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Little's Law

Why It Matters

It connects three things you care about (inventory, throughput, cycle time) with a hard constraint. You can't improve all three independently.

The Key Move

Pick two of the three variables. The third is determined. Want lower cycle time with same throughput? You must reduce inventory. Want higher throughput with same cycle time? Inventory must grow.

Classic Application

Call centers. Average customers on hold = arrival rate * average wait time. You can't reduce wait time without either reducing arrivals or adding capacity.

Surprising Application

Work-in-progress in knowledge work. If you have 10 projects and complete 1 per week, average project age is 10 weeks. Want faster delivery? Reduce concurrent projects, not just "work harder."

Failure Modes

  • Ignoring the law and expecting to improve all three variables
  • Applying to unstable systems (arrival rate exceeds capacity)
  • Measuring the wrong boundaries

Go Deeper

Any queuing theory text. Factory Physics by Hopp and Spearman.