Customizing theme to add image width and height automatically in WordPress

How to add the width and height attributes to all images in WordPress automatically

January 16, 2023

Fast page loading and keeping previously loaded text in place are important for site visitors and Google’s PageSpeed results.

If images are used on top of or among the text, the width and height of the images must be known so that the browser can define a suitable area for the image before it is loaded. This prevents a layout shift as the image downloads, which is an important factor for Chrome and Google PageSpeed’s cumulative layout shift metric.

Customizing theme to add image width and height automatically

Although WordPress and our frequently used plugin ShortPixel cannot automatically add width and height to images, you can make the theme generate the necessary information by editing the functions.php file in the WordPress theme.

Add the following code to the theme’s functions.php file. Before you begin customizing the code, please make a backup copy. Better safe than sorry.

function imgSize($content){
  $pattern = '/<img [^>]*?src="(https?://[^"]+?)"[^>]*?>/iu';
  preg_match_all($pattern, $content, $images);

  foreach ($images[0] as $index => $img) {
    if (strpos($img, "width=") !== false && strpos($img, "height=") !== false) {

    $imgUrl = $imgs[1][$index];
    $imgSize = @getimagesize($imgUrl);
    if (empty($imgSize)) {

    $replacedImg = str_replace("<img ", "<img " . $imgSize[3] . " ", $imgs[0][$index]);
    $content = str_replace($img, $replacedImg, $content);
  return $content;

add_filter("the_content", "imgSize");

Thanks for reading!