Reorder Point Calculator

What the reorder point calculator actually computes

The formula is reorder point = (average daily sales × supplier lead time in days) + safety stock. The first term is lead time demand: the units customers will buy between the moment you place a purchase order and the moment new stock is sellable on your shelf. Sell 6 units a day with a 21-day supplier and 126 units walk out the door while you wait. Order at 126 and a perfectly average stretch of sales lands you at exactly zero the day the shipment arrives — which is why the formula adds safety stock on top.

Safety stock is the buffer for the weeks that are not average: a demand spike, a customs delay, a supplier holiday. Add it to lead time demand and you get the trigger level. With 40 buffer units in that example, the reorder point is 166 — when stock on hand touches 166, the PO goes out.

Enter current stock as well and the reorder point calculator divides the gap above the trigger by your sales speed, returning two extra outputs: days until you hit the reorder point and the calendar date it happens. A stock level is easy to sail past; a date is something you can put in a calendar.

Where to pull each input

Everything the reorder point calculator asks for comes out of order history and one supplier invoice.

• Average daily sales: units sold over the last 30 to 90 days divided by the number of days. Use units, never revenue, and pick a window that reflects current demand rather than a promo month.
• Supplier lead time: count from the day the PO is sent to the day stock is sellable — production plus transit plus customs plus your own receiving and labeling. Check actual dates on your last two or three orders, not the supplier’s quoted figure.
• Safety stock: your buffer in units. If you are guessing, run the safety stock calculator first — it derives the buffer from your worst sales day and slowest delivery.
• Current stock on hand: optional. Adding it turns the trigger level into a countdown and a date.

Worked example: a candle studio

A candle studio sells 4.5 units of its bestseller per day. Between the wax supplier, pouring and cure time, a finished sellable batch takes 30 days from the day the order goes in, and the owner keeps 25 units of safety stock. Lead time demand is 4.5 × 30 = 135 units, so the reorder point is 135 + 25 = 160 units.

There are 250 units on the shelf today — 90 above the trigger. At 4.5 units a day, the studio crosses the line in 90 ÷ 4.5 = 20 days. The purchase order does not need to go out today, but in 20 days it does, and the reorder point calculator has attached a date to that fact instead of a vague intention to keep an eye on stock.

Using a reorder point calculator for Shopify

Shopify reports most of what the reorder point calculator needs. For average daily sales, open Analytics, pull units sold for the product over the last 60 days, and divide by 60. Current stock on hand sits under Products in the inventory column. Lead time still comes from your PO history, because no storefront knows your supplier.

A reorder point calculator for Shopify is worth re-running whenever velocity moves — after a price change, an ad push, or a feature in someone’s gift guide. Shopify’s low-stock alerts fire at a fixed unit threshold you set by hand; this math is how you decide what that threshold should be for each SKU, instead of picking a round number.

Common mistakes that break the trigger

When a reorder point calculator gives a bad answer, the fault is almost always in the inputs rather than the formula.

• Averaging revenue instead of units. Dollars flatten price changes and bundles; the formula needs physical units.
• Trusting the quoted lead time. Suppliers quote production days and forget transit, customs and your receiving queue. Measure door to shelf.
• Setting safety stock to zero. Order exactly at lead time demand and any bad week becomes a stockout.
• Calculating once and never again. A reorder point set in March is wrong by October — recheck monthly and before any peak.
• Ignoring seasonality. If Q4 doubles your sales speed, the reorder point roughly doubles too, and it needs raising before the ramp, not during it.

A trigger level is not a deadline

The reorder point tells you what stock level should fire the PO — it still relies on you noticing the crossing. The latest order date calculator flips the same inputs into the number most sellers actually want: the last calendar day you can order before a stockout becomes unavoidable. Pair the two, and let the safety stock calculator set the buffer they both depend on.

For one or two SKUs, re-running this reorder point calculator monthly works fine. Past a dozen, it quietly stops happening. The paid ShelfOwl forecaster exists for that case: upload a CSV of sales and stock, get the latest order date for every SKU plus email reminders before each deadline — $29 a month, with a 14-day trial that asks for no card. The free calculators stay free either way.