I'd like to add an image size where the resulting image will be the largest possible size while retaining a 4:3 aspect ratio.

Suppose I've added an image size like so:

add_image_size( 'cover-image', 2048, 1536, true );

By default, WP will only create an image with that size if the full size image is larger than those dimensions.

But suppose my full size image is only 1000px wide. I would still like to have an image cropped to the largest possible 4:3 ratio, which in this case would be 1000x750.

Can this be done?

1 Answer 1


The Approach

I think the best approach is to create an image size "on the fly", just before the images is resized.

You can do that using 'intermediate_image_sizes_advanced' filter hook. That allows you to edit the size to be generated, but being aware of the current image size, that is stored in the array $metadata passed by the filter as second argument.

The Math

First of all let's write a class that returns the biggest sizes for a specific ratio.

class ImageRatio {

  private $ratio;

  function __construct($ratioW = 4, $ratioH = 3) {
    $this->ratio = array($ratioW, $ratioH);

  function getLargestSize($imgW, $imgH) {
    $inverse = false;
    // let's try to keep width and calculate new height  
    $newSize = round(($this->ratio[1] * $imgW) / $this->ratio[0]);
    if ($newSize > $imgH) {
       $inverse = true;
       // if the calculated height is bigger than actual size
       // let's keep current height and calculate new width
       $newSize = round(($this->ratio[0] * $imgH) / $this->ratio[1]);

    return $inverse ? array( $newSize, $imgH ) : array( $imgW, $newSize );


Class usage

The usage of the class is pretty easy:

$ratio = new ImageRatio(4, 3)

$ratio->getLargestSize(1000, 500); // return: array(667, 500)
$ratio->getLargestSize(1000, 800); // return: array(1000, 750)

In Action

At this point, we can make use of the class to calculate on the fly an new images size, based on the image is being uploaded

add_filter( 'intermediate_image_sizes_advanced', function( $sizes, $metadata ) {

   if (! empty( $metadata['width'] ) && ! empty( $metadata['height'] ) ) {
      // calculate the max width and height for the ratio
      $ratio = new ImageRatio( 4, 3 );
      list($width, $height) = $ratio->getLargestSize( 
      // let's add our custom size
      $sizes['biggest-4-3'] = array(
        'width'  => $width,
        'height' => $height,
        'crop'   => true

   return $sizes;

}, 10, 2 );

Using the New Size

$image = wp_get_attachment_image( $attachment_id, 'biggest-4-3' );


Of course, this works for all the images that you upload after the code is in place. For older images, you should re-generate the thumbnails, on the fly when they are used, or in bulk using one of the plugins available on the web.

  • This works great, thanks! One minor correction I would make: I think it's unnecessary to set a default ratio in the constructor, as I might use this for other ratios in the future. Makes sense to always supply the desired ratio on instantiation. Commented Dec 24, 2015 at 14:45
  • The args in constructor are just default. You can pass any ratio you want, default are used only if you don't pass anything
    – gmazzap
    Commented Dec 25, 2015 at 11:11

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