# Safety Stock Calculator

> Calculate safety stock with the standard z × σ method from demand and lead-time variability at a chosen cycle service level, plus the reorder point.

Category: Commerce & retail · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/safety-stock-calculator

Safety stock is the buffer that covers demand and lead-time swings. The standard method multiplies the variability of demand over the lead time by a z value for your target service level.

Also called: Buffer stock calculator.

## Inputs
- Average daily demand (unit units/day; default 50)
- Standard deviation of daily demand (unit units/day; default 10)
- Average lead time (unit days; default 9)
- Standard deviation of lead time (unit days; default 0; optional)
- Cycle service level (options: 80% (z = 0.84), 85% (z = 1.04), 90% (z = 1.28), 95% (z = 1.64), 97.5% (z = 1.96), 98% (z = 2.05), 99% (z = 2.33), 99.5% (z = 2.58), 99.9% (z = 3.09); default 95)

## Results
- Safety stock
- Safety stock (rounded up)
- Reorder point
- z value used
- Std. dev. of lead-time demand

## Formula

```
Safety stock = z × √(L × σd² + d² × σL²);  Reorder point = d × L + Safety stock
```

Where:
- z: standard normal quantile for the cycle service level
- d, σd: mean and standard deviation of daily demand
- L, σL: mean and standard deviation of lead time in days

## Worked example
1. σ over the lead time = √(9 × 10²) = 30 units.
2. Safety stock = 1.644854 × 30 = 49.35, so hold 50 units.
3. Reorder point = 50 × 9 + 49.35 = 499.35 units.

**Result:** For a 95% cycle service level, demand of 50 ± 10 units a day and a 9-day lead time need about 50 units of safety stock.

## What the result means

A 95% cycle service level means about 1 replenishment cycle in 20 runs out before the delivery arrives. Going from 95% to 99% raises safety stock by about 41% (z 1.64 → 2.33).

## Assumptions and limits
- Daily demand is roughly normally distributed and independent from day to day.
- Demand and lead time vary independently.
- Service level means the probability of no stock-out during a replenishment cycle (cycle service level), not fill rate.
- Slow-moving or very lumpy items do not follow a normal distribution; this method under- or over-states their buffer.

## Questions

### How do I get the standard deviation of demand?

Take daily sales for a recent, typical period and use the Standard Deviation Calculator on them.

### Is service level the same as fill rate?

No. Cycle service level is the chance of not running out in a cycle; fill rate is the share of demand met from stock, which is usually higher.

## Related calculators
- [Stock Reorder Point Calculator](https://calculators.geekmidst.com/calculators/stock-reorder-point-calculator)
- [Economic Order Quantity (EOQ) Calculator](https://calculators.geekmidst.com/calculators/economic-order-quantity-calculator)
- [Standard Deviation Calculator](https://calculators.geekmidst.com/calculators/standard-deviation-calculator)

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/safety-stock-calculator