Inventory how-to

How to calculate weeks of supply

Weeks of supply equals the units you have on hand divided by your average weekly sales rate.

Short answer: Weeks of supply equals the units you have on hand divided by your average weekly sales rate. It tells you how many weeks your current stock will last if sales stay steady. Lower numbers mean you need to reorder soon, higher numbers mean you are overstocked relative to demand.

The basic formula

Weeks of supply = units on hand / average weekly sales. If you sell 70 units a week and have 350 on hand, that is 5 weeks of supply. Do this per variant, not per product, since sizes and colors sell at different rates.

Where to get the sales number

Use a recent, representative sales window rather than all-time average. A slow month after a promotion will understate demand, a holiday spike will overstate it. A rolling 28 day window, divided by 4 to get a weekly rate, works well for most small stores and smooths out day-to-day noise.

Include stock already on order

If you have stock on order from a supplier, decide whether you want weeks of supply to include it or not. Including it gives you a truer picture of future coverage: (on hand + on order) / weekly sales rate. Leaving it out tells you how long you can last with zero further deliveries, which is useful for spotting urgent gaps.

Turning it into a reorder decision

Weeks of supply is a monitoring number, not a reorder trigger by itself. Compare it to your supplier's lead time plus the safety time you want. If weeks of supply is close to or below lead time plus safety weeks, it is time to order.

Step by step

  1. Pick a sales window, for example the last 28 days, and get total units sold per variant.
  2. Divide that total by 4 to get average weekly sales for each variant.
  3. Get current on hand units per variant from your inventory.
  4. Divide on hand by weekly sales to get weeks of supply per variant.
  5. Optionally add units on order to on hand before dividing, if you want to include incoming stock.
  6. Compare weeks of supply to your supplier's lead time plus safety days, converted to weeks.
  7. Flag any variant where weeks of supply is at or below that threshold for reorder.

Worked example

A store sells a hoodie variant and had 240 units sold over the last 28 days, so weekly sales is 240 / 4 = 60 units. Current on hand is 180 units, giving weeks of supply of 180 / 60 = 3 weeks. The supplier lead time is 2 weeks and the store keeps 1 week of safety stock, so the reorder threshold is 3 weeks. Since weeks of supply equals 3, this variant is right at the line and should be ordered now, not next week.

How SteadyShelf helps

SteadyShelf does not display a "weeks of supply" figure directly, but it covers the same ground with daily sales forecasts, order-by dates, and days of stock per SKU based on your lead time and safety days. If you specifically want a weeks of supply number for reporting, you will need to calculate it manually from on hand and sales data as shown above.

$29/month flat, unlimited products and suppliers, 30-day free trial. Run the numbers now with the free reorder point calculator.

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FAQ

Should I use weeks of supply or days of stock

They are the same measure at different scales, just divide days of stock by 7 to get weeks. Use whichever unit matches how your suppliers quote lead times, if lead times are in weeks, weeks of supply is easier to compare directly.

What weeks of supply target should I aim for

There is no universal target, it depends on your supplier's lead time, how much demand swings, and how much cash you can tie up in stock. A common approach is to keep weeks of supply at roughly lead time plus a buffer of one to two weeks, adjusted per SKU based on how reliable that item's sales are.

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