📦 Safety Stock

Safety stock is the buffer that absorbs demand surprises while you wait for a replenishment order to arrive. Press play and watch a warehouse live through daily demand — then change the parameters and see how the buffer, the reorder point, and the cost all move.

Parameters

Std. deviation of daily demand
Time from placing an order until it arrives
Units you buy each time you reorder, before MOQ & rounding. Sets how much; the reorder point sets when. Find the cost-optimal size on the EOQ page.
Orders round up to a multiple of this (pallet / box size)
% of unit value per year to keep it on the shelf
On-hand inventory Safety stock zone Reorder point 🛒 Order placed 🚚 Delivery arrives Stockout
Day
0
Safety stock
–
Reorder point
–
Actual order qty
–
Service level (achieved)
–
Stockout days
0
Avg. on-hand
–
Holding cost / yr
–

What you're seeing

Each day, random demand eats into on-hand inventory. When the inventory position (on-hand + already ordered) drops to the reorder point, a new order is placed 🛒 — but it only arrives 🚚 after the lead time. Safety stock is the cushion sized so that demand during that wait only rarely exceeds what's left.

Safety stock = z × σ × √(lead time) =  – Here z is the service-level multiplier (1.64 for 95% service). Where that number comes from is explained on the Demand Variability page.
Reorder point = demand × lead time + safety stock =  –
Order qty = max(MOQ, target) rounded up to multiple =  –

Try this: double the lead time and watch the safety stock zone grow — uncertainty compounds with waiting time. Then set variability to 0 and see the buffer vanish. Crank up MOQ or rounding and notice how your average inventory (and holding cost) rises even though safety stock didn't change: batch size and buffer are two different levers.