Little's Law

Little's Law links the flow through a stable system to the amount of work sitting inside it. If items arrive at a steady average rate and spend a certain average time in the system, then the average number present must equal "rate × time" because each item contributes to the system for that long. This works for many queueing situations, even when the individual arrivals and service times vary, as long as the system is in long-run steady state.

Use this when you want to relate workload, waiting, and throughput in a queue or process, such as customers in a store, orders in a warehouse, or jobs in a production line. It is especially useful when two of the three quantities are known and you need the third.

• Mixing units, such as using calls per hour with minutes instead of converting time to hours
• Using the formula when the system is not in steady state, such as during a sudden surge or shutdown
• Confusing time in system with service time only, when the formula requires total time from arrival to departure