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Guide · Sizes

Which sizes really sell best in your store

The size with the most sales isn’t always the size customers want most. Sold-out days, returns and a mix of product types all bend the numbers. Five steps to see the real picture.

Published October 5, 2026 · By Sizo

Why the sales report misleads

  • Sold-out sizes stop selling. A size with no stock sells nothing, however many customers ask for it. Your fastest sizes sell out first, so they look weaker than they are.
  • Returns. A size that customers often send back sells less than its gross sales say.
  • Mixed product types. Dresses, jeans and knitwear each have their own size curve. Adding them together hides all three.
  • Several shops. A shop, the online store and each city can favour different sizes.

Five steps to find your best sizes

  1. Pick one product type and one season

    Compare like with like. A size curve for dresses says little about jeans, and summer says little about winter.

  2. Get units sold by size, after returns

    Shopify’s sales reports break sales down by product variant. Add up each size across the styles in your group, and count the units customers kept.

  3. Count the days each size was in stock

    This is the step most reports skip. Shopify records every inventory change for each variant, but no standard report counts the days a size sat at zero. Rebuild them from that history, or note when each size sold out and when it came back.

  4. Divide sales by days in stock

    That is how fast each size sells when customers can buy it. Each size’s share of the total is your size curve. The size curve calculator does this step for you, and shows how much it changes an order.

  5. Put it next to what you bought

    Compare the curve with the size mix of your last order. Sizes you bought less of than the curve are the ones that sold out; sizes you bought more of are the ones you marked down.

Then do it per shop

Run the same steps for each shop and for the online store. They often differ, and one curve for all of them sends too many of a size to a place that doesn’t sell it, while another place runs out.

What to do with the answer

  • Split the next order on the corrected curve, not on last season’s sales.
  • Allocate each shop’s share by how fast it sells each size.
  • Reorder earlier for the sizes that keep selling out: their real speed is higher than the report shows.
  • Buy fewer of the sizes you keep marking down.

New to the terms? The glossary explains size curve, size mix, sell-through, weeks of cover and the rest in plain English.

Questions

Is M always the best-selling size?

Not necessarily. It depends on the product, the fit and who buys from you, and it differs between shops. Your own sales, corrected for the days each size was sold out, are the reliable answer.

How often should I check which sizes sell best?

Every season, and before each big reorder. Curves shift with new fits, new product types and new customers.

Can I see best-selling sizes in Shopify’s reports?

Partly. Shopify’s sales reports break sales down by product variant, so you can see units sold per size. They don’t show how many days each size was out of stock, which is what hides your best sizes.

Can Sizo do this automatically?

Yes. Sizo reads sales, returns and stock history from Shopify, read-only, and works out how each size sells for every product and shop, counting only the days it was in stock. Start with the free audit.

Find out what the wrong sizes cost you

The free audit reads your last 12 months from Shopify, read-only, and ranks your biggest misses by product and size.

Get a free auditTry the calculator