Size curve calculator
A size curve is the distribution of demand across the sizes of a style. This calculator turns total units and a percentage per size into size-level quantities for a style, color, or assortment.
Size curves help planners translate style/color demand into size-level buys. They are especially important for apparel because demand is not evenly distributed across sizes.
- Definition — Size curve
- A size curve distributes total planned demand across the sizes of a style — for example XS through XXL. Each size's share should sum to 100%, so the curve converts a style or color buy into a clean size-level quantity for both D2C and wholesale orders.
- Size units = total units × size's % share of demand
- Used by: Merchandise planners, buyers, allocators
- Related: Assortment planning, buy planning, sell-through, demand forecasting
Merchandise planners, buyers, and allocators who need to translate a style or color buy into the right units per size for apparel assortment planning.
Use it when placing a buy, building an assortment plan, or allocating units across channels and door clusters that sell to different size mixes.
Spreadsheets are useful when the process is small and controlled. They become risky when multiple teams need the same version of the plan, when assumptions change frequently, or when decisions must flow into POs, production, and allocation.
- Confirm the percentages sum to 100% before you commit the buy.
- Sanity-check the curve against last season’s size sell-through, not just an even split.
- Use a different curve for styles or channels that sell to different size mixes.
- Reconcile the rounded size units back to the total buy quantity.
Want a reusable worksheet? Download the free size curve template, or read the guide on how to calculate size curves.
Frequently asked questions
- What is a size curve?
- A size curve is the distribution of demand across the sizes of a style — for example XS through XXL. It tells you what share of total units each size should represent, so you can translate a style or color buy into size-level quantities.
- How do you calculate size-level quantities?
- Multiply the total units by each size’s percentage share. For example, if you are buying 1,000 units and size M is 30% of the curve, M gets 300 units. Repeat for every size; the shares should sum to 100%.
- Should size curves be the same for every style?
- No. Size curves vary by fit, silhouette, category, and customer. A relaxed knit and a tailored bottom often sell to different size mixes, so applying one curve to every style usually leaves the wrong sizes short or overstocked.
- Why do size curves change by channel or region?
- Different stores, regions, and channels serve different customers, so their size demand differs. Curves shift with local fit preferences, climate-driven assortments, and how online versus store shoppers buy — which is why many planners hold separate curves by channel or door cluster.
Your calculator result is one number. RetailNorthstar keeps the whole plan connected — line plan, OTB, assortment, buy, POs, and production.
- A size curve is the percentage mix that splits total planned demand into size-level units.
- Size units = total units × each size’s share of demand, and the shares should sum to 100%.
- Curves vary by fit, category, channel, and region — one curve rarely fits every style.
- RetailNorthstar connects size curves to assortment plans, demand forecasts, buying, and purchase order creation so size-level decisions stay in sync.