Little’s Law

Little’s Law says that, in a stable system, the average number of items present equals how many items enter per unit time multiplied by how long each item typically stays. It works because, over a long enough period, "items in system" accumulate as the rate of arrivals times the average time each item spends there, so the total item-time must balance. This applies whether the items are customers, packets, jobs, posts, or any other flow through a queue-like process.

Use this when a system is running steadily and you know any two of: average arrival rate, average time in system, or average number in the system. It is common in queueing, operations research, networking, call centers, manufacturing, and performance analysis of servers or workflows.

• Mixing time units, such as using arrivals per hour with waiting time in minutes
• Using the formula for a non-steady system where arrival rate or waiting time is changing rapidly
• Confusing average time in system with only queue time and forgetting service or processing time if the total time is required