 4,000 machine hours a year, the production rate (p) is 40,000 parts per year, so d/p = 0.12. Here is how the same part compares on two different machines: | Cost item | Machine with spare capacity | Bottleneck machine | | --- | --- | --- | | Setup labor (2 hours at $45) | $90 | $90 | | First article inspection | $60 | $60 | | Output lost during the setup | None, the machine had idle time anyway | 20 parts × $35 = $700 | | **Cost of one setup (S)** | **$150** | **$850** | | **Economic batch quantity** | **About 452 parts** | **About 1,077 parts** | | Weeks of demand covered | About 5 | About 12 | The $35 is what each part earns once material and direct costs are paid. On the bottleneck, an hour of setup is an hour of sales lost, so the setup costs far more. Same part, same 2-hour setup, and the right lot size more than doubles depending on the machine. In practice, round both results to a workable quantity, such as a multiple of fixture capacity or of parts per bar. Then cap them by storage space and the risk of an engineering change. **Plan each day around your real bottleneck.** JITbase schedules jobs and operators from live machine data, so lot decisions match the capacity you actually have. [Explore scheduling and workforce management](https://www.jitbase.com/workforce-management) ## Lot Sizing at the Bottleneck vs Everywhere Else A practical rule follows from the example: size lots differently depending on the machine. - **At the bottleneck,** reduce the number of setups. Run longer lots and sequence part families that share fixtures and tools back to back. - **Everywhere else,** run smaller lots. The more parts waiting between machines, the longer every job takes to get through the shop. This is Little's Law: WIP = throughput × lead time. - **Between operations,** split transfer batches. Moving parts downstream in smaller groups lets the next operation start before the whole lot is finished. - **When the bottleneck moves,** revisit the rules. A new machine, a product mix change or a setup reduction project can shift the constraint. The [throughput boost blueprint](https://www.jitbase.com/blog/throughput-boost-blueprint-scheduling-kanban) shows how these rules combine with scheduling and kanban, and the [shop floor management guide](https://www.jitbase.com/blog/shop-floor-management-wip-production-monitoring) covers how to keep WIP visible once lot sizes change. ## Common Lot Sizing Mistakes - **One lot size for the whole shop.** The same rule cannot suit a bottleneck and a machine running at half capacity. - **Using quoted setup times.** Quotes are written to win the job, not to plan the floor. Measure instead. - **Chasing formula precision.** The total cost curve is flat near the optimum: a lot 20% larger or smaller than the economic quantity raises total cost by less than 3%. Round to practical quantities without worrying. - **Setting lot sizes once and never reviewing them.** Demand, setup times and the bottleneck all change. - **Confusing process and transfer batches.** A large process batch does not have to block the next operation. **What are extra setups costing your shop?** Estimate the return of better lot and schedule decisions with your own production data. [Calculate your ROI](https://www.jitbase.com/return-on-investment) ## The Bottom Line Lot sizing methods give a sound starting point. But in a CNC shop, the right batch size depends on two facts more than on the formula: where the machine sits in the flow, and how long setups really take. Measure setups and cycle times on the floor, run longer lots at the bottleneck and shorter ones elsewhere, and review the rules whenever demand or the constraint shifts. ## Frequently Asked Questions ### What is lot sizing in manufacturing? Lot sizing is the decision of how many units to produce in a single run between two setups. It balances two opposing costs: the time and money spent on each setup, and the cost of carrying inventory, which includes tied-up cash, floor space and longer lead times for the jobs waiting behind a large lot. Common methods include lot-for-lot, fixed order quantity, economic order quantity (EOQ), economic production quantity (EPQ) and periodic order quantity (POQ). In a CNC shop, the method matters less than the inputs: lot sizes calculated from measured setup and cycle times are far more reliable than those based on routing or quote estimates. ### What is the difference between EOQ and economic batch quantity? EOQ assumes the whole lot arrives at once, which fits purchased items delivered in a single shipment. The economic batch quantity, also called economic production quantity (EPQ), accounts for parts coming off the machine gradually while some are already being consumed, so average inventory stays lower during the run. Because of this, the economic batch quantity gives a slightly larger lot than EOQ for the same setup and holding costs. The gap depends on the ratio between demand rate and production rate: the faster the machine produces compared with demand, the closer the two results are. For parts made in-house on CNC machines, the economic batch quantity is the more accurate choice. ### Should a CNC job shop use lot-for-lot sizing? Lot-for-lot works well for make-to-order parts, expensive materials and machines with quick changeovers, because it keeps inventory close to zero and every part produced already has a customer order behind it. It also suits one-off jobs and parts with frequent engineering changes, where stock can quickly become obsolete. It is a poor fit for a bottleneck machine with long setups, where each additional setup removes output from the whole shop. Many job shops therefore combine approaches: lot-for-lot on machines with spare capacity, longer runs that group part families at the bottleneck, and fixed quantities where bar stock or fixture capacity sets a natural limit. ### How often should lot sizes be reviewed? Review lot sizes whenever demand, setup times or the bottleneck change significantly, for example after a setup reduction project, the arrival of a new machine, a change in product mix or a new long-term customer contract. Each of these events changes at least one input