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Lead time, cycle time and Little's Law calculator
How long does work really take, and how many cards should be in progress?
Little's Law
Fill in the two values you know. Work in progress = throughput x cycle time.
Cards started but not finished, on average.
Average cards finished in one week.
Enter the two values to see the answer.
Lead time, cycle time and work time
Lead time runs from the request to finished. Cycle time runs from started to finished, waiting included. Work time is the hands-on part. All in weeks.
In the queue, before anyone starts.
Blocked or waiting for review, after work started.
Someone is actively doing the work.
Enter the three times to see lead time, cycle time and how much of it is real work.
WIP limit starting points
Three common ways to pick a first limit. They are starting points, not rules.
Whole people, at least 1.
Only for the third formula.
Only for the third formula.
Enter the number of people to see the limits.
How to use it
- Pick a time unit. Choose days, weeks or months. Use the same unit for every number on the page.
- Choose what to find. Little's Law links three numbers. Choose the one you want, then fill in the other two.
- Split lead time from cycle time. Enter how long a card waits before work starts, how long it waits after it started, and the hands-on work. You get lead time, cycle time, work time and the share of the lead time that is real work.
- Try the WIP limits. Enter how many people do the work to get two starting limits. Add your throughput and the cycle time you want to get a third.
- Share it. Copy the link to send the same numbers, copy the result as Markdown, or download it as a CSV or an image.
How it works
Little's Law
WIP = throughput x cycle time. Rearranged, cycle time = WIP / throughput and throughput = WIP / cycle time. The result is rounded to 2 decimals. It describes averages over a long enough period for a stable flow; it does not predict a single card. The law was proved by John D. C. Little in "A Proof for the Queuing Formula: L = λW", Operations Research 9(3), 1961. Daniel Vacanti applies it to Kanban boards in "Actionable Agile Metrics for Predictability" (2015).
Lead time, cycle time and work time
lead time = waiting before start + cycle time, cycle time = waiting after start + work time and flow efficiency = work time / lead time. Many teams find that cards spend most of their lead time waiting, so a lower flow efficiency points at queues, not at slow people.
Three WIP limit formulas
- One card per person: limit = people. The strictest start.
- One and a half cards per person: limit = people x 1.5, rounded up. Leaves room when people wait on others.
- From Little's Law: limit = throughput x the cycle time you want, rounded, at least 1. It sets the limit from the speed you are aiming for.
The first two are rules of thumb taught in many Kanban courses; there is no single original source and no agreed multiplier, so treat them as a first guess. WIP limits are a core part of David J. Anderson's "Kanban: Successful Evolutionary Change for Your Technology Business" (2010). Start with one, observe, and adjust.
Want to count the days to a deadline instead? Use the days until calculator.
Try an example: 12 cards in progress, 4 finished per week
Questions
- What is the difference between lead time and cycle time?
- Lead time runs from the moment a request is made until it is finished. Cycle time runs from the moment work started until it is finished, waiting included. Work time is the hands-on part. Teams define the start point differently, so write down yours. This tool uses lead time = waiting before start + cycle time, cycle time = waiting after start + work time.
- What is Little's Law?
- For a flow that is stable over time, the average number of items in progress equals the average number finished per unit of time multiplied by the average time each item spends in progress. With 12 cards in progress and 4 finished per week, the average cycle time is 3 weeks.
- How many cards should be in progress at once?
- There is no single right number. A common start is one card per person, or a little more if people often wait on others. Pick a limit, watch the board for a few weeks, and change it. The tool shows three common starting points side by side.
- When does Little's Law not hold?
- It needs a stable flow: items start and finish at about the same rate, and the numbers are averages over the same period. If your backlog is growing fast, or you count a burst of work as one week, the answer will be off.
- Is anything sent or saved?
- No. The maths runs in your browser. The numbers you enter live only in the page address, so a copied link reproduces the result and nothing is stored on a server or in your browser.
Related
- Days until calculator: Count the calendar days, working days and weeks between today and any date.
- SLA calculator: Add up the time each stage gets and see if the turnaround you promised holds.
- What cycle time is
- What a WIP limit is
Set the limit on a real board
QodFlow lets you set a work-in-progress limit on each stage and shows how long jobs spend in each one.