# Safety Stock

**URL:** https://kaizumi.com/dictionary/safety-stock

**Description:** Safety stock is inventory held to protect against uncertainty in supply—delivery delays, quality rejections, or supplier disruptions.

**Category:** lean-tools

**Tags:** flow, foundational

## Definition

Safety stock is inventory held as insurance against supply variability—late deliveries, quality problems, or unexpected supply disruptions. While working stock (cycle stock) covers normal demand during normal replenishment time, safety stock covers the unexpected: the truck that's late, the batch that fails inspection, the supplier that has a fire. Safety stock quantity is calculated based on supply variability and acceptable stock-out risk. Lean systems minimize safety stock by improving supplier reliability and quality, but some safety stock is usually necessary for external supply sources.

## Examples

### Manufacturing

> A plant receives parts from a supplier with 95% on-time delivery. The 5% late deliveries average 2 days. Safety stock is set at 2 days of usage to cover typical delays. If supplier reliability improves to 99% on-time, safety stock can be reduced to 1 day.

### Healthcare

> A hospital pharmacy maintains safety stock of critical medications based on supplier reliability and criticality of the drug. Life-saving medications with single-source suppliers carry 30-day safety stock; common medications with multiple suppliers carry 7-day safety stock.

### Retail

> A retailer calculates safety stock based on supplier lead time variability and demand forecast accuracy. Items with reliable suppliers and predictable demand carry minimal safety stock; items with long, variable lead times or unpredictable demand carry more.

## Key Points

- Distinct from buffer stock (which protects against internal process variability)
- Sized based on supply variability and acceptable stock-out risk
- Should be systematically reduced as supply reliability improves
- Not free insurance—carries real costs that should drive improvement

## Common Misconceptions

**Safety stock means we can ignore supplier problems.** Safety stock buys time, not permanent protection. Chronic supplier issues consume safety stock and require permanent solutions.

**More safety stock is safer.** Excess safety stock ties up capital, hides problems, and may obsolete before use. The right level balances protection against carrying cost.

## Try It

The calculator below reports safety stock on its own line, separate from the working stock in the loop. Enter daily demand, replenishment lead time, container quantity, and a safety factor. The result shows how many units exist only to cover supply uncertainty. When a supplier gets more reliable, lower the factor and read off exactly how much inventory that releases.

<!--TOOL:kanban-->

## See Also

- [Buffer Stock](/dictionary/buffer-stock) - Protection against process variation
- [Supermarket](/dictionary/supermarket) - Where safety stock is held

Source: https://kaizumi.com/dictionary/safety-stock
Licence: free to quote and cite with attribution to Kaizumi.