Build a simple week-by-week sheet: forecast bags per week from your own sales history, subtract them from stock on hand, and mark the week in which stock falls below lead time plus safety cover. That week is your last safe order date. In 2026 Royal Mail's last recommended inland posting dates are 18 December for 2nd Class and 21 December for 1st Class and Tracked 24, so the heaviest packing weeks run from Black Friday (27 November) to around 18 December.
A single "how many do I need for Christmas" figure hides the real risk. You rarely run out across the whole season; you run out in one particular week, usually the one straight after Black Friday or the second week of December, when orders peak and a delivery is still on the road. A week-by-week sheet shows you that week in advance.
The 2026 dates frame the sheet. Black Friday is 27 November and Cyber Monday 30 November. Royal Mail's last recommended inland posting dates are 18 December for 2nd Class, 19 December for Tracked 48, 21 December for 1st Class and Tracked 24, and 23 December for Special Delivery Guaranteed. For most sellers that means the busiest packing runs for about three weeks, from the Black Friday weekend to around 18 December.
You need six numbers, all from your own data:
| Input | How to get it |
|---|---|
| Ordinary weekly orders | Average of a normal month, e.g. September |
| Bags per order | Usually 1; higher if you split orders or double-bag |
| Peak factor per week | Last year's orders in that week ÷ ordinary weekly orders |
| Stock on hand | A physical count, by bag size |
| Supplier lead time | Order to delivery at your door, in weeks |
| Safety cover | Extra weeks you want to hold in reserve |
If you have no history, estimate the peak factor and hold more safety cover. Write the assumption down, then compare it with the first peak week's real orders and correct the sheet.
Set up one row per week from now until mid-January, and one block per bag size. Then:
Here is a worked example for one size. All figures are for illustration only.
| Week starting | Peak factor | Bags used | Delivery | Stock at week end |
|---|---|---|---|---|
| 2 Nov | 1.0 | 500 | 5,500 | |
| 9 Nov | 1.2 | 600 | 4,900 | |
| 16 Nov | 1.4 | 700 | 4,200 | |
| 23 Nov (Black Friday) | 2.6 | 1,300 | 2,900 | |
| 30 Nov (Cyber Monday) | 2.4 | 1,200 | 5,000 | 6,700 |
| 7 Dec | 2.2 | 1,100 | 5,600 | |
| 14 Dec | 2.0 | 1,000 | 4,600 | |
| 21 Dec | 0.8 | 400 | 4,200 |
Opening stock 6,000; ordinary use 500 a week. Without the 5,000-bag delivery in the week of 30 November, stock would run out in the week of 7 December, right before the last 2nd Class posting date.
The reorder point is the stock level that triggers the next order:
Reorder point = weekly usage × (lead time in weeks + safety cover in weeks)
Use peak weekly usage for any week inside the peak. With 1,200 bags a week, four weeks' lead time and two weeks' safety cover, the reorder point is 7,200 bags. The same calculation with ordinary usage (500 a week) would give 3,000 and leave you short. Lead times also tend to stretch in the fourth quarter, because suppliers and hauliers are busy, so do not plan on your fastest delivery ever.
Keep it to the sizes that carry most of your orders. Two or three sizes are easier to keep in stock than six. For each size, check:
The mailing bag size table lists our common sizes in millimetres. Other sizes are made on request.
If the danger week is before your next delivery can arrive, you have three options, roughly in order of cost:
Then keep the sheet. After Christmas, enter the real weekly orders next to your forecast. The actual peak factor per week is the most valuable number you will have for planning 2027.
We produce mailing bags in our own factory in Istanbul, from film extrusion to printing and converting, and supply UK businesses directly. A trial order can start from one pallet, and every size, thickness and print is made to order, so the bags match your products instead of a catalogue list. If you are planning next peak's stock, start with our free sample kit and test the bags with your own products first.
For inland post, Royal Mail lists Friday 18 December for 2nd Class and 2nd Class Signed For, Saturday 19 December for Tracked 48, Monday 21 December for 1st Class, 1st Class Signed For and Tracked 24, and Wednesday 23 December for Special Delivery Guaranteed. Royal Mail says the dates can change, so check its page before you promise delivery dates.
Enough to cover supplier lead time plus a safety buffer, calculated at peak usage rather than normal usage. The buffer depends on how far last year's forecast was off and how reliable your supplier is.
Your average weekly orders in the peak weeks divided by your average weekly orders in an ordinary month. If you sent 400 parcels a week in September and 1,000 a week in early December last year, your peak factor was 2.5.
Only after testing it with your heaviest and sharpest products. A split bag in December means a reshipment and an unhappy customer at the worst time of year.
Yes. Royal Mail prices by format (letter, large letter, small parcel, medium parcel) as well as weight, so a bag that is larger than needed can push a parcel into a dearer format.
This guide is general information based on published sources at the time of writing. It is not legal or tax advice.
Free sample kit of our mailing bags, sent to your UK address. Test fit, strength and print before you plan your next order.