# Safety stock calculator with lead-time variability

> Calculate safety stock from service level, demand variation and lead-time variation, compare it with the demand-only formula and see reorder point and value.

Source: https://sourcesquid.co/tools/safety-stock/

Planning tool · free

# Safety stock calculator. The right buffer, not a guess.

Safety stock is the extra stock held to cover swings in demand and in supplier lead time. Enter your service level, average demand and lead time with their standard deviations and this calculator gives safety stock, the reorder point, the stock value and how the buffer changes with service level.

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Safety stock**0**

Reorder point**0**

Safety stock value**–**

Service level| z| Safety stock| Value  
---|---|---|---  
  
SS = z × √(LT × σd² + d² × σLT²), which combines demand variation over the lead time with lead-time variation at average demand, assuming the two are independent and roughly normal. z is the one-sided standard normal value for the cycle service level. Use the same time unit (days) for demand and lead time. Results are rounded up to whole units.

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How it works

## Three steps. One clear answer.

  1. 01

Choose the service level: the chance of not running out in each order cycle.

  2. 02

Enter average daily demand and lead time, each with its standard deviation.

  3. 03

Read your safety stock, reorder point, stock value and the table across service levels.

Method and sources

SS = z × √(LT × σd² + d² × σLT²), where z is the one-sided standard normal value for the cycle service level (80% 0.8416, 85% 1.0364, 90% 1.2816, 95% 1.6449, 97.5% 1.9600, 98% 2.0537, 99% 2.3263, 99.5% 2.5758, 99.9% 3.0902), d and σd are the average and standard deviation of daily demand, and LT and σLT the average and standard deviation of lead time in days. The demand-only formula z × σd × √LT is shown for comparison. Reorder point = d × LT + SS. Demand and lead time are treated as independent and roughly normal.

Results are estimates to support your decisions. For binding figures, ask our team or your customs broker.

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Safety stock questions

## Straight answers.

Anything else, ask us directly. A principal replies, not a bot.

[anirudh@sourcesquid.co](mailto:anirudh@sourcesquid.co)

### What is the safety stock formula?

When both demand and lead time vary: SS = z × √(LT × σd² + d² × σLT²). When only demand varies it simplifies to z × σd × √LT.

### Which service level should I choose?

Many businesses use 95% for regular items and 98% to 99% for key items or items with high margins. Each step up costs more stock, so the table shows the trade-off.

### Why does lead-time variation matter so much?

Each extra day of delay must be covered at full average demand. With imports from Asia, a few days of variation in production or shipping often drives most of the buffer.

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

Take daily sales for a recent, representative period and use =STDEV.S() in a spreadsheet. Use weekly data divided by √7 if daily data is noisy, and keep all figures in days.

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

No. Cycle service level is the chance of no stock-out during an order cycle. Fill rate is the share of demand met from stock, which is usually higher for the same safety stock.

Use it with

## Tools that work well together.

[Planning**Reorder point** The reorder point (ROP) is the stock level at which you place the next order so it arrives before you run out: average daily demand × lead time + safety stock.](https://sourcesquid.co/tools/reorder-point/)[Planning**EOQ** Economic order quantity (EOQ) is the order size that minimises the combined yearly cost of placing orders and holding stock.](https://sourcesquid.co/tools/eoq/)[Planning**Inventory turnover** Inventory turnover is how many times you sell through your average stock in a period: cost of goods sold divided by average inventory.](https://sourcesquid.co/tools/inventory-turnover/)[Suppliers**Supplier health check** A supplier health check is a structured screen of a new factory's identity, capability and working style before you order samples or tooling.](https://sourcesquid.co/tools/supplier-health-check/)

From numbers to factories

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