Safety Stock Calculator

Why safety stock matters

Safety stock is the buffer between your expected demand during lead time and the inventory you hold to protect service levels. Even well-planned supply chains face variability: customer demand changes week to week, supplier production lines have delays, and transportation schedules are affected by congestion or weather. Safety stock is the planned protection against those shifts so you can maintain a stable fill rate without constantly expediting or re-planning.

The standard safety stock formula uses a normal distribution approximation. It converts your desired service level into a Z-score and multiplies that by the standard deviation of demand over the lead time. If daily demand volatility is known, you scale it by the square root of lead time to account for variability over multiple days. Higher service targets or more variable demand both increase safety stock. If you carry too little, stockouts become more common. If you carry too much, capital and storage costs rise. The right safety stock balances service and cost in a way that aligns with your customer promise and operational risk tolerance.

In practice, safety stock is rarely static. Many teams review it monthly or quarterly, adjusting for seasonality, supplier changes, or new product introductions. If your lead time variability is significant, you can extend the model to include lead time variance as well, but the core formula here still provides a reliable baseline. Pair the output with your reorder point so the buffer is actually used in the replenishment trigger. That turns a statistical buffer into a practical policy that is easy to execute in an MRP or WMS system.

This calculator is designed for quick scenario analysis. You can move the service level slider, change demand variability, and immediately see the effect on safety stock. That makes it useful for negotiating service targets, evaluating forecast improvements, and planning inventory budgets. Because calculations are done locally in the browser, sensitive demand data stays private.

Formula

Safety stock: SS = Z * sigma_d * sqrt(L)

Lead time demand: d_avg * L

Lead time variability: sigma_d * sqrt(L)

Example calculation

Assume average daily demand is 120 units, daily demand standard deviation is 35 units, lead time is 14 days, and the service level target is 95% (Z=1.65). Lead time variability is 35 * sqrt(14) = 130.9 units. Safety stock is 1.65 * 130.9 = 216 units. You would carry about 216 units of safety stock above expected lead time demand.

FAQs

What is safety stock?

Safety stock is extra inventory held to protect against demand or lead time variability.

How does service level affect safety stock?

Higher service levels require a higher Z-score, which increases safety stock.

Does this assume normal demand?

Yes. The standard safety stock formula uses a normal distribution approximation for demand variability.

Is this calculator private?

Yes. All calculations are performed in your browser and no data is stored.

How it works

This calculator applies the standard Z-score safety stock formula using daily demand variability and lead time. All computation is client-side for privacy.

Disclaimer

Safety stock estimates assume normally distributed demand and stable lead time. Adjust based on operational realities and supplier performance.