What is Little's Law?

Little's Law (L = λW) is a theorem in queuing theory that says: average items in the system (L) = arrival/throughput rate (λ) × average time each item spends in the system (W). It holds for any stable, non-preemptive system regardless of the arrival distribution or service order.

What counts as WIP (L)?

WIP (Work In Progress) is the average number of items currently being processed — tasks in a sprint, customers waiting in line, orders on the shop floor, etc. It excludes items waiting to enter the system.

What is throughput (λ)?

Throughput (also called arrival rate or departure rate in a stable system) is the average number of items completed per unit of time — e.g., 5 features per week or 120 customers per hour.

What is cycle time (W)?

Cycle time (also called lead time or sojourn time) is the average time a single item spends inside the system from start to completion. The unit must match the time unit used for throughput.

Does the time unit matter?

Yes — throughput and cycle time must use the same time base. If throughput is in items/day, cycle time must be in days. The calculator lets you pick hours, days, or weeks so the units stay consistent.

When does Little's Law not apply?

The law requires a stable system (average arrivals = average departures over time). It breaks down during ramp-up/ramp-down phases, if WIP grows unboundedly, or if arrivals drastically exceed capacity. Use it as a steady-state model, not a real-time predictor.

Little's Law Calculator

Name: Little's Law Calculator
Availability: InStock
Author: Nham Vu

Solve for any variable in Little's Law — enter two known values (WIP, throughput, or cycle time) and instantly get the third.

Little's Law: L = λW

Enter any two values. Select what you want to solve for.

Solve for

WIP — Average items in system (L)

Number of items currently in flight (tasks, tickets, customers…)

Throughput — Items completed per time unit (λ)

Cycle Time — Avg time per item in system (W)

Time unit for throughput & cycle time

Select what to solve for, enter two values, and click Calculate.

Formula Reference

L = λ × W

Solve for WIP: multiply throughput by cycle time.

λ = L ÷ W

Solve for Throughput: divide WIP by cycle time.

W = L ÷ λ

Solve for Cycle Time: divide WIP by throughput.

All three variables must use consistent units. If throughput is in items/day, cycle time must also be in days.

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Summary