Calculate the largest text font size that will fit a bounding box, without a web browser. Works anywhere Satori does, e.g. Node.js, web browser, Vercel edge runtime, etc.

# Using NPM
npm install @altano/satori-fit-text
# Using Yarn
yarn add @altano/satori-fit-text
# Using PNPM
pnpm add @altano/satori-fit-text

import { findLargestUsableFontSize, type Font } from "@altano/satori-fit-text";

async function getInter(): Promise<Font> {
  const interSemiBoldBuffer = await fetch(`https://cdn.jsdelivr.net/npm/@fontsource/inter@5.0.17/files/inter-latin-600-normal.woff`).then((res) => res.arrayBuffer());

  const font = {
    name: "Inter",
    data: interSemiBoldBuffer,
    weight: 600,
  };
}

const largestUsableFontSize = await findLargestUsableFontSize({
  lineHeight: 1,
  font: await getFont(),
  text: "Some text I want to be as big as possible",
  maxWidth: 1136,
  maxHeight: 429,
});

console.log(largestUsableFontSize);

• React Component (sandbox)
• Next.js Open Graph Card - Simple (codesandbox)
• Next.js Open Graph Card - Complex
• Node.js CLI App (codesandbox)

The library tries various font sizes until it finds the ideal one that works. Each iteration is tested by generating an SVG with Satori and calculating the bounding box of the SVG.

The font size search is a binary search between 1 and 1000 with O(log maxFontSize-minFontSize) runtime, or a default of O(log 1000). You can improve performance by restricting the search space by reducing the difference between minFontSize and maxFontSize, which reduces the number of guesses.

If you're doing something like producing Open Graph cards then you probably don't need to worry about performance.