Format bytes as KB, MB, GB
Convert a byte count into a short human-readable size such as 1.5 MB.
- JavaScript
- GreenSnippets
- MIT
- Executed with Node.js 22
function formatBytes(bytes, decimals = 1) {
if (!Number.isFinite(bytes) || bytes < 0) throw new RangeError("bytes must be a non-negative number");
if (bytes === 0) return "0 B";
const units = ["B", "KB", "MB", "GB", "TB", "PB"];
const i = Math.min(Math.floor(Math.log(bytes) / Math.log(1024)), units.length - 1);
const value = bytes / 1024 ** i;
return `${Number(value.toFixed(decimals))} ${units[i]}`;
}Example / expected output
formatBytes(1536) // "1.5 KB"
formatBytes(5_000_000) // "4.8 MB"How it works
Uses base-1024 steps with the common KB/MB/GB labels. The number of decimals is configurable and trailing zeros are dropped. Negative or non-finite input throws instead of returning nonsense.
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