Post-calculation in machining: actual machine time next to your estimate
A post-calculation compares the actual times and costs of a finished Production Order with what you estimated. In machining, that comparison is mostly about machine time. This is how to do it, and how it improves your prices and margins.
Updated: 11 October 2026
What is a post-calculation?
Before a Production Order starts, you make an estimate: how much setup time, how many minutes per part, which machine-hour rate. The price in your quote is based on it. The post-calculation is made once the Production Order is finished. You put the actual times and costs next to the estimate and look at where they differ.[1]
Many machine shops skip this or do it only now and then. Actual times are hard to collect: an operator writes them on a job card, someone retypes them, and short interruptions get lost. The next estimate then remains a guess based on the previous guess.
Why it pays off
- You see which parts lose money. A part that keeps running longer than estimated earns less with every new Production Order. Without post-calculation you notice only in the annual results.
- You quote sharper where you can. If a part runs faster than estimated, there is room in the price. That can decide the next request for quote.
- Your estimates learn from practice. Measured times from earlier Production Orders are a better basis than rules of thumb.
- You find process losses. A large gap is not always an estimating error. Sometimes the process got slower: a worn tool, a lower feed, a long setup.
What to measure per Production Order
For a post-calculation of machine time you need these times per Production Order. They follow the status of the machine: Setup (magenta), Active (green), Change (orange) and Not Productive (gray).
| Time | What it is and when it weighs most |
|---|---|
| Setup time | The magenta time before the first part: the machine is being set up, with clamping, tools and zero point. Once per batch, so it weighs most in small batches. |
| Active time | The green time in which the machine is machining the workpiece. The largest part of the cycle time. |
| Workpiece change time | The orange time in which the workpiece is swapped between two parts, not the tool change. The second part of the cycle time: small each time, large in total. |
| Cycle time | Active time plus workpiece change time: the time per part. Counts per part, so it weighs most in large batches. |
| Downtime | The gray time in which the machine stands still during the Production Order: waiting for material, for approval, a fault. Rarely in the estimate, always in the lead time. |
You also need the number of good parts and the scrap. A Production Order of 100 parts with 6 to be remade costs more machine time than the estimate says, even if every cycle was on time.
Compare per part and per batch separately
Do not convert setup time into a time per part before comparing. Setup time depends on how often you set up, cycle time (active time plus workpiece change time) on the number of parts. If you add everything up and divide by the quantity, a small batch shows a part time that is too high and a large batch one that is too low, and you cannot tell where the gap comes from.
Worked example
The numbers are a made-up example. A Production Order of 200 parts on a machining center. The estimate assumes 45 minutes of setup and a cycle time of 6:00 minutes per part, active time and workpiece change together. The machine-hour rate is €80.
| Estimate | Actual | |
|---|---|---|
| Setup time | 45 min | 70 min |
| Cycle time per part | 6:00 min | 7:30 min |
| 200 parts | 1,200 min | 1,500 min |
| Total | 1,245 min | 1,570 min |
The difference is 325 minutes, a little over 5.4 hours. At €80 per hour that is €433 that was not in the price. Of those 325 minutes, 300 come from the cycle time and 25 from the setup. So the first question is not why the setup ran over, but why every part took a minute and a half longer. Measured as active time and workpiece change time, you see whether the machining or the change took longer.
If the same part runs at 7:30 again on the next Production Order, the estimate was wrong and you change it. If it was a one-off, for example because of different material, you note the cause and leave the estimate as it is.
How to act on it
- Look at the largest gaps every week. Sort the Production Orders finished that week by the difference between actual and estimated time. Take the three biggest losers and the three biggest winners.
- Ask for each loser: estimate or process? If the estimated time is wrong, change the estimate. If the process is slower than it can be, work on the process.
- Use the winners for your pricing. If a part keeps running faster than estimated, you can quote sharper or keep the margin on purpose. Both are a choice, not luck.
- Treat setup time separately. Shorten long setups with SMED: everything that can be done while the machine is still running is done beforehand.[2]
- Update the estimate with measured times. For a new Production Order of a known part, use the average measured time of earlier batches, not the old estimate.
Common mistakes
- Comparing only the total time. You see that something went wrong, but not what.
- Using times from job cards. Manual recording misses short stops and rounds off.
- Forgetting scrap. Every rejected part costs a full cycle nobody pays for.
- Taking one outlier as the new standard. Look at several batches before changing an estimate.
Post-calculation with measured machine times
Setup time, active time, workpiece change time and downtime come straight from the control on CNC machines; the cycle time is active time plus workpiece change time. Smart Factory measures them automatically per machine, per batch and per NC program. For each NC program you see the paid time, the ideal time and the average actual time side by side. The paid time comes from the ERP (Ultimate plan) or you enter it by hand. After every batch you see whether the work stayed within the estimate, without operators writing down times.
Frequently asked questions
What is the difference between an estimate and a post-calculation?
The estimate is made before the Production Order starts, with expected times and costs, and the price is based on it. The post-calculation is made afterwards, with the actual times and costs, and compared with the estimate.
How often should I do a post-calculation?
Record the actual times for every finished Production Order and review the gaps once a week. That way you spot patterns before the same part is ordered again.
Should I calculate setup time per part?
Not when comparing. Compare setup time per batch and cycle time per part, so you see where a gap comes from. Only for the price do you spread the setup time over the quantity.
Do I need an ERP system for post-calculation?
No. You can also record the estimated times by hand. With an ERP connection they arrive automatically with the Production Order.
About Smart Factory
Smart Factory measures setup time, active time, workpiece change time and downtime straight from the CNC control and puts them next to the paid time for every NC program. More: Smart Factory and Pricing.
Sources
- Duden Wirtschaft von A bis Z, 6. Auflage 2016: Kalkulation (Lizenzausgabe der Bundeszentrale für politische Bildung)
- Shigeo Shingo: A Revolution in Manufacturing. The SMED System. Productivity Press, 1985
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