How to Calculate Weeks Cover for Your Shopify Store

Every Shopify merchant can tell you how many units of each product they have in stock. Almost none can tell you how many weeks that stock will last. That gap — between knowing what you have and knowing how long it will last — is where stockouts happen, cash gets tied up in excess inventory, and purchasing decisions become guesswork. Weeks cover bridges that gap. It takes two numbers you already have (stock quantity and sales velocity) and produces one number that is immediately actionable. A product with 500 units sounds comfortable until you realise you sell 100 per week — that is only 5 weeks of cover. A product with 20 units sounds alarming until you realise you sell 1 per week — that is 20 weeks of cover. Raw stock quantities are meaningless without context. Weeks cover provides that context.

The basic weeks cover formula is:

Weeks Cover = Current Stock Quantity / Average Weekly Sales

Step 1: Get your current stock quantity from Shopify. Go to Products > Inventory in your Shopify admin. Note the total available quantity for the SKU you want to analyse.

Step 2: Calculate your average weekly sales. Export your order data for the last 8-12 weeks (avoid shorter periods as they may not capture normal patterns). Count the total units sold for that SKU. Divide by the number of weeks.

Step 3: Divide stock by weekly sales.

Example: You have 240 units of a blue dog collar in stock. Over the last 12 weeks, you sold 360 units. Average weekly sales = 360 / 12 = 30 units per week. Weeks cover = 240 / 30 = 8 weeks.

At current sales velocity, you have 8 weeks of stock remaining for that product.

The lookback period — how many weeks of sales data you use to calculate average weekly sales — significantly affects your weeks cover number. Too short (2-4 weeks) and a single promotional week or stockout week skews your average dramatically. Too long (26-52 weeks) and your average may not reflect current demand trends. For most Shopify brands, 8-12 weeks is the sweet spot. It captures enough data to smooth out weekly variations while staying recent enough to reflect current demand. However, if your products are highly seasonal, you may need to use the same period from the previous year as a comparison. Be especially careful with the lookback period for products that were out of stock during part of that period. If a product was unavailable for 3 of the last 12 weeks, using 12 weeks of sales data understates the true weekly velocity. You should either exclude out-of-stock weeks from the calculation or use a period where the product was continuously available.

Let us work through a real example from Bailey & Coco, a dog accessories brand with 2,845 active SKUs.

Product: Classic Floral Collar — Size Medium Current stock: 186 units Sales data (last 12 weeks): 23, 19, 28, 31, 22, 25, 27, 20, 24, 26, 21, 30 Total sales: 296 units over 12 weeks Average weekly sales: 296 / 12 = 24.67 units/week Weeks cover: 186 / 24.67 = 7.5 weeks

Now here is why this matters for Bailey & Coco specifically: they source from China with a 70-day production lead time and 120-day sea freight. That is 190 days — roughly 27 weeks — from placing a purchase order to receiving stock. With only 7.5 weeks of cover, they are already 19.5 weeks behind where they need to be. If they have not already placed a reorder, this product will be out of stock for at least 12 weeks. At 24.67 units per week with an average selling price of £18.95, that represents approximately £5,600 in lost revenue. One SKU. One miscalculation. That is the power of weeks cover — it makes the urgency visible instantly.

There is no universal answer — ideal weeks cover depends on your supply chain lead time. The rule of thumb is:

Minimum weeks cover = Total lead time (in weeks) + Safety buffer

For a UK brand with domestic suppliers and 1-2 week delivery: target 4-6 weeks cover.

For a brand importing from Europe with 4-6 week lead times: target 8-12 weeks cover.

For a brand importing from China with 20-27 week total lead times: target 30-35 weeks cover on core products.

Anything below your lead time means you are already at risk of a stockout before your next order can arrive. Anything above double your lead time means you may be tying up too much cash in inventory. The sweet spot is lead time plus a 20-30% safety buffer.

If you want to calculate weeks cover manually for your entire catalogue:

1. Export your inventory from Shopify (Products > Export > select inventory fields) 2. Export your sales by product from Shopify Analytics (Analytics > Reports > Sales by product) 3. In a spreadsheet, create columns: SKU, Product Name, Current Stock, Total Units Sold (last 12 weeks), Average Weekly Sales (total / 12), Weeks Cover (stock / weekly sales) 4. Sort by weeks cover ascending — the products at the top are your most urgent reorder candidates 5. Add a column for supplier lead time and calculate the gap: Weeks Cover minus Lead Time in Weeks. Negative numbers mean you are already behind.

This process works but it is manual, time-consuming, and needs to be repeated every week to stay accurate. For brands with hundreds or thousands of SKUs, automating this calculation is essential.