Lead time demand calculator.

Worked example

Average demand of 25 units a day and a 21-day lead time: lead-time demand = 25 x 21 = 525 units. If daily demand has a standard deviation of 6 units, the standard deviation over the lead time is 6 x sqrt(21) = 27.5 units, so about two thirds of cycles should fall between 497.5 and 552.5 units if demand is roughly normal.

How it works

Lead-time demand is expected demand during the replenishment lead time: L x E(D), the first term of the reorder point formula ROP = L x E(D) + safety stock that the Wikipedia safety stock article gives for a constant lead time.

When daily demands are independent with the same standard deviation, variance adds across days, so the standard deviation over L days is s_d x sqrt(L). That is the same sqrt(L) term used in the safety stock formula. The one-sigma range is the mean plus or minus that value (the low end is floored at zero).

Use the same time unit for demand and lead time. Add the review period if you order on a fixed schedule rather than continuously. Seasonality, promotions and supplier delays are not modelled.

Sources (opened 2026-10-18): Safety stock - Wikipedia. The formula wording above follows these pages; the example figures and all arithmetic are ours.

Inputs are user-entered figures. Results are planning estimates, not financial advice or a forecast. Everything runs in your browser. Nothing you type is sent or stored.

FAQ

What is lead time demand?

The number of units you expect to sell between placing a replenishment order and receiving it. It is average demand times lead time.

How is lead time demand used?

It is the base of the reorder point: reorder point = lead-time demand + safety stock.

Does lead time need to be in days?

Any unit works as long as demand per period uses the same unit. Weekly demand pairs with lead time in weeks.

Why does the spread use a square root?

Independent daily variations partly cancel, so variance grows with the number of days and the standard deviation grows with its square root.