# Ensure an image upload is smaller than a specific size

> Learn how to make sure an image upload stays under a size limit like 3MB by checking the files size in a React onChange handler and rejecting bigger files.

Author: [Flavio Copes](https://flaviocopes.com/about/) | Published: 2023-02-16 | Topics: [JavaScript](https://flaviocopes.com/tags/js/) | Canonical: https://flaviocopes.com/how-to-ensure-an-image-upload-is-smaller-than-a-specific-size/

I had a form with a file input box, to let people upload an image:

```javascript
<input
  name='image'
  type='file'
  accept='image/*'
```

I needed this image to be smaller than 3MB.

So here’s what I did to implement this requirement in a [React](https://flaviocopes.com/react/) app, using the `onChange` event:

```javascript
<input
  name='image'
  type='file'
  accept='image/*'
  onChange={(event) => {
    if (event.target.files && event.target.files[0]) {
      if (event.target.files[0].size > 3 * 1000 * 1024) {
        alert('Maximum size allowed is 3MB')
        return false
      }
      setImage(event.target.files[0])
      setImageURL(URL.createObjectURL(event.target.files[0]))
    }
  }}
/>
```

Notice the `3 * 1000 * 1024` part: mixing 1000 and 1024 is easy to get wrong. KB and KiB are different things, and I made a [byte size converter](https://flaviocopes.com/tools/byte-size/) to sort that out.