How to resize to an exact width and height
Upload the image and choose Pixels. If the target shape matches your image, for example a 4:3 photo going to 800 × 600, leave Keep aspect ratio on and type the width. The height will match exactly.
If the target has a different shape, uncheck Keep aspect ratio and type both numbers. The image is resized to exactly that width and height. Because the shape changes, the content is stretched or squeezed to fit, which is fine for small changes but noticeable on faces and circles.
Exact dimensions without stretching
To change the shape without distortion, crop first, then resize. The Image Cropper crops to common ratios such as 1:1, 4:5 and 16:9. Crop to the ratio of your target size, then come back here and type the final dimensions with the aspect ratio locked.
For social media sizes, the Social Media Image Resizer does both steps at once. It scales the image to cover the preset size and trims the edges evenly from the center.
Checking the ratio of your target size
Divide the width by the height for both your image and the target. A 3000 × 2000 photo has a ratio of 1.5. A 1920 × 1080 target has a ratio of about 1.78, so going straight there would stretch the photo sideways. The Image Aspect Ratio Calculator works this out for you and tells you the matching height for any width.
Common exact sizes
Some sizes come up again and again: 1920 × 1080 for desktop wallpapers, slides and video frames, 1080 × 1080 for square social posts, 1200 × 630 for link previews on social networks, and 600 × 600 or 800 × 800 for product and profile photos. Forms often specify their own sizes, so follow their instructions when they do.