Kanban and WIP Limits: Finish More by Starting Less

Kanban is a Japanese word meaning signboard or card. At Toyota, kanban cards signal when to make or move more parts, so nothing is made until it is needed. Most small businesses meet kanban today as a board with three columns (to do, doing and done) and a limit on how many jobs can sit in the doing column at once.

That limit, the WIP (work-in-progress) limit, is the part that makes the difference. Without it, a kanban board is just a to-do list.

Little's Law

In 1961 John Little published a proof of a simple relationship that holds for any stable queue:

If you finish 20 jobs a week and 60 are in progress, each job takes three weeks on average from start to finish. Nobody has to work any faster for lead time to fall. Starting fewer jobs at once is enough.

Worked example

A small joinery takes on jobs as they are ordered. It has 60 jobs in progress and finishes about 20 a week:

• Lead time: 60 ÷ 20 = 3 weeks, or 15 working days.
• Target: 7 working days.
• WIP limit for the target: 20 jobs a week ÷ 5 days × 7 days = 28 jobs.

If the joinery stops starting new jobs until work in progress falls to 28, average lead time falls to around seven days. Throughput stays the same, and customers wait less than half as long.

Calculate your WIP limit

Count every job started but not finished, including those waiting for parts or a customer reply.

Setting up a simple kanban board

1. Draw three columns: to do, doing and done. Add columns for real stages only, such as "waiting for parts", if they matter.
2. One card per job. A sticky note or a card in a free tool.
3. Write the WIP limit at the top of the doing column.
4. Stop starting, start finishing. When the column is full, help finish something instead of starting something new.
5. Review weekly. Look at the cards that have been stuck longest and ask why.

Why it feels wrong at first

Limiting work in progress can feel like turning work away. It is not: the jobs still get done, just started later and finished sooner. Customers care about when the job is finished, not when it was started. Fewer jobs in progress also means less switching between them, which is a waste of its own.