Sizing Analytics: The Data Your Store Is Already Producing
You already have the data to diagnose most of your sizing problems. It's sitting in your orders and returns, unread. Here's what to look at and what each signal means.
Quick answers
- Do I need special tools?
- No — start with your orders and returns data
- What reveals a chart people distrust?
- Orders containing two sizes of the same item
- What reveals an item that runs small?
- Returns skewed to 'too small'
- What reveals bad grading?
- Returns concentrated in your biggest/smallest sizes
Signals Hiding in Your Data
| Signal | What it means |
|---|---|
| Returns skew 'too small' on a product | Item runs small |
| Same item ordered in 2 sizes | Shoppers don't trust the chart |
| Size mix differs from your curve | You're buying the wrong quantities |
| One size always sells out | Under-buying that size |
| High returns at range extremes | Grading problem |
Notes
Size mix is a demand signal you're probably ignoring. Compare the size distribution of what you sold against what you bought. If mediums sell out while XLs sit, you're not buying to your actual curve — and you're losing sales at one end while carrying dead stock at the other.
Stock-outs distort everything. If a size is constantly out of stock, its sales look low — so you buy less of it next time, and the problem compounds. Always read size demand against availability, not raw units sold, or you'll starve your best-selling size.
The directional return signal is the most valuable one you have. Nothing else in your data tells you as directly what to change. Returns skewed 'too small' on one product is an instruction: correct the chart, add a 'runs small' note. It's free, it's specific, and most merchants never look.
You don't need a data team. All of this is available from Shopify exports and a spreadsheet. The barrier isn't tooling, it's that nobody has looked. An hour with your orders and returns CSVs will usually surface two or three fixable products.
FAQs
What sizing data should I look at in Shopify?
Return reason direction by product (too small vs too big), orders containing the same item in two sizes (bracketing), your sold size mix versus what you purchased, size-level stock-outs, and whether returns concentrate at the extremes of your range.
How do I know if an item runs small?
Look at the return reasons for that specific product. If they skew heavily toward 'too small,' the item runs small. Correct the size chart and add an explicit 'runs small — consider sizing up' note to the product page.
Do I need an analytics tool for sizing?
No. Everything you need to start is in your Shopify orders and returns exports plus a spreadsheet. The obstacle is usually that nobody has looked, not that the data is missing.
Need this on your store?
Tailor Size Guide ships pre-built size charts for Shopify.