Stock health guide
What is weeks of cover, and how do you calculate it?
By Andrew Simpson, founder of Canopy Inventory. Last updated 6th November 2026: two earlier guides merged into this page and rewritten
Quick answer
Weeks of cover is how long your stock lasts at your current sales rate. Weeks of cover = units in stock / average units sold per week. If you have 120 units and sell 15 a week, you have 8 weeks of cover. Compare it with your supplier lead time: once weeks of cover gets close to the lead time, it is time to reorder.
Weeks of cover (also called stock cover or weeks of supply) turns a stock count into time. 500 units sounds safe until you see you sell 100 a week: that is 5 weeks. 20 units sounds alarming until you see you sell 1 a week: that is 20 weeks. This guide covers the formula, a worked example, how it relates to days sales in inventory and stock on order, and how to work it out in Shopify. For the exact rules our app uses, see how Canopy Inventory calculates.
The weeks of cover formula
Weeks of cover = units in stock / average units sold per week
Average units sold per week = units sold in your sales window / days in the window x 7
Use units, not revenue. A product taking £500 a week might be 2 units or 200 units, and the reorder urgency is completely different. Work at variant level: "Navy collar, size M" and "Navy collar, size XL" sell at different rates and run out at different times.
Worked example
Example (made-up round numbers)
Weekly rate = 60 / 30 x 7 = 14 units a week.
Weeks of cover = 120 / 14 = 8.6 weeks.
Your supplier takes 10 weeks from order to delivery. 8.6 weeks of cover is less than the lead time, so even if you order today, this SKU is likely to run out for about 1.4 weeks before the delivery lands.
Weeks of cover, stock cover and weeks of supply
These are the same measure under different names: weeks of cover, stock cover, weeks cover, weeks of stock, weeks of supply and weeks on hand. All of them answer one question: at the rate we are selling, how long does this stock last?
Days of cover vs weeks of cover
Days of cover = units in stock / average units sold per day
Weeks of cover = days of cover / 7
Days of cover suits fast sellers and short lead times, where a few days matter. Weeks of cover is easier to read when you import with lead times of several weeks or months, because it lines up with how suppliers quote lead times.
Example (made-up round numbers)
Days sales in inventory (DSI): meaning and formula
Days sales in inventory (DSI, also called days sales of inventory or days inventory outstanding) is the accountant's version of cover. It is worked out in money for the whole business over a past period.
DSI = (average inventory value / cost of goods sold) x days in the period
Example (made-up round numbers)
| Weeks of cover | Days sales in inventory | |
|---|---|---|
| Unit | Units of stock | Money (inventory value) |
| Level | Each SKU | The whole business |
| Direction | Forward: how long today’s stock lasts | Backward: how long stock sat over a past period |
| Best for | Deciding what to reorder and when | Finance reporting and comparing periods |
A healthy DSI can hide a best seller about to run out and a slow line with a year of stock, because they average out. That is why reorder decisions are made per SKU.
Pipeline inventory: counting stock on order
Pipeline inventory is stock you have ordered but not yet received, mostly in transit: on the water or waiting at the port. It does not sell today, but it changes what you need to order next.
Cover including stock on order = (units in stock + units on order) / average units sold per week
Example (made-up round numbers)
Keep the two figures apart. Stock on order only helps if it arrives before you run out, so a late delivery can still leave you with a gap even when the combined figure looks healthy.
Choosing a sales window
The sales window is how many days of sales you average. It changes the answer more than anything else.
- 7 or 14 days: reacts fast to a product that is taking off, but one busy day can swing it.
- 30 days: a sensible default for most steady sellers.
- 45, 60 or 90 days: smoother, and needed for slow sellers that do not sell every week. "Two months of sales" is the 60-day window.
Whichever window you choose, check what happened in it. A big promotion makes the rate too high. Days when the SKU was out of stock make it too low, because nothing could sell. For seasonal products, compare with the same weeks last year as well (see our guide to seasonal weeks cover adjustments).
What is a good weeks of cover?
There is no single right number. It depends on your lead time, how steady demand is and how much cash you want in stock. Two limits help:
- The floor is your lead time plus the safety stock you want to hold. Below that, an order placed today arrives after you have started using your safety stock; below your lead time alone, it arrives after you run out. Our reorder point calculator and the safety stock formula guide help you size it.
- The ceiling is the floor plus how long you want each order to last. Well above that, cash is tied up in stock that will not sell for months.
For a step-by-step version of this decision, read how to know when to reorder stock.
