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What is an AI size chart?

"AI size chart" means one of two things: an AI tool that recommends a size to a shopper on your product page, or a size chart written so AI shopping assistants such as ChatGPT, Copilot and Gemini can read it. Most size-chart tools do the first. Very few stores do the second: in our check of 1,383 UK and US Shopify clothing stores, 80% of the products we sampled sent AI assistants no size information at all.

By Sienna Stones · Published 2 November 2026

The two kinds side by side

	AI size recommender	AI-readable size chart
Where it lives	On your product page, as a widget or quiz	In the product data your store shares with AI assistants
Who reads it	A shopper on your site	An assistant answering a shopper somewhere else
What it answers	"Which size am I in this?" on your site	"What size should I get?" asked in ChatGPT, Copilot or Gemini
What happens without it	The shopper reads the chart themselves, or guesses	The assistant guesses, often a middle size

They solve different problems and work together. A recommender helps shoppers who are already on your page. An AI-readable chart helps the shoppers who ask an assistant first.

Why AI assistants can't read most size charts

Assistants don't browse your product page the way a shopper does. On Shopify, they receive your products through Shopify Catalog: the title, description, images, variants, category and similar fields. Size charts usually live somewhere else:

In a pop-up or tab drawn by a size-chart app in the shopper's browser, which never enters the product data.
In an image, which carries no text an assistant can read as measurements.
On a separate size guide page, which the product data may link to but doesn't contain.

Even a chart typed into the description as a table can arrive badly. In our test on a demo store, Shopify's preview of what AI channels receive turned a normal size table into a row of bare numbers, XS 68.58 93.98 90.17 16.51, with no labels to say which number is the chest and which is the length. Written as text, the same row reads XS: Chest 93.98 cm, Length 68.58 cm.

What happens when an assistant can't read it

It still names a size. In our 2 October 2026 test, ChatGPT matched the brand's own chart for 14 of 15 products where the chart was readable. Where it wasn't, it named a size every time, and 13 of 14 letter-size answers were Medium. Six times it said it couldn't find the brand's measurements and recommended a size anyway.

A small, non-random sample run on one day with two mid-range shopper profiles, where Medium is often a reasonable answer. It shows the assistant falls back on a default, not how often that default is wrong.

How common it is

On 21 and 22 October 2026 we looked up one product from each of 1,383 UK and US Shopify clothing stores in Shopify Catalog, the data AI channels receive:

80% (1,104) carried no size chart or measurements.
13% (180) carried some measurements, but not a full chart.
3% (40) pointed to a size guide elsewhere without its contents.
4% (59) carried a full size chart.

Most of these stores do show a chart to shoppers: in our Shopify size chart report of 829 stores, 32% published a text chart on the product page and 27% a picture. The chart exists; it just doesn't travel. Method and the anonymised data are in the AI sizing study 2026.

How to make your size chart AI-readable

Write the chart as text, not an image. One line per size, each measurement labelled with its unit: "M: Chest 102 cm, Waist 86 cm, Length 72 cm".
Say what it measures. State whether the numbers are body or garment measurements; assistants can't tell from the numbers alone.
Add a fit note and the model's size. "Relaxed fit; model is 178 cm and wears M" is one of the few size references an assistant can use.
Put it where AI channels look. On Shopify, Catalog mapping (Sales channels › Agentic) lets you send a product metafield as the description AI channels receive, so your product page can stay as it is.
Keep it current. New products and new sizes need the same treatment, or the gap reopens.

Our guides on how to make a size chart and adding one in Shopify cover the chart itself, and the agentic commerce product page checklist covers the other fields assistants use.

Check what AI sees for your store

The free AI size chart checker looks up your products the way an assistant does and shows whether a size chart, fit advice and model size come through. No install, no sign-up.

Frequently asked questions

What is an AI size chart?

The term is used in two ways: an AI tool that recommends a size to a shopper on the product page, or a size chart written into a store's product data so AI shopping assistants such as ChatGPT, Copilot and Gemini can read it.

Can ChatGPT read my size chart?

Only if it reaches the product data the assistant receives. Charts in pop-ups, images or theme tabs usually stay on the product page. In our October 2026 check of 1,383 UK and US Shopify clothing stores, 80% of the products sampled carried no size information in that data.

Does an AI-readable size chart replace my size-chart app?

No. A size-chart app shows the chart to shoppers on your product page. An AI-readable chart puts the same measurements into the product data assistants read. A store can use both.

Does making my size chart AI-readable change my product pages?

It doesn't have to. On Shopify, Catalog mapping can send a product metafield to AI channels as the description, so the text shoppers see on your product page can stay as it is.

Is an AI size chart the same as a size recommender?

A size recommender is the first kind: it asks the shopper questions and suggests a size on your site. An AI-readable size chart is the second kind: it lets assistants elsewhere answer size questions from your own measurements.

Sources: Rendered Fits check of 1,383 UK and US Shopify clothing stores via Shopify Catalog, 21–22 October 2026 (method and data); Rendered Fits ChatGPT size test, 2 October 2026; Shopify size chart report 2026 (829 stores, 6 October 2026); Shopify Help Center: mapping your product data sources for Shopify Catalog.

About Rendered Fits. Rendered Fits is a Shopify app for fashion brands that recommends a size for supported products from the brand's own size chart, and can show shoppers an optional AI try-on preview. There is a Free plan, and Complete is $349 a month with a 14-day trial.