Little's Law Calculator: WIP, Throughput and Lead Time
Little's Law says average lead time equals average WIP divided by average throughput. It is the quickest sanity check a scheduler has: if lead time is too long, WIP is too high, whatever the dispatch list says. This calculator applies it to a cell, a department or a plant.
Your numbers
Average jobs or units open between the first and last operation, not a single snapshot on a bad day.
Completed units leaving the last operation per scheduled day, averaged over several weeks.
Sum of value adding and necessary run and setup time per unit across all operations.
Hours the area is open per day, for example two 8 hour shifts.
Your results
Little's Law holds for a stable system in steady state with consistent definitions of WIP and throughput. It does not predict the lead time of a single order.
Lead time (days) as WIP grows
Sweeps Average WIP in the area from 20 to 600, holding your other inputs fixed.
Automate WIP and lead time reporting from your ERP
Optional: we can review how your ERP records job quantities and show how lead time by work center could be reported automatically.
Optional. The tool and every export work without this. No spam.
How to use the calculator
Enter the average WIP in the area you are measuring and the average daily throughput over the same period. Add the touch time per unit and the hours the area runs per day to see how much of the lead time is actual work. Then enter the lead time goal to get the WIP cap that supports it at the current throughput. Use averages over several weeks, because a single day of heavy WIP will mislead you.
- Count WIP between the same two points where you count throughput
- Use completed units out, not units started
- Keep units consistent: jobs with jobs, pieces with pieces
- Use scheduled hours, not clock hours, for flow efficiency
The formula behind it
Lead time = WIP / throughput. If 240 units sit in the area and 80 units leave per day, the average unit spends 3 days inside. Flow efficiency is touch time divided by lead time in the same hours. Six hours of touch time against 48 scheduled hours of lead time is 12.5 percent, which is typical for a job shop. The WIP cap is the same law run backwards: goal lead time times throughput.
Reading the chart and the result
The line is straight because the law is linear: every extra unit of WIP adds the same delay at fixed throughput. In practice throughput also falls when WIP is starved or flooded, so treat the chart as the best case. Cutting WIP is usually done by limiting releases to the floor, which is a planning parameter in most ERP systems, not a shop floor exhortation.
Common mistakes
The most frequent error is mixing calendar days and working days. If throughput is per scheduled day, lead time is in scheduled days. Another is counting finished goods or parts waiting on a kit as WIP in the area. Finally, do not apply the result to individual orders: Little's Law is about averages, so a hot job can still move through in a fraction of the average.
Frequently Asked Questions
Does Little's Law need a steady state?
Yes. WIP and throughput should be roughly stable over the period you average. During a ramp up or a big release spike the result drifts, so use a longer window or measure before and after the change separately.
What counts as WIP?
Any unit that has started processing and not yet left the boundary you chose. Raw material in stores and finished goods are outside the boundary. Parts on hold for quality or waiting for a missing component are inside it, because they still delay the unit.
How is this different from takt time?
Takt time is the pace customers demand, calculated from demand and available time. Little's Law describes the pace the system actually delivers and the time units spend inside. They answer different questions and are often used together.
Can I use it for a single machine?
Yes, as long as you count the queue plus the unit being processed as WIP and the output rate of the machine as throughput. The result is the average time a job spends waiting and running at that machine.
If your ERP already holds open job quantities and completion transactions, we can pull WIP and throughput per work center automatically and show lead time trends without anyone keying numbers.
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