Little's Law Calculator (WIP, Throughput, Lead Time)
Little's Law for any process: solve for work in progress, throughput or lead time from the other two — with WIP limits for a target lead time, queue length and how much inventory a cycle-time reduction frees.
How it works
WIP = throughput × lead time (L = λW); lead time = WIP ÷ throughput; throughput = WIP ÷ lead time — valid for any stable system measured over the same period
Little’s Law states that, for any stable process, the average number of items inside it equals the average arrival (or completion) rate multiplied by the average time each item spends inside. It holds for factories, hospitals, help desks, software teams and supermarket queues without any assumption about the arrival pattern, which makes it the most useful sanity check in operations: if you know two of WIP, throughput and lead time, the third is fixed. Its practical lesson is that lead time falls only when WIP falls or throughput rises — so starting more work does not finish it sooner; it lengthens every queue. Kanban WIP limits, order release rules and “stop starting, start finishing” all follow from this relationship.
Worked example
120 orders in the system and 15 completed a day: average lead time 8 days. To promise 4 days at the same rate, WIP must fall to 60 orders — freeing 60 × $500 = $30,000 of work-in-progress — or throughput must double to 30 a day.
Frequently asked questions
Does it work for a single unstable week?
No — the law describes long-run averages of a stable system (arrivals ≈ departures). Over short windows with growing backlogs it under-predicts lead time.
How does this relate to inventory turns?
Turns = throughput ÷ inventory = 1 ÷ lead time, so a 30-day dwell is 12 turns a year: the same law in financial clothing.
Related calculators
Operations planning estimate based on the figures entered; validate against measured shop-floor data before changing staffing, equipment or customer commitments.