Christmas stock planning: how to adjust weeks of cover for a seasonal peak
By Andrew Simpson, founder of Canopy Inventory. Last updated .
Quick answer
Weeks of cover worked out from your recent sales rate will overstate how long stock lasts in a seasonal peak such as Christmas. To adjust it by hand: divide last year's weekly sales in the peak by last year's weekly sales in the weeks before it to get a seasonal factor, multiply this year's current weekly sales by that factor, and divide your stock by the result. Then work back from the date you need the stock by your lead time to find when to order.
Why weeks of cover misleads you before a peak
Weeks of cover is how long your stock lasts at your current sales rate. In October, that rate comes from October sales. If a product sells three times as fast in December, the cover you see in October is about three times too optimistic for December. After the peak the opposite happens: January cover worked out from December sales looks too short, and you risk reordering stock you do not need.
The fix is not a different formula, it is a different sales rate for the weeks you are planning.
Seasonal weeks of cover formulas
- Seasonal factor = last year's units per week in the peak ÷ last year's units per week in the base weeks before it
- Peak weekly rate = this year's current units per week × seasonal factor
- Peak weeks of cover = available stock ÷ peak weekly rate
- Units needed for the peak = peak weekly rate × weeks in the peak
- Shortfall to order = units needed + safety stock − (stock you expect to have when the peak starts + units already on order)
- Order-by date = date you need the stock on sale − lead time days
Worked example: planning one product for Christmas
Example (round made-up numbers). Last year this product sold 20 units a week in September and October and 60 a week across the 8 weeks of November and December. The seasonal factor is 60 ÷ 20 = 3.
This year it sells 25 a week, so plan for 25 × 3 = 75 a week in the peak. You expect to have 400 units available on 1st November, after the sales between now and then, and 100 on order.
At today's rate, 400 units is 16 weeks of cover. At the peak rate it is 400 ÷ 75 ≈ 5.3 weeks. The 8-week peak needs 75 × 8 = 600 units. With 50 units of safety stock, the shortfall is 600 + 50 − (400 + 100) = 150 units.
If you need the stock on sale by 1st November and your lead time is 45 days, the order must be placed by mid-September. Most of the planning for a Christmas peak happens before the season starts.
Practical tips for Christmas stock
- Products with no history last year: borrow the seasonal factor from a similar product or category, and order cautiously with a plan to top up.
- Treat Black Friday week separately if your sales jump that week, rather than averaging it into the whole peak.
- Check the last order date for each supplier, including supplier holidays and carriers' Christmas cut-offs, and work back from those.
- Split the order where you can: a main order early, and a smaller top-up once the first weeks of the peak show whether you guessed high or low.
- Plan the run-down: decide what you want left on 1st January and do not size the order to sell out of best sellers in mid-December.
- After the peak, wait until sales settle before trusting weeks of cover again, or use a window that excludes the peak.
How to do it in Shopify
- Open the Inventory sold daily by product report and set the period to last year's peak weeks. Note the average units per day for each variant, and multiply by 7 for units per week.
- Run the same report for last year's base weeks, and again for the most recent weeks this year.
- Put the three figures side by side in a spreadsheet and apply the formulas above. Our weeks of cover spreadsheet template has the columns to start from.
- Use Products > Inventory > Export for current available stock, and your own list of open purchase orders for stock on order.
How Canopy Inventory does it
Canopy Inventory, a read-only Shopify app coming soon to the Shopify App Store, does not adjust for seasonality. Its sales rate is simply units sold in the window divided by the days in the window. You can view weeks of cover over a 7, 14, 30, 45, 60 or 90-day window to see how a recent burst or a quiet spell changes the picture. The stockout date, reorder-by date and suggested buy quantity use the last 30 days, or all the history it has if that is shorter.
So before a peak, use the by-hand method above for your seasonal lines. One practical way to carry it into Canopy is to raise the target cover or safety stock days for those products, which raises the suggested buy quantity. Settings are shop-wide on the Free plan and can be set per supplier or per SKU on Growth and Pro. See how Canopy calculates.
See weeks of cover for every SKU across six sales windows
Canopy Inventory shows how many weeks of stock each SKU has left, when it runs out and what to reorder. Read-only. Free for your 50 most recently sold SKUs.
Sources
- Shopify Help Center, inventory reports (Inventory remaining per product, Inventory sold daily by product, Products by sell-through rate, Month-end inventory value): help.shopify.com/en/manual/reports-and-analytics/shopify-reports/report-types/default-reports/inventory-reports
- Shopify Help Center, exporting inventory with a CSV file: help.shopify.com/en/manual/products/inventory/setup/inventory-csv
- How Canopy Inventory calculates each figure: How Canopy calculates
Last updated 6th November 2026: rewritten as a by-hand Christmas planning method with a worked example, and a clear note that Canopy Inventory does not adjust for seasonality. Unsourced figures removed.
Frequently Asked Questions
Work back from the date you need the stock on sale. Subtract your full lead time, including production, shipping and receiving, and allow for supplier holidays and carrier cut-offs. For long lead times, that can mean ordering in late summer.
Work out a seasonal factor from last year (peak weekly sales divided by the weekly sales before the peak), multiply this year's weekly rate by it, multiply by the weeks of the peak, and subtract the stock you have and have on order.
A seasonal factor (or seasonal index) compares sales in a season with normal sales. A factor of 3 means the product sells three times as fast in that season.
No. Canopy Inventory works out the sales rate as units sold in the window divided by the days in the window. You can view different windows, and raise target cover or safety stock days for seasonal products, but it does not forecast seasonal peaks.
Use the seasonal factor of a similar product or category, order a cautious first quantity, and plan a top-up order once the first weeks of the peak show how it is selling.
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