Should I use today's WIP count or a period average?

Use today's count to decide today's releases, because days of supply describes the stock you hold now. Use a period average for flow time or WIP turns over a month.

Is WIP days the same as manufacturing lead time?

Only on average and when flow is steady. By Little's law, average WIP divided by throughput equals the average time a unit spends in process; a single count can sit above or below that average.

Where does a WIP days target come from?

From your own line. Set it below today's figure and lower it in steps while daily output holds; the step at which the bottleneck starts waiting for work marks the practical floor.

Will holding releases hurt on-time delivery?

Not while the WIP left on the floor keeps the bottleneck busy: the line keeps shipping from work already released while it drains. Orders wait in the office instead, where they can still be resequenced.

Lean Manufacturing & Operations calculator

WIP Days of Supply Calculator

Find how many production days of output your current WIP represents and what it holds at cost. You need a WIP count, daily throughput, WIP value at cost and your target.

What this calculator does

Production days of output held as WIP on the floor now, checked against your target, with the value at cost above it.

Formula used

WIP days of supply = current WIP ÷ daily throughput
WIP at target = target WIP days × daily throughput; WIP above target = MAX(0, current WIP − WIP at target)
Average cost per WIP unit = WIP value at cost ÷ current WIP
WIP value above target = WIP above target × average cost per WIP unit
WIP value per day of supply = average cost per WIP unit × daily throughput

Inputs explained

Current WIP: Units on the floor now, from first release to the last operation.
Daily Throughput: Average good units completed per production day over recent weeks.
WIP Value at Cost: Open work orders at cost from the ERP, same operations.
Target WIP Days: Your WIP cap, in production days of output.

How to use the result

Best suited to deciding whether to hold new order releases, setting a WIP cap in days of output, showing finance the cash tied up in WIP.
Removed units are valued at the average WIP cost; the units you actually take out may carry more or less. A single count can catch a spike; count at the same time each day. Cash is freed only if material buying follows releases; otherwise the value moves into raw stock.

Current U.S. benchmarks

U.S. manufacturing runs at 75.7% of capacity (Federal Reserve, Aug 2026). New factory orders are up 8.5% year over year (Census).

Common questions

Should I use today's WIP count or a period average? Use today's count to decide today's releases, because days of supply describes the stock you hold now. Use a period average for flow time or WIP turns over a month.
Is WIP days the same as manufacturing lead time? Only on average and when flow is steady. By Little's law, average WIP divided by throughput equals the average time a unit spends in process; a single count can sit above or below that average.
Where does a WIP days target come from? From your own line. Set it below today's figure and lower it in steps while daily output holds; the step at which the bottleneck starts waiting for work marks the practical floor.
Will holding releases hurt on-time delivery? Not while the WIP left on the floor keeps the bottleneck busy: the line keeps shipping from work already released while it drains. Orders wait in the office instead, where they can still be resequenced.

Related guides

Last reviewed 2026-10-01.