$ snippet format-bytes-numbers

Format bytes and numbers for humans

'1.2 MB', '3,456,789' and '$1,234.50' using Intl.NumberFormat — correct for every locale.

JavaScript

/** 1536 → "1.5 KB" (decimal, like macOS/disks) or "1.5 KiB" with binary: true. */
function formatBytes(bytes, { decimals = 1, binary = false } = {}) {
  if (!Number.isFinite(bytes) || bytes < 0) return "—";
  const base = binary ? 1024 : 1000;
  const units = binary ? ["B", "KiB", "MiB", "GiB", "TiB", "PiB"] : ["B", "KB", "MB", "GB", "TB", "PB"];
  const i = bytes === 0 ? 0 : Math.min(units.length - 1, Math.floor(Math.log(bytes) / Math.log(base)));
  const value = bytes / base ** i;
  return `${value.toFixed(i === 0 ? 0 : decimals)} ${units[i]}`;
}

/** Thousands separators, localized: 1234567.891 → "1,234,567.89" (en) / "1.234.567,89" (de). */
function formatNumber(n, locale = undefined, options = {}) {
  return new Intl.NumberFormat(locale, { maximumFractionDigits: 2, ...options }).format(n);
}

/** Currency: formatCurrency(1234.5, "USD") → "$1,234.50" */
function formatCurrency(n, currency, locale = undefined) {
  return new Intl.NumberFormat(locale, { style: "currency", currency }).format(n);
}

/** Compact: 12800 → "12.8K", 3400000 → "3.4M" */
function formatCompact(n, locale = undefined) {
  return new Intl.NumberFormat(locale, { notation: "compact", maximumFractionDigits: 1 }).format(n);
}

How it works

Never build number strings with regex hacks like replace(/\B(?=(\d{3})+(?!\d))/g, ",") — it breaks for locales that use spaces or periods as separators, and for negative decimals. Intl.NumberFormat is built in, fast when you reuse the instance, and handles currency symbols, compact notation and rounding rules.

Gotchas

Decide decimal vs binary bytes deliberately: file systems and browsers mostly show decimal (1 KB = 1000 B); memory and Linux tools often use binary (1 KiB = 1024 B).
Passing undefined as the locale uses the user's browser language — usually what you want in a UI, rarely what you want in a log file or CSV.
Create the formatter once and reuse it in loops; constructing Intl.NumberFormat per call is slow.

Try it

Result

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