Inventory glossary

What is Days of supply?

Days of supply is the number of days your current stock will last at the rate you are currently selling it.

Days of supply tells you how urgent a reorder is. A SKU with 60 days of supply can wait. A SKU with 5 days of supply needs a purchase order today, especially if the supplier's lead time is longer than that.

Use it alongside lead time. If days of supply is lower than lead time plus a safety margin, you will run out before new stock arrives. Checking this per SKU, not just for your store overall, catches the fast movers before they go to zero.

Formula

Days of supply = Units on hand / Average daily sales

Example

A candle store has 180 units of a scented jar candle in stock. It sells 9 units a day on average. Days of supply = 180 / 9 = 20 days. If the supplier's lead time is 14 days, the store has a 6-day cushion before it needs to reorder.

Common mistakes

  • Using total inventory value instead of units for a single SKU, which blends fast and slow movers together.
  • Ignoring recent sales trends and using an outdated average daily sales figure.
  • Forgetting that days of supply must be compared against lead time, not looked at alone.

Days of supply and SteadyShelf

SteadyShelf calculates days of supply per variant and compares it to each supplier's lead time to tell you the order-by date for every SKU.

Work it out for your own products with our free tool: Days of stock calculator.

SteadyShelf is launching on the Shopify App Store. Join early access to get the install link first.

Related terms

Average daily sales · Lead time · Reorder point · Safety stock · Weeks of supply

Full inventory glossary

FAQ

Is days of supply the same as weeks of supply?

They measure the same thing on different scales. Weeks of supply is days of supply divided by 7.

What days of supply number means I should reorder?

There is no fixed number. Compare days of supply to your supplier's lead time plus a buffer. If days of supply gets close to that total, it is time to order.

Let SteadyShelf do this math for every SKU

Order-by dates per supplier, seasonal forecasts and draft POs for your Shopify store. $29/month after a 30-day free trial.

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