What is a size curve in fashion buying?
The planned distribution of units across sizes for a style, reflecting how demand typically varies by size.
In short
A size curve is the planned distribution of units across sizes for a style, reflecting how demand typically varies from the smallest to the largest size. Instead of buying equal quantities in every size, buyers and merchandisers use size curves based on past sales to put more units in the sizes that sell most.
How does it work in practice?
A buyer ordering a knit top will not order the same number of extra-small and medium pieces. Using sales history for the category, the store or the customer base, they apply a curve that weights the core sizes more heavily. The order is then expressed as a total quantity per colourway, split by the curve into units per size.
Brands often support this with prepacks or ratio packs, where a fixed mix of sizes is shipped together. Retailers with very different customer profiles may need their own curves rather than one standard ratio.
Why does it matter?
Getting the size curve right has a direct effect on margin and customer experience:
- Too few core sizes leads to lost sales and broken size runs early in the season
- Too many outer sizes ends up as markdown stock
- Accurate curves reduce transfers between stores
- Good curves make replenishment and re-orders more predictable
How is AI changing it?
Machine learning models can estimate size demand at a much finer level, for example by store, region, category and fit, rather than using one curve for the whole business. They can also correct for lost sales, recognising that a size which sold out quickly may have had higher demand than the sales data shows. These recommendations can then be applied directly when buyers build orders.
Common pitfalls
- Using one curve for all categories, fits and store formats
- Basing curves on sales without accounting for stock-outs
- Ignoring differences between brands that size differently
- Applying prepacks that do not match a store's actual demand
Frequently asked questions
How do you calculate a size curve?
A common starting point is to look at historical sales by size for a comparable category and store group, adjusted for sizes that sold out. The resulting percentages are then applied to the total quantity of a new order.
What is a broken size run?
A broken size run is when key sizes of a style are sold out while others remain in stock. It usually means the size curve or the depth of buy did not match demand, and the remaining stock is harder to sell.
