WordPress Image Size Calculator

How does WordPress resize uploaded images?

Every time you upload an image, WordPress creates several resized copies (“sub-sizes”) so themes can serve an appropriately sized file instead of the multi-megabyte original. Out of the box that means thumbnail (150×150, cropped), medium (300×300), medium_large (768 wide), large (1024×1024) and, since WordPress 5.3, 1536x1536 and 2048x2048. Themes and plugins add more with add_image_size(), which is why a single upload can turn into ten or more files.

Proportional sizes are bounding boxes: the image is shrunk until it fits inside the width and height, keeping its aspect ratio (wp_constrain_dimensions()). Cropped sizes are resized to cover the box and then cut to the exact dimensions from the chosen crop position (image_resize_dimensions()). WordPress never upscales: if the original is not larger than a size in either dimension, that file is simply not created.

Uploads larger than the big image size threshold (2560px by default) are scaled down to a -scaled copy that becomes the “full” size used on the front end; the original is kept, and sub-sizes are still generated from it. This calculator runs ports of those core functions in your browser, so the dimensions match what WordPress actually produces.

How to use the WordPress Image Size Calculator

1. Enter the width and height of the image you plan to upload, or choose a preset such as a 4032×3024 phone photo. Pick PNG if applicable — PNGs are never scaled by the threshold.
2. Adjust the default sizes to match Settings → Media, untick sizes you want to disable, and change or turn off the big image threshold.
3. Add your theme’s custom sizes with width, height, hard crop and crop position.
4. Read the table of generated files and srcset widths, then copy the PHP to register the sizes, disable unused ones and adjust the threshold.

Example

A 4032×3024 photo with the default settings creates 8 files:

photo.jpg               4032×3024  (original, kept)
photo-scaled.jpg        2560×1920  (full size)
photo-150x150.jpg        150×150   thumbnail
photo-300x225.jpg        300×225   medium
photo-768x576.jpg        768×576   medium_large
photo-1024x768.jpg      1024×768   large
photo-1536x1152.jpg     1536×1152  1536x1536
photo-2048x1536.jpg     2048×1536  2048x2048

How srcset uses these files

When an image is output with wp_get_attachment_image() or inserted in a post, WordPress adds a srcset attribute listing every generated file with the same aspect ratio as the displayed size, up to 2048px wide (max_srcset_image_width). The browser then picks the smallest file that fills the slot on the visitor’s screen. Cropped sizes with a different ratio, such as a square thumbnail of a landscape photo, are never mixed in — which is why a good set of proportional sizes matters more for performance than many cropped ones.

Tips for choosing image sizes

• Register only the sizes your theme actually uses: each size multiplies storage and upload processing time.
• Disable sizes you never output with the intermediate_image_sizes_advanced filter — the generated PHP does this for any size you untick.
• Use hard crops for grids and cards that need identical shapes; use proportional sizes for content images so srcset can work.
• After adding or changing sizes, regenerate existing images with wp media regenerate --only-missing (WP-CLI) or a regenerate-thumbnails plugin.
• Need the dimensions for a specific ratio? Use the Aspect Ratio Calculator, and register sizes alongside other theme code with the functions.php generator.