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Safety Stock Calculator

Free, in-browser. Computes safety stock three ways — demand variability only, lead-time variability only, and the combined formula — so you can see which source of uncertainty is actually costing you the buffer. Nothing leaves your machine.

Safety stock: 24.7 units
That is roughly 2.1 days of demand held as buffer.
Breakdown — demand variability only: 14.8 units · lead-time variability only: 19.8 units · combined (used above): 24.7 units. If the lead-time-only number dominates, the cheapest fix is a more reliable supplier, not more stock.

The formulas

Demand variability only:    SS = z × σ_d × √LT
Lead-time variability only: SS = z × d × σ_LT
Combined (recommended):     SS = z × √(LT × σ_d² + d² × σ_LT²)

Use the combined formula unless you have a reason not to: real operations have both wobbly demand and wobbly suppliers, and the two simple formulas each understate the buffer when the other source of variability exists. The breakdown in the result matters more than the total — if the lead-time-only term dominates, your buffer is paying for supplier unreliability, and the cheapest fix is a conversation (or a second source), not more inventory. The z-score sets your tolerance for stockouts; safety stock is the price you pay for the service level you chose.

How to use this

  1. Pull daily demand mean and standard deviation from your POS (30–90 days; Excel: =AVERAGE(...), =STDEV.S(...)).
  2. Measure lead time from your last several POs — order date to arrival date — and its standard deviation, not the supplier's promise.
  3. Add unit cost to see the buffer as capital: safety stock is money standing still on purpose.
  4. Then plug the result into your reorder point or PAR level.

Safety stock is per-item, and demand patterns differ per item — smooth sellers and lumpy sellers need different math entirely (check yours with the demand pattern classifier). Computing it across the whole catalog, nightly, from measured demand and measured lead times, is what LineNow does. Free for 90 days, $100/month after.

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