Safety stock calculator
Work out how much buffer stock to hold so a demand spike or a late shipment doesn't sell you out. Enter four numbers. The formula is shown in full, with no black box.
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The safety stock formula
Safety stock is the buffer you hold on top of your normal stock to absorb two kinds of surprise: selling faster than usual, and a supplier delivering later than usual. Hold too little and a busy week plus a late shipment sells you out; hold too much and cash sits idle on the shelf. The most practical formula sizes the buffer from your own peaks:
The first term is the worst realistic case: your busiest days running for the longest a shipment has taken. The second is the normal case you already plan for. The gap between them is the cover you need when both go wrong at once, which is exactly what safety stock is for. In the default example, peak exposure is 18 × 21 = 378 units, the normal plan covers 10 × 14 = 140, and the difference, 238 units, is your safety stock.
The service-level method, briefly
A more statistical approach multiplies the standard deviation of demand over lead time by a service-level factor (a Z-score: about 1.65 for 95% service, 2.33 for 99%). It is more precise when you have clean demand history and want to target an exact stockout probability, but it needs more data and is easy to get wrong. The max-minus-average method above gets most stores a sound number from figures they can actually estimate.
From safety stock to a reorder point
Safety stock is only half of knowing when to order. The trigger level, the reorder point, is your lead-time demand plus this safety stock. Feed this number into the reorder point calculator to get the exact stock level that should prompt your next order, and read the inventory replenishment guide for how the pieces fit together.
Doing this once is easy; doing it for every variant, every week, is not. Restockly calculates safety stock and reorder points across your whole Shopify catalog automatically, using the same transparent math you see here, so you can still check any number by hand.