Aspect ratio is width divided by height, written as two whole numbers. A 1920 x 1080 image is 16:9 because both numbers divide by 120. The tool finds the largest number that divides both your width and height and shows the simplified result. Sizes with no common factor stay as they are; 1366 x 768, a common laptop screen, simplifies to 683:384, which is close to but not exactly 16:9.
What an aspect ratio is
Calculating new dimensions
After you load an image, its width and height fill the two boxes. With the lock on, typing a new width calculates the height that keeps the same shape (rounded to whole pixels), and the other way round. Unlock it to type both freely.
The preset buttons (16:9, 4:5, 9:16, 4:3 and 1:1) keep the current width and work out the height for that ratio. This tells you what size to crop or resize to; it does not change your image. To actually produce those dimensions, crop with the Crop tool and then resize with the Image Resizer.
Rounding and odd sizes
Pixel counts are whole numbers, so an exact ratio is not always possible. For example, 1000 px wide at 16:9 needs a height of 562.5, which rounds to 563. The difference is less than one pixel and invisible, but some upload checks compare ratios strictly. Choosing a width that is a multiple of the first number (16, 32, 48... for 16:9) avoids fractions altogether.
Where the processing happens
The image is only read to get its dimensions. It is not modified or uploaded.
Questions about this tool
What is the aspect ratio of my photo?
Load the photo and the tool shows its ratio in the simplest form, for example 4:3 or 16:9. Nothing in the image is changed.
I need my photo at 16:9 for a slide but want to keep the width. What height should it be?
Load the photo and press the 16:9 button. The tool keeps your width and shows the matching height. Then crop to that shape with the Crop tool.
I want to make my image 800 pixels wide without changing its shape. What height do I need?
Keep the lock on and type 800 in the width box. The height that keeps the same shape appears, rounded to whole pixels.
Why does my 1366 x 768 screen show 683:384 instead of 16:9?
Those numbers are close to 16:9 but not exactly. The tool shows the exact ratio, so sizes that are almost 16:9 show their true values.
Does this tool resize my image?
No. It only calculates. To change the size, use the Image Resizer with the numbers you worked out here.