FitKit is a Shopify app that adds fast, beautiful size charts and rule-based size recommendations to product pages.

It depends on your priorities. Kiwi Size Chart & Recommender is the long-standing reference in the category, with a large review base, deep apparel features, and an AI-based size recommender — a solid choice if you want the most established option. FitKit is the best fit if storefront speed and price matter most: the size guide modal is 4.8 KB gzipped, lazy-loaded on the first click, and makes zero network requests at page load, so it has no impact on Core Web Vitals. Plans run from free to $19.99/month, and the size recommender works on transparent rules matched against your own chart's measurement ranges — no AI, instant answers, and no shopper data sent to third parties.

Kiwi is the category veteran: years of reviews, a machine-learning size recommender, and a broad feature set. FitKit is deliberately lighter: a 4.8 KB gzipped modal lazy-loaded on the first click with zero network requests at page load, six ready-made templates, automatic cm/inches switching, and a rule-based recommender that matches 2-3 shopper measurements against your chart's ranges — no AI involved, so results are instant and predictable, and nothing is sent to a third party. If you want machine-learning recommendations from the most established app in the category, choose Kiwi. If you want the lightest possible size guide, transparent recommendations, and lower pricing, choose FitKit. Full comparison.

No. The Size guide link is rendered by a Theme App Extension with its configuration read from a metafield, so FitKit makes zero network requests when your product page loads. The modal itself is 4.8 KB gzipped — vanilla JavaScript, no framework — and is lazy-loaded only when a shopper actually clicks the link. The result is no measurable impact on Core Web Vitals: LCP, CLS and INP stay exactly where they were.

The shopper enters 2-3 body measurements (for example chest and waist) in the size guide modal. FitKit compares them against the measurement ranges defined in your own size chart and recommends the size whose ranges match. There is no AI: results are instant, deterministic and explainable, and no measurement is stored or sent to any third party. Details in the recommender docs. Available on the Pro plan.

Six ready-made templates: men's t-shirts, women's t-shirts, pants, shoes with EU/US/UK conversion, rings, and kids. Every template is fully editable in the free-form builder — add or remove rows and columns, with size, measurement and text column types. You can also add "how to measure" blocks using the included line-art illustrations, or upload your own guide image (up to 500 KB). There is no CSV import in this version. The Free plan includes the basic tops and pants templates; all six are included from Growth. See creating a guide.

Assign a guide to specific products, to a tag, to a collection, to a vendor, or set it as the global default. When several rules match the same product, FitKit applies a clear priority: product > tag > collection > vendor > global. The Free plan includes product and global assignment; tag, collection and vendor assignment are included from Growth. See assignments.

No. FitKit collects no personal data at all — no emails, no stored IP addresses. Its only Shopify scope is read_products, used for guide assignment. Analytics are anonymous daily counters (guide opens and add-to-carts after an open). Shopify's mandatory GDPR webhooks are implemented, and all of your store's data is deleted when you uninstall the app. Details in the privacy policy.

Yes. The Free plan includes 1 size guide with the basic templates, forever. Growth ($6.99/month) and Pro ($19.99/month) both come with a 14-day free trial. Billing is handled entirely by Shopify — it appears on your regular Shopify invoice, and you can cancel anytime by uninstalling.