Production Parameters
Total time a unit spends in the production system.
Cost Parameters
Average material + labor cost of one unit while in process.
Typically 20% to 30% of inventory value per year.
Improvement Target
Goal cycle time after process improvement. Used to calculate target WIP and excess inventory.
Little's Law Reference
WIP = Throughput Rate x Cycle Time
Reducing cycle time directly reduces WIP
Lower WIP improves cash flow and quality visibility
World-class WIP turnover: 12 to 50+ turns/year
WIP Analysis
Enter production data to calculate WIP levels.
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Introduction
Work-in-progress inventory is the inventory most manufacturers underestimate and most lean practitioners target first. It sits on the production floor, consuming warehouse space, tying up working capital, and hiding quality problems until they have multiplied. If you have 4 hours of WIP in your system and a defect occurs, you might build 4 hours worth of defective product before anyone catches it. If you have 4 days of WIP, you discover the problem after producing thousands of bad units. John Little's 1961 theorem provides the mathematical foundation: average WIP equals throughput rate multiplied by average cycle time. It is one of the most broadly applicable equations in operations management. In 2026, manufacturers applying Little's Law to WIP reduction consistently report 20% to 40% reductions in in-process inventory and corresponding improvements in cash flow, lead time, and defect detection.
What This Calculator Does
This Work In Progress inventory calculator applies Little's Law to determine the steady-state WIP level from throughput rate and cycle time. It calculates inventory value, holding costs, WIP turnover rate, and days of supply. The calculator also models improvement targets by comparing current WIP against a target cycle time to quantify excess inventory and tied-up capital. In 2026, WIP reduction remains a primary lever for improving cash flow, quality visibility, and lead time in both discrete manufacturing and process industries.
The Formula
Little's Law states that the long-term average number of items in a stable system equals the long-term average arrival rate multiplied by the average time an item spends in the system. In manufacturing, this means WIP equals the throughput rate (units per hour) times the cycle time (hours a unit spends in the system). Reducing cycle time directly reduces WIP, which reduces tied-up capital, holding costs, and the time to detect quality issues.
Step-by-Step Example
Enter production parameters
Throughput: 200 units/day. Operating hours: 16/day. Average cycle time: 4 hours.
Enter cost parameters
Average unit cost in WIP: $150. Annual holding cost rate: 25%.
Set improvement target
Target cycle time after kaizen: 3 hours.
Review WIP analysis
Current WIP: 50 units ($7,500 value). Target WIP: 38 units. Excess WIP: 12 units ($1,800). Annual holding cost: $1,875. Reducing cycle time from 4 to 3 hours would free $1,800 in working capital.
Real-World Use Cases
Lean Manager Reducing Inventory
Use Little's Law to quantify the WIP reduction achievable through cycle time improvement projects, providing financial justification for kaizen events and capital investments.
CFO Analyzing Working Capital
Calculate the dollar value of WIP on the production floor and the annual holding cost to identify opportunities for freeing cash through process improvements.
Supply Chain Analyst Setting Targets
Establish WIP targets for each production cell based on throughput requirements and target cycle times, creating visual management standards for daily monitoring.
Comparison
| Industry | Typical WIP Turns/Year | WIP as Days of Supply | World-Class Target |
|---|---|---|---|
| Automotive Assembly | 50 - 200+ | 1 - 7 days | Hours of supply |
| General Discrete Mfg | 12 - 50 | 7 - 30 days | Under 5 days |
| Electronics Mfg | 20 - 60 | 6 - 18 days | Under 3 days |
| Process Industry | 6 - 20 | 18 - 60 days | Under 14 days |
| Job Shop / Custom Mfg | 4 - 12 | 30 - 90 days | Under 20 days |
Common Mistakes to Avoid
Confusing cycle time with processing time. Cycle time in Little's Law is the total time a unit spends in the system, including queue time, not just the time spent being actively worked on. Queue time often accounts for 80% to 95% of total cycle time.
Applying Little's Law to an unstable system. The law requires a system in steady state (average throughput in equals average throughput out over time). During ramp-ups, shutdowns, or major demand changes, the calculation may not reflect reality.
Using replacement cost instead of WIP cost. The unit cost for WIP should reflect the average investment at the midpoint of the production process, not the finished goods selling price.
Ignoring the quality impact of high WIP. Beyond financial costs, high WIP means defects are discovered much later, increasing rework cost and complicating root cause analysis.
Frequently Asked Questions
Accuracy and Disclaimer
This calculator uses Little's Law to estimate steady-state WIP levels. Actual WIP varies with demand fluctuations, equipment reliability, quality issues, and scheduling changes. Use these calculations for planning and target-setting. Regular physical inventory counts are needed to validate calculated levels.
Conclusion
WIP reduction follows directly from cycle time reduction. Shorten how long a unit spends in the system and your WIP drops proportionally. The Cycle Time Calculator identifies which station is adding the most time to your system cycle, making it the natural companion to this tool. For the raw material side of the inventory equation, the EOQ Calculator optimizes order quantities to prevent excess raw material from building up before WIP even starts.
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