Little's Law
Definition
Little's Law is the queuing relationship stating that average inventory in a system equals average throughput multiplied by average flow time (WIP = throughput × lead time), linking the three core measures of any process.
In Practice
Proved by John Little in 1961, the law holds for any stable process — a factory, warehouse, order desk, or port. Its power is that knowing two of the three values gives the third: a line completing 50 units a day with 400 units of work-in-process must have an average flow time of 8 days.
The planning consequence is blunt: for a fixed output rate, lead time is proportional to WIP. Releasing more orders into a full shop does not increase throughput — it just lengthens queues and lead times. That insight underlies CONWIP and kanban release control, drum-buffer-rope, and every lead-time-reduction program that starts by draining excess work-in-process instead of buying capacity.
Frequently Asked Questions
How is Little's Law applied in practice?
Use any two known values to find the third. A distribution center shipping 2,000 orders daily with 6,000 orders in process has a 3-day average cycle time. To promise 2-day fulfillment at that volume, work-in-process must be held near 4,000 orders — a direct target for release control.
Why does releasing more work not increase output?
Throughput is capped by the bottleneck's rate. By Little's Law, adding work-in-process beyond what keeps the constraint busy leaves throughput unchanged while flow time grows proportionally. A shop with double the necessary WIP delivers the same units per week at twice the lead time.
Does Little's Law require steady demand?
It holds for long-run averages in any stable system regardless of arrival patterns, which is why it is so widely applicable. During transients — ramp-ups, seasonal surges, drawdowns — instantaneous values deviate, but over a representative period the relationship between average WIP, throughput, and flow time still governs.
Related Calculators
Related Terms
Throughput is the rate at which a system produces sellable output, such as units per hour or orders shipped per day. In theory-of-constraints usage, it means the rate at which the business generates money through sales.
Lead TimeLead time is the elapsed time between initiating a process, such as placing an order, and its completion, such as receiving the goods. It is one of the most important inputs to inventory, planning, and customer promise decisions.
Pipeline InventoryInventory that has been ordered or shipped but has not yet arrived at its destination, such as goods in transit between a supplier and a warehouse. It is owned but not yet available to sell.
BottleneckA bottleneck is the resource or process step with the least capacity in a system, which limits the output of the whole chain. Total throughput can never exceed the bottleneck's rate.