Inventory how-to

How to plan size curves for apparel

A size curve is the split of a style's total order across sizes, based on how each size actually sells.

Short answer: A size curve is the split of a style's total order across sizes, based on how each size actually sells. Build it from your own per-variant sales history, not a generic bell curve, and reorder each size on its own forecast rather than treating the style as one unit.

Why a flat split is wrong

Most apparel stores sell more M and L than XS and XXL, but the exact ratio varies by style and by customer base. If you order 20 units of each size for every style, you will overstock the slow sizes and run out of the fast ones every time. The fix is to track sell-through per variant (each size is its own SKU) and let that history set the ratio.

Build the curve from sales data

Pull sales by size for a style over a representative window, ideally a recent full season or at least a few months, not just the last week. Turn each size's unit sales into a percentage of total units sold for that style. That percentage is your size curve. Recalculate it every season or after a few reorder cycles, because curves drift as your customer base changes.

Apply the curve to new orders

When you reorder or place a new style, split the total order quantity using the current curve, then round to each supplier's case pack or minimum per size. Some sizes may round up more than others relative to their share, which is normal with small case packs. Check that the rounded quantities still roughly match the curve rather than letting rounding flatten it back out.

Watch for curve breaks

New styles have no history, so start with a flat or industry-typical curve and correct it after the first few weeks of real sales. Also watch for sizes that sell out early and then show zero sales, that zero is a stockout, not a lack of demand, so do not let it drag down the size's curve percentage.

Step by step

  1. List sales by size for each style over the last full season or a few months
  2. Convert each size's units sold into a percentage of the style's total units
  3. Use that percentage split to divide your next order quantity across sizes
  4. Round each size's quantity to the supplier's case pack or minimum
  5. Exclude stockout periods from the sales history so dead stock doesn't masquerade as low demand
  6. Recheck and update each style's curve every season or every few reorder cycles
  7. Flag any size with unusually fast or slow sell-through for a manual look before the next order

Worked example

A store sells a t-shirt style and looks at the last 90 days of sales by size: XS 20 units, S 60, M 100, L 80, XXL 40, total 300 units. That gives a curve of roughly 7% XS, 20% S, 33% M, 27% L, 13% XXL. For a new order of 300 units of the same style, the store would order about 20 XS, 60 S, 100 M, 80 L, and 40 XXL, then round each to the supplier's case pack of 10, which keeps the ratios close to the original curve.

How SteadyShelf helps

SteadyShelf forecasts and reorders each size as its own SKU using its own 28-day sales window, which naturally reflects your size curve without you building one by hand. It does not calculate or display a size curve percentage as a report, and it has no bundle or style-level grouping, so you still need to eyeball the per-size suggested quantities before approving a purchase order, especially for new styles with little history.

$29/month flat, unlimited products and suppliers, 30-day free trial. Run the numbers now with the free reorder point calculator.

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FAQ

Should I use the same size curve for every style?

No. Different styles, especially different fits like slim versus relaxed, often sell in different size ratios. Track and calculate the curve per style, or at least per style category, rather than applying one store-wide curve to everything.

How do I set a size curve for a brand new style with no sales history?

Start with the curve from your closest comparable existing style, or a flat split if you have nothing similar, then correct it after the first few weeks of real sales once each size has its own sell-through data.

Let SteadyShelf do this math for every SKU

Order-by dates per supplier, seasonal forecasts and draft POs for your Shopify store. $29/month after a 30-day free trial.

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