Little's Law is a queueing theory relationship that holds for any stable system: average work in progress equals average throughput multiplied by average cycle time. In product and delivery teams, it shows why starting fewer things makes each item finish sooner: with the same throughput, less WIP means less waiting and a shorter cycle time.
How to use
- Fill in two of the three values: work in progress, throughput or average cycle time.
- Choose the units (items per day or per week; days or weeks) and whether a week has 7 calendar days or 5 working days.
- Read the calculated value and, if you fill in all three, the consistency check between them.
- Enter a WIP you want to test and compare the reduction scenarios in the table.
- Export the result or move on to the WIP limit simulator by column.
What each field asks for
| Field | What to enter |
|---|---|
| Work in progress (WIP) | Average number of items started and not yet finished. Leave blank to calculate. |
| Throughput | Average number of items finished per period. Leave blank to calculate. |
| Throughput unit | Items finished per day or per week. |
| Average cycle time | From the start of work to the delivery of each item. Leave blank to calculate. |
| Cycle time unit | Days or weeks. |
| A week has | Use the same basis the team uses to count cycle time. |
| WIP you want to test | A candidate limit: it is highlighted in the scenario table. |
Example
A product team has on average 12 items in progress and finishes 4 items a week. By Little's Law, each item takes on average 3 weeks, or 21 calendar days, from start to delivery. The team wants to test a limit of 8 items: with the same throughput, the average cycle time falls to 14 days, 7 days less per item (one third). The table also shows the cuts in between: with 10 items, 17.5 days; with 6, 10.5 days.
When to use
- To explain to the team or to leadership why starting less work makes deliveries come out sooner.
- To estimate average cycle time when you only have counts of items in progress and items delivered.
- Before setting WIP limits, to see how much cycle time could fall with each cut.
- To check whether the board metrics are consistent with one another and whether the system is stable.
Frequently asked questions
What does Little's Law say?
That, in a stable system, average work in progress equals average throughput times average cycle time. With two values, the third follows. It holds for long-run averages, not for a single item.
Which units should I use?
Items for WIP, items per day or per week for throughput, and days or weeks for cycle time. The calculator converts units, but the day basis must match how the team measures: 7 calendar days or 5 working days per week.
Why doesn't the calculation match my data?
Usually because the system is not stable (WIP grew or shrank over the period) or because the figures come from different windows. Measure all three over the same period, ideally 8 to 12 weeks, and check again.
Does reducing WIP always reduce cycle time?
With the same throughput, yes, in the same proportion. In practice, throughput tends to hold or even rise with less WIP, because there is less context switching. The limit is the point where people run out of work to pull.
Can I use Little's Law to forecast a date?
It describes system averages and helps you understand flow. To answer when a batch of items will be done, with a probability, use the simulation-based delivery forecast built on historical throughput.
What counts as work in progress?
Every item that has started and has not yet been delivered, including items that are blocked or waiting for review. Leaving blocked items out underestimates WIP and makes cycle time look shorter than it is.