sitespeed.io/lib/support/helpers/co2.js

sitespeed.io is an open-source tool for comprehensive web performance analysis, enabling you to test, monitor, and optimize your website’s speed using real browsers in various environments.

const kg = 1000;

export function co2(grams) {
  if (grams === 0) return '0';
  else if (grams === undefined) return 0;
  else if (grams < kg) {
    return Number(grams).toFixed(4) + ' grams';
  } else {
    return Number(grams / kg).toFixed(3) + ' kg';
  }
}