Size Curve Planning: How Many of Each Size to Order

To plan a size curve, split each order across sizes in the same proportions that your own past sales and returns show, then adjust for sizes that sold out early. A size curve is the share of units you buy in each size, and it is the main way to avoid ending up with too many of the wrong sizes.

This guide is for fashion store owners and buyers placing stock orders. If you also want fewer fit problems on the site, AI size recommendation for fashion stores is one way to help shoppers choose the right size from the stock you buy.

What is a size curve?

A size curve shows how a purchase is divided across sizes. If you order 20 units and buy 2 small, 6 medium, 8 large and 4 extra large, your curve is 2 : 6 : 8 : 4.

It is also called a size ratio or size breakdown. The best curve is not the same for every product, brand or customer base.

Why does the curve matter?

Clothing is sold by size, so a poor curve leaves you with two problems at once. Popular sizes sell out and shoppers leave, while unpopular sizes sit unsold and need discounts.

The curve also affects returns. If a size runs out, some shoppers buy the next size and return it. The guide to fit-related returns covers where those come from.

What data do you need to build one?

Gather these before you place an order:

• Sales by size for similar past products, over a full season.
• Stockout dates for each size, because a size that sold out in week two hides demand.
• Returns by size and reason, especially "too small" and "too large".
• Your size chart, so you know what each label means.
• Customer mix, such as how many buyers are gifts.

Use your own data first. Industry averages fit a typical shopper, not yours.

How do you build a size curve step by step?

1. Choose a comparable product. Pick a past style with the same fit, price and audience.
2. List units sold per size. Use the full selling period.
3. Adjust for stockouts. If a size sold out early, raise its share. Look at how fast it sold before the stockout and estimate what it could have sold.
4. Convert to shares. Divide each size's units by the total.
5. Round to whole units. Apply the shares to your planned order.
6. Check minimums. Factories often require minimum units per size, per color.

Worked example: a 40-unit order

This is an illustration, not real data.

A store sold 100 units of a similar top last season. Sales by size were: S 10, M 30, L 40, XL 20. The large sold out in week five, and the others stayed in stock.

The store estimates that it would have sold about 10 more large units. That makes the adjusted total 110: S 10, M 30, L 50, XL 20.

For a 40-unit order, the shares become about 4 small, 11 medium, 18 large and 7 extra large. The store rounds to 4, 11, 18 and 7, which adds to 40.

The same method works for any number of sizes. The numbers above are made up to show the steps.

How do you handle sizes at the ends of the range?

The smallest and largest sizes sell less, but they matter to the shoppers who need them. Dropping them completely can lose those customers for good.

Options for end sizes:

• Buy a smaller quantity rather than none.
• Sell them online only, if your shop space is limited.
• Take them on pre-order for slow sizes. The guide to selling pre-orders with try-on shows how a pre-order can fit with fit tools.

How should the curve change by product type?

Product type	What to watch
Fitted tops and dresses	Wider spread, since fit is less forgiving
Oversized or relaxed items	Narrower curve, since one size covers more shoppers
Trousers and jeans	Waist and length combinations, not just one label
Gifts and seasonal items	Skew to middle sizes, since the buyer may not know the size

This table is a working guide, not a standard. Test it against your own sales.

How do you use returns to improve the next curve?

Returns tell you where the curve or the size chart is off. If many returns say "too small" in one size, your chart may be labeling that size too tight.

Tag every return by size and reason. The article on returns and size data feedback loops explains how to feed that back into the next order.

Warehouse errors can also look like sizing problems. Read avoiding wrong-size shipments in pick and pack before you blame the curve.

How can product pages support the curve?

A clear product page spreads demand across the sizes you stocked. Fit notes, a size chart and a size suggestion help shoppers choose the size that fits, not just the one in the middle.

The product page checklist for fashion stores lists what to include. ZeDaq.ai offers a Free plan to test size recommendation and try-on, with 3 try-ons and 3 measurements a month.

FAQs

What is a good size curve?

There is no single good curve. Build it from your own sales for similar products, then adjust for stockouts.

Should every size get the same quantity?

No. Middle sizes usually sell more than the ends, so a flat split leaves too many of some sizes and too few of others.

How often should I update my size curve?

Review it each season, or whenever returns or stockouts show a pattern.

What if I have no sales history?

Start with a modest curve based on a similar brand or product, buy small and reorder fast sellers.