Reorder point calculator: lead time, safety days and stock on order

A reorder point is the stock level at which you have to place the next order, so that the new stock arrives before the old stock runs out. Most online stores already have a number like this somewhere: a low-stock threshold in the store admin, or a row in a spreadsheet that turns red. The trouble is that the number is usually fixed, while the two things it depends on, how fast the product sells and how long the supplier takes, keep changing.

This guide gives you the calculation in plain terms, a worked example, and spreadsheet formulas you can paste in. It also covers the two inputs most calculators leave out: the stock you have already ordered, and how often you place orders.

The reorder point formula

The classic formula has two parts:

Reorder point = (daily sales × lead time in days) + safety stock

• Daily sales is how many units you sell on an average day, recently.
• Lead time is the number of days from placing the order to having the stock on the shelf, ready to ship. That includes the supplier's production time, transit, and the day or two it takes you to receive and shelve it.
• Safety stock is extra stock to cover a supplier that runs late or a week that sells faster than usual. Our safety stock calculator guide covers how to size it.

When the stock you have, plus what is already on its way, falls to the reorder point or below, you order.

A worked example

Say you sell a medium kraft mailer, one per order, and you ship about 50 orders a day. Your packaging supplier takes 21 days from order to delivery, and you keep 7 days of safety stock.

If you have 600 mailers on the shelf and none on order, you are well below the reorder point. At 50 a day, 600 mailers last 12 days, and the next delivery is at least 21 days away. A store that uses a fixed low-stock alert of, say, 200 units would not hear about this for another 8 days, which is already too late.

Stock on order changes the answer

A reorder point compares against the stock you have plus the stock already ordered and not yet received. Without that, you will reorder the same product every week until the first delivery lands.

Inventory position = available + on order

"Available" means stock you can still sell: on hand minus anything already committed to unshipped orders.

Order when the inventory position is at or below the reorder point. If you placed an order for 1,200 mailers last week, your inventory position is 600 + 1,200 = 1,800, which is above 1,400, so you wait.

Only count orders the supplier has actually confirmed. A draft you never sent is not stock on order.

How much to order: add the order cycle

The reorder point tells you when. To decide how much, you need one more number: how often you place orders with that supplier. If you order from them every week, the delivery has to last until the next delivery after it, so it needs to cover the order cycle as well as the lead time.

Order quantity = daily sales × (lead time + order cycle + safety days) − on hand − on order

Then round up to the supplier's case pack and minimum order quantity.

For the mailers, ordering weekly:

• 50 × (21 + 7 + 7) = 1,750 units of cover needed
• 1,750 − 600 on hand − 0 on order = 1,150 units
• Sold in cases of 200, so round up to 1,200

Getting daily sales right

This is the input that goes wrong most often.

• Use a recent window. The last 28 days reflects what is selling now. A longer window, such as 56 days, smooths out a single big week. A blend of the two, weighted toward the recent one, is a reasonable middle.
• Leave out cancelled and refunded orders. They took stock off the shelf in the report but not in reality.
• Watch for stockouts. If a product was out of stock for ten of the last 28 days, its sales over those days were zero because you had nothing to sell, not because nobody wanted it. Dividing by 28 understates its pace. Either divide by the days it was in stock, or flag it and check by hand.
• Count packaging per order, not per product. A mailer, a sheet of tissue and a thank-you card go out with every order, so their daily use is your daily orders times the units used per order.

The calculator as a spreadsheet

One row per product. Put these columns in order, starting at column A:

Daily sales, weighting the last 28 days twice as heavily as the 56-day average:

=(2*(B2/28)+(C2/56))/3

Days of stock left, counting what is on order:

=IF(D2=0,"",(E2+F2)/D2)

Reorder now, when the stock will not last through lead time, order cycle and safety days:

=IF(D2=0,"No sales",IF(J2<G2+H2+I2,"Order now","OK"))

Order quantity, rounded up to the case pack:

=IF(K2<>"Order now",0,CEILING(MAX(0,D2*(G2+H2+I2)-E2-F2),M2))

Sort by column J, smallest first, and the top of the sheet is your order list for the week.

Where a spreadsheet runs out

The math above is simple. Keeping it current is the work. Every week you have to export orders, paste them in, update stock, and remember which purchase orders went out and which arrived. Lead times change when a supplier gets busy. New products appear without a supplier assigned. If any of those steps slips, the sheet keeps producing confident numbers from stale data.

Where Reorder Day fits

Reorder Day runs this calculation for you every week. It reads the last 56 days of orders and your stock from WooCommerce, or from two Shopify exports you upload. It blends the 28-day and 56-day pace, compares each product's days of stock left with its supplier's lead time, order cycle and safety days, counts the purchase orders you have marked sent (or Shopify's Incoming), and emails you a draft purchase order per supplier, rounded to case packs. You check each draft and decide what to send.