How to calculate weeks of cover in Shopify by hand
Shopify does not show weeks of cover as a column, although its Inventory remaining per product report estimates days of inventory remaining from the last 28 days of sales. You can work out weeks of cover with its reports and a spreadsheet:
- Get stock per variant. Use the Available quantity, not On hand: on hand includes stock already committed to unfulfilled orders.
- Get units sold per variant for your window. In Analytics > Reports, open a sales by product or product variant report, set the date range to your window (for example the last 30 days, ending yesterday) and use net items sold. Shopify's inventory reports, such as "Inventory sold daily by product", can also help.
- Put both in a spreadsheet with one row per variant SKU: SKU, available, units sold, days in window.
- Add weekly rate = units sold / days in window x 7, then weeks of cover = available / weekly rate.
- Add a lead time column in weeks and a gap column: weeks of cover minus lead time. Sort by the gap, smallest first. Negative means you are already late.
This works, but it is a snapshot. It is out of date as soon as you sell something, and it has to be rebuilt every week.
Common mistakes
- Using on hand instead of available stock, so committed units look like cover.
- Working it out per product rather than per variant, which hides the size or colour that is about to run out.
- Using revenue instead of units.
- Forgetting that a stockout or a promotion in the window distorts the sales rate.
- Adding stock on order to today's cover without checking when it actually arrives.
- Working it out once a month, when it changes with every sale and delivery.
How Canopy Inventory does it
Canopy Inventory is a read-only Shopify app, coming soon to the Shopify App Store. It works out weeks of cover for every SKU from your Shopify data, using a 7, 14, 30, 45, 60 or 90-day sales window. It uses Shopify's available stock across your sellable locations, counts units sold inside bundles towards each component, and shows the stockout date, reorder-by date, suggested buy quantity and cash to buy at your Shopify cost per item. Incoming purchase orders (on the Growth and Pro plans) are kept out of weeks of cover and used in the stockout date and the buy quantity.
Every rule, including what it does not adjust for, is written out on how Canopy Inventory calculates. It is free for your 50 most recently sold SKUs; see pricing for the paid plans.
Weeks of cover for every SKU, without the spreadsheet
Canopy Inventory is coming soon to the Shopify App Store. One email the day it's live. Free for your 50 most recently sold SKUs.
Questions
How do I calculate weeks of cover?
Divide the units you have in stock by the units you sell in an average week. 120 units selling 15 a week is 120 / 15 = 8 weeks of cover. Work it out per variant SKU, in units, using available stock.
Is weeks of cover the same as stock cover or weeks of supply?
Usually, yes. Weeks of cover, stock cover, weeks of stock and weeks of supply all mean how many weeks your current stock lasts at your current sales rate. Days of cover is the same measure counted in days.
What does DSI mean?
DSI stands for days sales in inventory (also days sales of inventory). It is an accounting ratio: average inventory value divided by cost of goods sold, multiplied by the days in the period. It tells you how many days, on average, the business held its stock over a past period.
What is the difference between weeks of cover and days sales in inventory?
Weeks of cover is worked out per SKU, in units, from today's stock and recent sales, so it looks forward. DSI is worked out for the whole business, in money, from average inventory and cost of goods sold over a past period, so it looks back.
What is pipeline inventory?
Pipeline inventory is stock you have ordered but not yet received: mostly goods in transit. Adding it to your stock shows how long you are covered once those orders arrive.
How many weeks of sales should I use?
Long enough to smooth out odd days, short enough to reflect current demand. 30 days suits most steady sellers; slow sellers need 60 or 90 days to show a rate at all; fast or trending products can use 7 or 14 days. Avoid a window that includes a big promotion or a stretch when the SKU was out of stock.
What tool calculates days of cover and weeks of cover across all my Shopify SKUs?
Shopify's Inventory remaining per product report includes a days of inventory remaining estimate, based on the last 28 days of sales, and its other inventory reports can be turned into weeks of cover in a spreadsheet. Canopy Inventory, a read-only Shopify app coming soon to the Shopify App Store, works out weeks of cover for every SKU, with a stockout date, reorder-by date and suggested buy quantity.
Sources
- Shopify Help Center, Inventory states (available, on hand, committed and incoming). Checked 6th November 2026.
- Shopify Help Center, Inventory reports (including "Inventory sold daily by product" and "Inventory remaining per product"). Checked 6th November 2026.
- Canopy Inventory's own calculation rules: how Canopy Inventory calculates.
Related guides and tools
Every formula the app uses, with worked examples.
Work out a reorder point and safety stock for one product.
Turn weeks of cover and lead time into a reorder date.
Stockout date, reorder-by date and buy quantity for one SKU.
How big a buffer to hold for demand and delivery swings.