Reorder point calculator for a small online shop: when to order, how much, and the cash it ties up

The spreadsheet that most small shops keep has a column for stock and no column for time. The question that matters is not "how many do I have" but "on what date does this run out, and when do I have to order so that it does not". The calculator answers both and adds the one that stops people ordering enough: how much cash the order ties up.

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Reorder point calculator: when to order, how much, and the cash it ties up

Average units sold a dayLast 30 or 90 days of sales divided by the days Supplier lead time (days)From placing the order to the stock being on your shelf Safety stock (days of sales) Units in stock now Units already on order Days of cover an order should give Supplier minimum order Unit cost (£)

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The formula, and what goes into it

Reorder point = daily sales × (lead time in days + safety stock in days). When stock on hand plus stock already on order falls to that number, you order.

Daily sales. The last 30 or 90 days of units sold, divided by the days. Use the shorter window for a product that is growing or in season, the longer for a steady one. A product that has just been in a promotion will overstate; one that was out of stock for a fortnight will understate, because it could not sell. Correct for both before trusting the number.

Lead time. From the day you place the order to the day the stock is on your shelf and sellable, not the day it leaves the supplier. Include their processing time, the shipping, customs if any, and your own day to book it in. A supplier who quotes "two weeks" usually means two weeks to dispatch.

Safety stock. Days of sales held against a late delivery or a good week. Seven days is a sensible floor for a supplier who is usually on time; fourteen for one who is not, or for imports.

The two numbers the formula does not give you

The stock-out date. Stock on hand divided by daily sales, counted forward from today. This is the date to write down, because it is the one that turns a reorder from an abstraction into a diary entry. The calculator also tells you the date to order by, which is the stock-out date minus the lead time and the safety days.

The cash. Order quantity multiplied by unit cost. For a shop run on a current account rather than a credit line, this is the actual constraint. The calculator takes the days of cover you want, lifts the quantity to the supplier's minimum if that is higher, and prints the sum. A sixty-day order that ties up £400 is a different decision from one that ties up £4,000, and the formula alone does not know that.

When you have already missed it

The calculator says "order now" and tells you when the order lands. If that date is after the stock-out date, you have a gap, and the honest options are to expedite the order, to pause the ads on that product so it sells down more slowly, or to accept a few days out of stock and set the product page to show a dispatch date. Hiding the product is worse than showing a date; the customer who wanted it looks elsewhere and does not come back.

After Stocky

Shopify's Stocky app, which held reorder points and purchase orders for a lot of small Shopify stores, is unavailable after 31 August 2026, and Shopify points users to the inventory features in the admin. Those track quantity well and lead time less well. Our guide on what to do now Stocky has gone sets out the three choices; for a shop with fewer than a few hundred products, the reorder point in this calculator and a weekly run through the list is the whole system.

Keeping it honest

Rerun the number before every order. A reorder point is a function of a sales rate that moves, and the shops that go out of stock in November are the ones that computed their reorder points in June. The weekly check is the job: list the products, update the daily rate, note the ones below the reorder point, place the orders, write the landing dates down. It takes an hour, which is less than one afternoon of "sorry, we're out of that" emails, and it is the job an AI Stock Clerk set up by Overlyft does from the sales data every Monday.

Suppliers' minimums and the cash to meet them are the other half of the problem, and the fee calculator is a reminder of what is left of each sale to pay for the next order.