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Get size of the selected file

Written byPhuoc Nguyen
Level 1 — Basic
10 Mar, 2020
Last updated
07 May, 2021
In the markup below, we have two elements defined by different `id` attributes. The `id="size"` element will be used to display the size of selected file from the `id="upload"` element.
<input type="file" id="upload" />
<div id="size"></div>
We listen on the `change` event of the file input, and get the selected files via `e.target.files`. The file size in bytes of the selected file can be retrieved from the `size` property of the first (and only) file.
The size element is shown up or hidden based on the fact that user selects a file or not.
// Query the elements
const fileEle = document.getElementById('upload');
const sizeEle = document.getElementById('size');

fileEle.addEventListener('change', function (e) {
const files = e.target.files;
if (files.length === 0) {
// Hide the size element if user doesn't choose any file
sizeEle.innerHTML = '';
sizeEle.style.display = 'none';
} else {
// File size in bytes
sizeEle.innerHTML = `${files[0].size} B`;

// Display it
sizeEle.style.display = 'block';

Display a readable size

There is a room for improving the output of file size. Instead of displaying in bytes, we can transform it to a readable format in kB, MB, GB, and TB depending on how big it is.
The following `formatFileSize` helper method is created for that purpose:
// Convert the file size to a readable format
const formatFileSize = function (bytes) {
const sufixes = ['B', 'kB', 'MB', 'GB', 'TB'];
const i = Math.floor(Math.log(bytes) / Math.log(1024));
return `${(bytes / Math.pow(1024, i)).toFixed(2)} ${sufixes[i]}`;

// Display the file size
sizeEle.innerHTML = formatFileSize(files[0].size);

Use case

  • Validate the file size before uploading to the server.


See also

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Phước Nguyễn