formatFileSize function

String formatFileSize(int bytes, { int decimals = 1, })

Formats bytes as a human-readable size using binary (1024) units up to TB.

decimals controls the maximum fraction digits; trailing zeros are trimmed. Whole values and sizes of 10 or more drop the fraction. Negative inputs are prefixed with -.

Example:

formatFileSize(1536); // '1.5 KB'
formatFileSize(0); // '0 B'

Audited: 2026-06-12 11:26 EDT

Implementation

String formatFileSize(int bytes, {int decimals = 1}) {
  if (bytes < 0) return '-${formatFileSize(-bytes, decimals: decimals)}';
  if (bytes == 0) return _kSizeZeroB;
  const List<String> units = _kFileSizeUnits;
  int i = 0;
  double v = bytes.toDouble();
  while (v >= 1024 && i < units.length - 1) {
    v /= 1024;
    i++;
  }
  String fmt;
  if (v >= 10 || v == v.truncateToDouble()) {
    fmt = v.truncate().toString();
  } else {
    fmt = v.toStringAsFixed(decimals);
    // Strip trailing zeros only AFTER a decimal point. With decimals: 0,
    // toStringAsFixed rounds (9.5 -> "10") and produces no '.', so the bare
    // `0+$` strip turned "10" into "1" — wrong magnitude. Guarding on '.' keeps
    // integer-part zeros.
    if (fmt.contains('.')) {
      fmt = fmt.replaceAll(RegExp(r'0+$'), '').replaceAll(RegExp(r'\.$'), '');
    }
  }
  return '$fmt ${units[i]}';
}