Inventory planning
Service Level
By the Supply Chain Manager team · Updated October 16, 2026
Service level (cycle service level) is the probability of not running out of stock during a replenishment cycle, the target you set when sizing safety stock.
A 95% cycle service level means that in about 95 of 100 replenishment cycles the shelf never empties before the next delivery. It is a planning target, and it is not the same as fill rate, which measures the share of demand actually shipped.
In the statistical safety stock formula, the target service level becomes a z-score: about 1.28 for 90%, 1.65 for 95% and 2.33 for 99% under a normal distribution.
How to calculate service level
Safety Stock = z × σ(demand during lead time) z = the normal-distribution score for the target service level
Service level example
Illustrative numbers: if demand during the lead time has a standard deviation of 40 units, safety stock is about 51 units for 90% (1.28 × 40), 66 units for 95% and 93 units for 99%. Each extra point of service costs more stock than the last.
Why service level matters for distributors
Setting one service level for every SKU wastes money. Distributors usually set higher targets for A-class and critical items and lower ones for C-class items, where a rare stockout costs little.
Common service level mistakes
- Promising 99%+ service on everything without pricing the inventory it requires.
- Mixing up cycle service level with fill rate when reporting to customers.
Examples on this page use made-up round numbers to show the calculation. They are not benchmarks or results from real businesses.