Set up SizeCast

1. Install and open the app

Install SizeCast from the Shopify App Store. On first open you'll be asked to approve the permissions it needs — products, metaobjects, and themes. Approve them, and the SizeCast dashboard opens inside your Shopify admin. The dashboard's setup guide tracks your progress through the steps below.

2. Create a size chart

In the Size charts section, click a preset that's closest to your products — for example Men's tops (S–XXL) or Women's tops (XS–XL). A ready-made chart opens in the editor with sensible height, weight, chest, and waist ranges for each size.

• Adjust any number to match your own garments. Every change raises a save bar at the bottom — click Save when you're done.
• Prefer to bring your own data? Use Import from CSV to paste a chart, or start from Blank chart.
• You can Export CSV at any time — your chart data is always yours to take.

3. Assign the chart to products

Still in the chart editor, use Assign products to pick the products this chart applies to, or Assign a collection (Pro) to attach it to an entire collection at once. On the dashboard you can also set one chart as the store default (Pro), which covers any product that doesn't have its own chart.

Every product uses exactly one chart, so there are no priority rules to manage. The dashboard's Connected products list shows precisely which chart each product will display.

4. Add the "Find my size" button

On the dashboard, click Add the button in the theme editor. This opens your theme editor with the SizeCast block ready to place. Two ways to position it:

• As a block (recommended): inside the Product information section, click Add block → Apps → Size calculator and drag it right under the variant picker.
• As a section: use Add section → Apps → Size calculator for a standalone placement.

Click Save in the theme editor. The button only renders on products that have a chart assigned, so it's always safe to place.

5. Style it to your theme

Select the block in the theme editor to open its settings. By default the button copies your theme's own button style automatically, including hover states, so it matches your Add-to-cart button. You can also switch to a solid, outline, or text-link style; set the accent color, alignment, width, and padding; or paste custom CSS under Advanced.

6. Test the shopper experience

Open one of your assigned products on the storefront and click Find my size:

• Enter a height and weight, then Calculate my size. A recommended size appears with a confidence bar and a note if you're between sizes.
• Click Use this size — the recommended variant is selected on the product form automatically.
• Switch to the Size chart tab to see the full table with a cm/in and kg/lb toggle and the recommended row highlighted.
• Open the popup on another product — for a returning shopper it jumps straight to the pre-calculated result, all computed in their browser.

Need a hand?

Email hello@spell.agency and we'll help you get set up. Common questions are answered on the FAQ.