NPM Bundle Size Estimator

Bundle size is the amount of JavaScript that gets shipped to the browser when a user loads your app. Larger bundles mean longer download times, slower parse/execute times, and worse Core Web Vitals scores. A package that is 200 KB minified might seem small, but on a 3G connection it adds noticeable load time. Keeping total JS under 170 KB gzipped is a good baseline for most apps.

Minified size is the size after whitespace, comments, and long variable names are stripped. Gzip size is the minified bundle after gzip compression, which is what actually travels over the network when servers have compression enabled (most do). Gzip exploits repeated patterns in text, so it compresses JavaScript very well — typically 60–70% smaller than minified. Brotli compresses even further (10–20% smaller than gzip) and is supported by all modern browsers.

Tree-shaking is a dead-code elimination technique used by bundlers like Webpack, Rollup, and esbuild. When a package exports many functions but you only import a few, a tree-shaking-capable bundler removes the unused exports. A package with 'sideEffects: false' in its package.json is fully tree-shakable. Packages with side effects (like polyfills or CSS-in-JS) cannot be safely eliminated, so you always get the full bundle.

Under 10 KB gzipped is green — lightweight, no concern. 10–100 KB gzipped is yellow — evaluate whether you truly need it or if a smaller alternative exists. Over 100 KB gzipped is red — this is a heavy dependency. Examples: lodash-es is ~72 KB minified but tree-shakable; moment.js is ~232 KB minified with no tree-shaking; date-fns is ~78 KB but fully tree-shakable so real cost is just the functions you use.

Yes. Enter the package name with a version using the @ syntax, for example: [email protected] or [email protected]. If you omit the version, the latest published version is used. This is useful when your project pins an older version and you want to compare bundle sizes between versions before upgrading.

The package names are sent only to the bundlephobia.com public API (https://bundlephobia.com/api/size) to fetch size data. No other server receives your input. Bundlephobia is a well-known open-source project maintained by Paskal Paknikar that provides accurate npm package size data.