Fill Rate from Safety Stock Calculator
Cycle service level and fill rate are different promises. One counts cycles without a stockout, the other counts units shipped from stock. This calculator shows the fill rate your safety stock and lot size really give you.
Your numbers
Daily sigma times the square root of lead time days, if demand is independent.
Your results
Normal demand over lead time and an order quantity that is large relative to sigma are assumed. Results are planning estimates.
Fill rate and cycle service level vs safety stock
Sweeps Safety stock from 0 to 600, holding your other inputs fixed.
Fill rate by item from your ERP
Optional: we can map actual shipments to planned service levels for your item base.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the mean and standard deviation of demand over the replenishment lead time, your safety stock and the typical order quantity. The output shows the cycle service level, expected units short per cycle and the fill rate. Use the slider to see how much each extra block of safety stock buys.
- Sigma is for the lead time, not per day
- Use the real order quantity, including supplier minimums
- Compare against the target in your customer contract
The method
Expected shortage per cycle equals sigma times L(z), where L(z) = phi(z) - z(1 - Phi(z)) is the unit normal loss function and z is safety stock divided by sigma. The fill rate is one minus expected shortage divided by order quantity. Large lots give high fill rates for the same safety stock, because each cycle has fewer exposure points.
Reading the result
A cycle service level of 90% can still mean a fill rate above 98% when lots are large. The chart shows fill rate rising faster and flattening sooner than cycle service level. If your customers measure line fill or on-time-in-full, base safety stock on fill rate and you will usually carry less than a cycle service target would suggest.
Common mistakes
Do not mix the two measures in one report. Do not apply the formula to a tiny order quantity, where the shortage can exceed the lot and the estimate loses meaning. In ERPs the planning parameter is usually a safety stock quantity or days of supply, so use this to justify the number, not to replace the field.
Frequently Asked Questions
What is the difference between fill rate and service level?
Cycle service level is the probability of no stockout during a replenishment cycle. Fill rate is the share of demand units filled from stock. Fill rate is usually higher than cycle service level for the same safety stock.
What is L(z)?
The unit normal loss function gives the expected shortage per standard deviation of demand when the safety factor is z. Multiply by sigma to get units short per cycle. It equals phi(z) minus z times (1 minus Phi(z)).
Why does a larger order quantity raise fill rate?
Shortages only happen near the end of each cycle. A larger lot means fewer cycles per year and fewer exposure windows, so a fixed expected shortage per cycle is spread over more units sold.
Can I use this for make to order items?
Not directly. It assumes stocked replenishment with a known lot. For make to order work use promise date and capacity based measures such as on-time delivery instead.
If you want fill rate tracked per item against actual shipments in your ERP, Netray can build that report on your own data.
Related Tools
Periodic Review Order-Up-To Level Calculator
Set the order-up-to level S for items you review on a fixed schedule, with safety stock sized to the review period plus lead time.
Supply Chain & InventorySafety Stock Calculator with Demand and Lead Time Variability
Compute safety stock and reorder point when both demand and supplier lead time vary, and see how lead time variability inflates the buffer.
Supply Chain & InventoryNewsvendor Single-Period Order Quantity Calculator
Find the best one-time order quantity for seasonal, short-life or last-time buys using the critical ratio.
Supply Chain & InventorySupplier Delay Stockout Exposure Calculator
Test what a late purchase order does to coverage: days short, units short and the expedite quantity needed.
More Supply Chain & Inventory tools
See all- Oracle Fusion Inventory Turns and Carrying Cost Calculator
- Distribution ERP Fill Rate Calculator
- Distributor Rebate Tracking Worksheet
- Branch Transfer Planning Worksheet
- Distributor Slow-Moving Inventory Review Worksheet
- Distributor Vendor Performance Scorecard Worksheet
- Distributor GMROI Calculator
- Distributor Backorder Cost and Fill Rate Calculator
- EOQ with Vendor Minimum Order Calculator