Work-in-Process (WIP) Calculator
Calculate WIP inventory value across production stages using Little's Law and estimate carrying costs.
About this calculator
This calculator applies Little's Law -- one of the most fundamental relationships in operations management -- to estimate how much inventory value is tied up in a production line at any given moment. Little's Law states that Work-in-Process (WIP) equals Throughput multiplied by Lead Time: if a line finishes 200 units a day and each unit spends 5 days moving through production, roughly 1,000 units are somewhere in process at any instant, regardless of how many workstations or process steps sit between raw material and finished goods. Daily Throughput is the dominant driver of WIP Inventory Value because it's the input with by far the widest realistic range across manufacturing operations -- a single work cell might finish a handful of units a day while a high-volume line finishes hundreds of thousands -- and WIP value scales directly and linearly with it.
Average Cost per Unit converts the WIP quantity into a dollar figure (the capital literally parked on the shop floor mid-process), and Annual Carrying Cost applies a standard percentage-of-value holding-cost assumption covering capital cost, storage, obsolescence risk, and handling. What this calculator does NOT model: variability in lead time (Little's Law is a long-run average relationship; real WIP fluctuates around this mean as bottlenecks and queues change), WIP valued at different completion percentages (every WIP unit is priced at the same average cost, even though a unit that just started costs far less than one nearly finished), or the operational tradeoffs of actually shortening lead time, such as smaller batch sizes or more frequent changeovers, which carry their own costs this tool ignores.
Inputs
Results
WIP Inventory Value
$25,000.00
≈ 13 gaming PCs
How to Use This Calculator
- Enter the average daily throughput in units produced per day.
- Input the average production lead time in days (Little's Law: WIP = throughput × lead time).
- Enter the average accumulated cost per WIP unit and the annual carrying cost percentage (typically 20-30%).
- Review the estimated WIP units, WIP inventory value, and annual and monthly carrying cost.
- Compare WIP turns per year and days of inventory, and see the potential savings if lead time is halved.
How the result changes with Daily Throughput
| Daily Throughput | WIP Inventory Value |
|---|---|
| 100 | $12,500.00 |
| 150 | $18,750.00 |
| 300 | $37,500.00 |
| 500 | $62,500.00 |
What each input means
- Daily Throughput
- Average number of finished units produced per day.
- Production Lead Time
- Average time a unit spends in production from start to finish.
- Average Cost per Unit
- Average accumulated cost per WIP unit (material + labor + overhead to midpoint).
- Annual Carrying Cost
- Annual inventory carrying cost as percentage of value (typically 20-30%).
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersDaily Throughput = 200, Production Lead Time = 5, Average Cost per Unit = 25, Annual Carrying Cost = 25 = 4 input(s) provided
- Calculate WIP Inventory ValueWIP Inventory Value25000 = $25,000
- Calculate WIP UnitsWIP Units1000 = 1000
- Calculate Annual Carrying CostAnnual Carrying Cost6250 = $6,250
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
How is WIP Inventory Value calculated from throughput and lead time?
This calculator multiplies Daily Throughput by Production Lead Time to get WIP Units (Little's Law), then multiplies WIP Units by Average Cost per Unit to get WIP Inventory Value. At the default 200 units/day and a 5-day lead time, 1,000 units are in process; at $25 average cost per unit, that's $25,000 of capital sitting on the floor before a single unit reaches a customer.
Does the Annual Carrying Cost percentage change how much WIP inventory I need?
No -- Annual Carrying Cost only scales the Annual Carrying Cost and Monthly Carrying Cost outputs; it has zero effect on WIP Units, WIP Inventory Value, WIP Turns per Year, or Days of Inventory, which are driven entirely by Daily Throughput, Production Lead Time, and Average Cost per Unit. It's purely a cost-of-holding assumption applied after the physical WIP quantity is already fixed.
Why does WIP Turns per Year fall as lead time increases?
WIP Turns per Year is 365 divided by Production Lead Time in days, so a longer lead time always produces fewer turns -- a 5-day lead time turns WIP roughly 73 times a year, while a 30-day lead time turns it only about 12 times. Fewer turns generally signal more capital tied up in process relative to output, which is why lean manufacturing initiatives target lead-time reduction directly.
What happens to WIP value if I cut lead time in half?
Because Little's Law makes WIP Units directly proportional to lead time, halving Production Lead Time roughly halves WIP Units and therefore WIP Inventory Value -- the calculator reports this directly as Savings if Lead Time Halved. This is the core financial argument behind lean initiatives like smaller batch sizes, faster changeovers, or eliminating queue time between operations.
Is this calculator accurate for a production line with highly variable cycle times?
Only as a long-run average. Little's Law holds exactly for the long-run average relationship between throughput, lead time, and WIP, but it says nothing about variability -- two lines with identical average throughput and lead time can carry very different peak WIP if one has erratic cycle times and bottleneck queuing while the other runs smoothly, so treat this output as a planning baseline, not a real-time WIP target.
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