How to Add a cm/inch Unit Switch to Shopify Size Charts

Who uses which unit

The US is the main market where shoppers measure their bodies in inches. Canada and the UK are mixed: many people know their measurements in inches, but schools, doctors and most official uses are metric, so younger shoppers often think in centimetres. Most of the rest of the world, including Europe, India, Australia, and most of Asia and Latin America, uses centimetres for body measurements.

If you sell into more than one of these, a single-unit chart is going to be the wrong unit for a big share of your shoppers.

The conversion, and why rounding matters

Since 1959, one inch has been defined as exactly 2.54 centimetres. That number never changes, so the maths is simple:

• inches × 2.54 = centimetres
• centimetres ÷ 2.54 = inches

The trouble isn't the maths. It's the rounding. A chart that says "chest 38.10 cm" looks precise, but nobody measures their chest to a tenth of a millimetre. False precision makes a chart harder to read and suggests an accuracy you don't have. Garment factories typically work to a tolerance of around a quarter to half an inch on most measurements anyway.

Practical rounding rules that work for size charts:

A few examples:

The most important rule: one source of truth

Pick one unit as the master for each chart, the unit your factory or pattern-maker actually uses, and convert from it. Never convert a converted number back again.

Here's what goes wrong if you do. Start with 34 inches. Convert to cm and round: 86.5 cm. Someone later "fixes" the inch column by converting 86.5 cm back: 34.06 inches, rounded to 34. Fine so far. Do it a few more times with ranges and different rounding, and you get charts where the inch and cm columns disagree by a size. Shoppers notice. They pick the column they trust, and half of them pick the wrong one.

So: store the source values once, generate the other unit automatically, and round only for display.

Ranges, not single numbers

Most size charts show ranges: "Size M, chest 38 to 40 inches." Convert both ends of the range separately, and round each one:

• 38 to 40 in → 96.5 to 101.5 cm

Make sure the ranges don't overlap or leave gaps after rounding. If size M ends at 101.5 cm and size L starts at 101.5 cm, a shopper who measures exactly 101.5 cm has two answers. Adjust the boundary so each measurement falls in one size only.

Option 1: a size chart app with a unit toggle

This is the quickest route and the one most stores take. With Tailor Size Guide, for example, you enter each chart once in your source unit, switch on the unit toggle, and the app shows a cm/inch switch above the table and converts the values for shoppers. (It's our app, so weigh that, but most good size chart apps work along similar lines.)

What to look for in any app:

• Enter once, convert automatically. You shouldn't have to type every chart twice.
• Sensible rounding you can control.
• A default unit you can set, ideally by market or language.
• The toggle remembers the shopper's choice across products.
• Accessible controls that work with a keyboard and screen reader.

Setup on most Online Store 2.0 themes is: install the app, create your charts, add the app block next to the variant picker in the theme editor, and switch on the unit toggle in the app's settings. Our guide to adding a size chart to the Dawn theme shows the theme editor steps.

Option 2: custom code in your theme

If you'd rather not use an app, a developer can build a toggle into your theme. The usual approach:

1. Store the source values in the table itself, as data attributes on each cell, so the page always has the original numbers.
2. Add two buttons above the table, "in" and "cm," marked up as a proper toggle so screen readers announce which unit is active.
3. When a shopper clicks one, a small script rewrites each cell from its source value using the 2.54 conversion and your rounding rules.
4. Save the choice in the browser's local storage, so the next product page opens in the same unit.
5. Set the default from the shopper's market. Shopify's Liquid can tell your theme which country the shopper has selected, so you can start in inches for the US and centimetres elsewhere.

This is free apart from development time, and it's fully under your control. The downsides are maintenance (theme updates can overwrite your changes, so keep the code in its own snippet and note where it lives) and that every new chart has to be entered in the right format. If you have more than a handful of charts, that adds up.

Option 3: both units in one table

The simplest option of all: show inches and centimetres in the same table, either as separate columns or as "38 in / 96.5 cm" in each cell.

It works without any code and it's what most stores start with. The catch is width. A table with three measurements in two units has seven columns, which is too wide for a phone screen and forces sideways scrolling. If your chart has more than two or three measurements, a toggle is easier to read on mobile.

Designing a toggle people actually use

A toggle only helps if shoppers notice it and understand it. Small details make a big difference:

• Put it directly above the table, not in a menu or at the bottom.
• Label it plainly: "in | cm" or "Inches | Centimetres." Avoid flags; country doesn't equal unit, and flags confuse people in mixed markets like the UK and Canada.
• Show which one is active clearly, with a filled or highlighted state.
• Keep the table the same size when switching, so nothing jumps around.
• Convert everything in the chart, including the how-to-measure notes. "Measure 1 inch below your armpit" should become "2.5 cm" too.
• Remember the choice on every product page, so shoppers set it once.
• Make it work with a keyboard, and announce the change to screen readers. Our guide to size chart accessibility covers the details.

Body measurements vs garment measurements

Before you convert anything, be sure your chart says which kind of measurement it shows. It's the single most common source of confusion on size charts, in any unit.

• Body measurements describe the shopper: "chest 38 in" means a person whose chest measures 38 inches should choose this size.
• Garment measurements describe the item laid flat: "chest 21 in" on a T-shirt usually means the shirt measures 21 inches across from armpit to armpit, which is 42 inches all the way round.

Both are useful, and many good charts show both. But if the label just says "chest," shoppers will compare their body to a garment number, and the result is wrong in either unit. Label each table clearly ("Body measurements" or "Garment measurements, laid flat"), and if you show flat garment widths, say so, because a flat width has to be doubled to compare with a body measurement. A unit toggle should convert both kinds the same way, but it can't fix a chart that doesn't say what it's measuring.

Units and international selling

A unit switch is one part of selling internationally. Shoppers in other countries also need sizes they recognise, such as UK and EU dress sizes or EU shoe sizes. Our guides to localizing size charts by country, translating size charts, and size charts with Shopify Markets cover the rest. The international size converter is useful for checking your conversions.

Testing before you go live

Before publishing, check:

1. Every value converts correctly in both directions. Spot-check five or six cells against a calculator.
2. No range overlaps or gaps after rounding.
3. The default unit is right for at least two markets.
4. The choice persists from one product page to another.
5. The table fits on a phone screen in both units.
6. Screen readers announce the unit change.

Does a unit switch reduce returns?

On its own, a toggle doesn't make your chart more accurate. What it does is remove a conversion step that shoppers often get wrong or skip. When someone in Germany sees "chest 40" and assumes it's centimetres, or a US shopper guesses at a metric chart, they're likely to order the wrong size. Making the chart readable in their own unit takes away that guess. It works best alongside an accurate chart, clear measuring instructions, and fit notes, which together make the biggest difference to wrong-size returns.

Frequently asked questions

Sources and standards

1. International yard and pound agreement (1959)Defined the international inch as exactly 2.54 centimetres, the conversion used in this guide.
2. Shopify Help Center: Online store themesShopify's documentation for adding app blocks and editing themes.
3. W3C Web Content Accessibility Guidelines (WCAG) 2.2The accessibility guidelines behind the keyboard and screen-reader recommendations for the toggle.
4. Apparel manufacturing tolerancesGarment specs typically allow a tolerance of around a quarter to half an inch on most measurements, which is why finer precision on a size chart isn't meaningful.