Days of inventory: formula, free calculator and reorder point

The formula

daily sales = units sold in the last 30 days ÷ 30
days of inventory = units available ÷ daily sales
reorder point = daily sales × supplier lead time (in days)
quantity to order = daily sales × (lead time + days to cover) − units available

Example: 40 units left and 60 sold in the last 30 days, so 2 a day. That is 20 days of inventory. With a 10-day lead time, the reorder point is 20 units. To cover 30 days of sales after delivery, order 2 × (10 + 30) − 40 = 40 units.

Use a 30-day window. Last week alone is too noisy: one promotion, one holiday or one wholesale order and the number is wrong. Ninety days hides a product that just took off.

Calculator

Enter the numbers for one variant. Units sold come from Shopify Analytics, report “Sales by product variant”, last 30 days. Nothing leaves your browser.

How to read the result

• Days of inventory above the lead time plus a week: fine for now. Note the reorder point and check again next week.
• Days of inventory within a week of the lead time: order this week. Anything that goes wrong at the supplier will cost you the sale.
• Days of inventory at or below the lead time: you are already late. Order today, and expect a gap unless the supplier is early.
• No sales in 30 days: the stock is not running out, but the product may be dead. That is a different problem.

The reorder point is the unit threshold that matches your lead time. If you alert on units rather than days, this is the number to use instead of a round figure like 5 or 10, and it is different for every product.

Where to find these numbers in Shopify

1. Units available: Products › Inventory, column “Available”. Sum the locations you sell from.
2. Units sold: Analytics › Reports › “Sales by product variant”, date range “Last 30 days”, column “Net items sold”.
3. Days of inventory, pre-computed: Analytics › Reports › “Days of inventory remaining”. It uses the same formula, but it is a report you have to open, not an alert.
4. Lead time: your last three purchase orders, from order date to delivery. Take the longest, not the average.

Three mistakes that make the number lie

• Counting units reserved by unfulfilled orders as available. Use “Available”, not “On hand”.
• Mixing variants. A T-shirt sells 2 a day in total, but size M sells 1.2 and size XS 0.1. Compute per variant, which is also how you run out.
• Ignoring seasonality. Thirty days in November says nothing about January. Before a peak, use last year's peak month as the sales figure.

Getting this as an email instead of a spreadsheet

This page does the maths for one variant at a time. Stock Alert App, which we make, runs the same formula on every variant every day and emails you when one drops under the days of stock you chose, with the quantity to order. On Monday it sends the list of everything to reorder this week. It is free with no variant limit, reads inventory only, and never touches orders or customers. If a spreadsheet and this calculator are enough for your catalog, keep them.