of the calculation, and sometimes moves the constraint to another machine. Outside those events, a quarterly review based on measured setup and cycle times is a reasonable rhythm for most shops. Start with the part numbers that run most often or consume the most bottleneck hours, since that is where an outdated lot size costs the most capacity. #### SHARE [mailto:email@example.com?subject=Mail%20from%20our%20Website](mailto:email@example.com?subject=Mail%20from%20our%20Website) ## Related Articles [![CNC operator organizing tooling and machined parts on a shop floor, illustrating Kaizen experiments to reduce setup time, improve cycle time, and optimize production efficiency with JITbase.](https://www.jitbase.com/hs-fs/hubfs/blog-images/5-kaizen-experiments-cut-setup-cycle-time-hero.jpg?width=352&name=5-kaizen-experiments-cut-setup-cycle-time-hero.jpg)](https://www.jitbase.com/blog/5-kaizen-experiments-cut-setup-cycle-time) [Production planning and scheduling](https://www.jitbase.com/blog/tag/production-planning-and-scheduling) ### [5 Kaizen Experiments to Cut Setup and Cycle Time in Small Machine Shops (With Metrics to Track)](https://www.jitbase.com/blog/5-kaizen-experiments-cut-setup-cycle-time) Read more [![Deux techniciens CNC évaluant des pièces usinées et des outils de coupe devant un centre d'usinage — comparatif logiciels CNC pour ateliers de précision.](https://www.jitbase.com/hs-fs/hubfs/blog-images/best-cnc-software-platforms-precision-machining-hero.jpg?width=352&name=best-cnc-software-platforms-precision-machining-hero.jpg)](https://www.jitbase.com/blog/best-cnc-software-platforms-precision-machining) [Production planning and scheduling](https://www.jitbase.com/blog/tag/production-planning-and-scheduling) ### [15 Best CNC Software Platforms for Precision Machining in 2026](https://www.jitbase.com/blog/best-cnc-software-platforms-precision-machining) Read more [![CNC shop floor with machined parts, measurement tools, and connected machines, illustrating overall equipment effectiveness (OEE) software, machine performance monitoring, and production efficiency with JITbase.](https://www.jitbase.com/hs-fs/hubfs/blog-images/best-overall-equipment-effectiveness-software-top-10-hero.jpg?width=352&name=best-overall-equipment-effectiveness-software-top-10-hero.jpg)](https://www.jitbase.com/blog/best-overall-equipment-effectiveness-software-top-10) [Production planning and scheduling](https://www.jitbase.com/blog/tag/production-planning-and-scheduling) ### [Best Overall Equipment Effectiveness Software: Top 10 Picks for 2026](https://www.jitbase.com/blog/best-overall-equipment-effectiveness-software-top-10) Read more [![Machinist reviewing a color-coded job schedule on a tablet beside CNC machines -- work-in-process inventory audit with JITbase](https://www.jitbase.com/hs-fs/hubfs/blog-images/wip-self-audit-cnc-shops-hero.jpg?width=352&name=wip-self-audit-cnc-shops-hero.jpg)](https://www.jitbase.com/blog/wip-self-audit-cnc-shops) [Production planning and scheduling](https://www.jitbase.com/blog/tag/production-planning-and-scheduling) ### [Is Your WIP Draining Cash? 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It balances two opposing costs: the time and money spent on each setup, and the cost of carrying inventory, which includes tied-up cash, floor space and longer lead times for the jobs waiting behind a large lot. Common methods include lot-for-lot, fixed order quantity, economic order quantity (EOQ), economic production quantity (EPQ) and periodic order quantity (POQ). In a CNC shop, the method matters less than the inputs: lot sizes calculated from measured setup and cycle times are far more reliable than those based on routing or quote estimates." }, "name" : "What is lot sizing in manufacturing?" }, { "@type" : "Question", "acceptedAnswer" : { "@type" : "Answer", "text" : "EOQ assumes the whole lot arrives at once, which fits purchased items delivered in a single shipment. The economic batch quantity, also called economic production quantity (EPQ), accounts for parts coming off the machine gradually while some are already being consumed, so average inventory stays lower during the run. Because of this, the economic batch quantity gives a slightly larger lot than EOQ for the same setup and holding costs. The gap depends on the ratio between demand rate and production rate: the faster the machine produces compared with demand, the closer the two results are. For parts made in-house on CNC machines, the economic batch quantity is the more accurate choice." }, "name" : "What is the difference between EOQ and economic batch quantity?" }, { "@type" : "Question", "acceptedAnswer" : { "@type" : "Answer", "text" : "Lot-for-lot works well for make-to-order parts, expensive materials and machines with quick changeovers, because it keeps inventory close to zero and every part produced already has a customer order behind it. It also suits one-off jobs and parts with frequent engineering changes, where stock can quickly become obsolete. It is a poor fit for a bottleneck machine with long setups, where each additional setup removes output from the whole shop. Many job shops therefore combine approaches: lot-for-lot on machines with spare capacity, longer runs that group part families at the bottleneck, and fixed quantities where bar stock or fixture capacity sets a natural limit." }, "name" : "Should a CNC job shop use lot-for-lot sizing?" }, { "@type" : "Question", "acceptedAnswer" : { "@type" : "Answer", "text" : "Review lot sizes whenever demand, setup times or the bottleneck change significantly, for example after a setup reduction project, the arrival of a new machine, a change in product mix or a new long-term customer contract. Each of these events changes at least one input of the calculation, and sometimes moves the constraint to another machine. Outside those events, a quarterly review based on measured setup and cycle times is a reasonable rhythm for most shops. Start with the part numbers that run most often or consume the most bottleneck hours, since that is where an outdated lot size costs the most capacity." }, "name" : "How often should lot sizes be reviewed?" } ] } ```