Size-Curve Planning for Apparel: From Sell-through by Size to Pack Ratios

A style can be the right design at the right price and still fail because the sizes were wrong — mediums sell out in a week while extra-smalls sit until the sale. A size curve is the percentage of units you buy in each size. Getting it right is one of the cheapest ways to improve full-price sell-through.

Build the curve from sales, not purchases

The most common mistake is to copy last season's buy ratio. That ratio reflects what you bought, not what customers wanted. Use units sold by size, preferably at full price, for comparable styles — same category, same fit, same customer, same channel.

• Group styles by fit block. A slim-fit shirt and a relaxed-fit shirt will not share a curve.
• Separate channels if their customers differ (online often has a wider spread of sizes than a single store).
• Separate regions where body-size distribution or customer mix differs.

Adjust for stock-outs

If a size sold out early, its sales understate demand. Two practical adjustments: compare its rate of sale in the weeks it was in stock with the other sizes, or look at the share of that size in the first weeks when everything was available. The workbook keeps raw sales separate from the curve sheet so you can overwrite a size's share with judgement and leave a note.

Using the workbook

1. Size history: enter units sold by size for each comparable style or category. Each row shows its own curve, and the total row gives the combined curve.
2. Curve: the planning curve, taken from the totals, with each cell editable.
3. Buy split: enter planned units per style; units by size follow the curve and a rounding-difference column shows where whole numbers do not add up exactly.
4. Pack ratio: set the pack size (for example a 12-piece carton or store pack); the sheet proposes units per size in each pack and tells you if rounding has made it one unit off.

Pack ratios and minimums

Factories and warehouses work in packs; stores receive packs. A pack ratio such as 1-2-3-3-2-1 for XS to XXL keeps distribution simple, but small stores may not sell a full pack of a slow style. Two approaches: a smaller pack for small stores, or allocate single units from a warehouse for tail sizes. Whichever you choose, check that the total across packs reproduces the curve you intended.

Edge sizes

Extreme sizes are where both lost sales and leftovers concentrate. Options:

• Offer edge sizes online only, where demand is pooled across the country.
• Buy edge sizes in a smaller number of styles (core colours) rather than across the range.
• Use made-to-order or quick repeats for edge sizes in high-value categories.

Review after the season

Compare the planned curve with actual sell-through by size, note where you sold out and where you were left with stock, and update the curve. Keep a curve per fit block and channel; over two or three seasons it becomes one of your most reliable planning inputs.

If you use Bebaak Loom, size breakdowns flow from orders into cutting plans, and Online and Retail sales feed sell-through by size back into planning.

Worked example (illustrative)

Last season a women's kurta sold 40 XS, 160 S, 260 M, 240 L, 150 XL, 70 XXL and 30 3XL — 950 in total, so the curve is about 4%, 17%, 27%, 25%, 16%, 7% and 3%. M and L sold out three weeks before season end; looking at the weeks when everything was in stock, the team lifts M and L by a point each and trims S. For a planned 1,200 units the buy split gives 51, 196, 336, 312, 189, 88 and 38 after rounding, and the pack ratio for a 12-piece pack comes out as 0-2-3-3-2-1-0 with edge sizes allocated from the warehouse.

Common mistakes

• Copying the buy ratio instead of the sales ratio.
• One curve for every fit and category.
• Ignoring sizes that sold out early.
• Packs that small stores cannot sell through.
• Never updating the curve after the season.