Free tool

Safety stock and reorder point calculator

Work out how much buffer stock to hold and the stock level at which to place your next order. Accounts for both demand swings and late suppliers.

Showing an example. Replace the numbers with yours.

Demand

Use daily units for one SKU.

Enter average and variationPaste daily sales

Average daily units sold

Standard deviation of daily unitsHow much daily sales swing. In Excel: =STDEV.S().

Daily units sold, one number per dayPaste at least 30 days from a spreadsheet column. Separators can be commas, spaces or new lines.

Supply

Lead time runs from placing the PO to stock being available to sell.

Average lead time (days)

Lead time variation (days) (optional)Standard deviation. A supplier that is usually a week either way is about 7.

Service levelChance of not running out during a lead time.

Typical order quantity (units) (optional)Used for the chart only.

Where you are now

Stock on hand plus on order (units) (optional)

Landed cost per unit ($) (optional)

The formula

Safety stock = z × √(lead time × σ_demand² + average demand² × σ_{lead time}²). Reorder point = average demand × lead time + safety stock. The z value comes from the service level you choose: 1.65 for 95%, 2.33 for 99%.

The first term covers demand being higher than average during the lead time. The second covers the supplier being late. Many spreadsheets leave the second term out, which is why stockouts so often follow a delayed shipment.

What service level means

A 95% service level means that in 95 out of 100 replenishment cycles you will not run out before the new stock arrives. Each step up costs more stock than the last, so it pays to use higher levels for your top sellers and lower ones for the long tail.

For the background, read Safety stock and reorder points.

Frequently asked questions

What service level should a DTC brand use?

95% is a sensible default. Use 97.5 to 99% for hero SKUs where a stockout costs you ad momentum, and 85 to 90% for slow movers where holding stock is expensive.

How do I calculate the standard deviation of daily demand?

Export daily units sold for the SKU for the last 90 days and use =STDEV.S() in Excel or Google Sheets, or paste the numbers into this calculator.

Does the reorder point include stock already on order?

Compare the reorder point with stock on hand plus anything already ordered but not yet received. Otherwise you will double order.

Is my data stored?

No. The calculation runs in your browser. Nothing is sent anywhere unless you choose to download the spreadsheet, in which case only your email address and the headline result are sent to us.