I picked 4:5 and got 799 x 999 instead of an exact 4:5. Is the cropper broken?
It is not broken. A pixel cannot be split, so when the exact ratio would need a fraction of a pixel the tool rounds to the nearest whole number, and the result can be off by at most one pixel. For a perfectly exact ratio, type a width that divides by the ratio's first number, such as 800 for 4:5.
Where the difference comes from
A 4:5 ratio means the height is the width times 1.25. A photo measuring 1000 × 999 limits the height to 999 pixels, and the width that matches that height is 799.2 pixels. No image is 799.2 pixels wide, so the Image cropper rounds the width to 799.
A 1000 × 999 photo at 4:5: the box is 799 × 999, starting at X = 100.
Typing a width of 805 at 4:5: the exact height would be 1006.25, so the output is 805 × 1006.
Typing a width of 804: the height is exactly 1005, and 804 × 1005 is a true 4:5.
In the first example the width is off by a fifth of a pixel, and nobody will ever see it. It can matter, though, if a platform or upload form checks dimensions strictly, or if you are preparing images that will sit side by side in a grid.
How to get an exact ratio every time
The rule is simple: choose a width that divides evenly by the first number of the ratio, and the height comes out as a whole number with no rounding.
- 4:5: a width that is a multiple of 4, such as 800 for a height of 1000, or 1080 for 1350.
- 3:4: a multiple of 3, such as 900 for a height of 1200.
- 16:9: a multiple of 16, such as 1920 for a height of 1080.
- 1:1: any number works, since the height always equals the width.
If you type the height instead, the same idea runs the other way: a height of 1351 at 4:5 needs a width of 1080.8, which rounds to 1081, while a height of 1350 gives exactly 1080.
Steps
- Open the photo in the cropper and pick the ratio, for example 4:5.
- Type a width that divides by 4 in 'Width (px)'. The height is filled in for you.
- Position the box by dragging or with the X and Y fields, without changing its size.
- Check the 'Output:' line under the preview. That is the size the file will actually have.
When rounding cannot be avoided
If the photo is smaller than the size you asked for, the tool shrinks the box to keep it inside the image, and the shrunken width may not divide evenly. In that case type a smaller multiple that fits inside the photo. Remember that the box is always measured in the original image's pixels, not in the size you see on screen.
The same logic applies to a custom ratio: with 3 and 2 as the ratio numbers, pick a width that is a multiple of 3.
Common questions
Will my store reject a 799 px image instead of 800?
That depends on the platform. If it checks for exact dimensions, type the exact size yourself instead of relying on the largest automatic box.
Does the rounding distort the picture?
No. The crop takes pixels as they are, without stretching; rounding only moves the edge of the box.
Where do I see the final size?
In the 'Output:' line under the preview, and in the downloaded file name, which gets the size added, such as crop-804x1005.