Image aspect ratio describes the proportional relationship between width and height. It is useful when you need to resize an image without stretching it, decide whether a crop will fit a destination, or create several assets with the same shape. Common ratios include 1:1, 4:3 and 16:9.
Calculate aspect ratio from pixel dimensions
Take the image width and height in pixels and simplify the two numbers by their greatest common divisor (GCD). For a 1920 × 1080 image, the GCD is 120, so 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9. The simplified aspect ratio is therefore 16:9.
Use Tervilo's Image Dimensions tool
- Open the Tervilo Image Dimensions Checker.
- Select the image.
- Review its width and height.
- Use those dimensions to identify the ratio shown by the tool.
- Use the Image Resizer or Image Cropper when the destination shape is different.
Example: 1200 × 800
The GCD of 1200 and 800 is 400. Divide both dimensions by 400 to get 3:2. An image with a 3:2 ratio can be resized proportionally to 900 × 600, 600 × 400 or another pair that keeps the same relationship.
Example: 1080 × 1080
Both dimensions are equal, so the ratio is 1:1. This is a square image. Reducing it proportionally to 800 × 800 keeps the same shape.
Example: 2560 × 1080
The GCD is 40, producing 64:27. This is a much wider shape than the common 16:9 ratio. If the destination requires 16:9, simply resizing the whole image to the new ratio would distort it. Cropping is safer.
Aspect ratio vs resolution
Aspect ratio describes shape. Pixel dimensions describe how many pixels the image contains. A 1920 × 1080 and a 1280 × 720 image are both 16:9 but have different pixel dimensions.
Aspect ratio vs file size
Two images can share the same aspect ratio and dimensions but have different file sizes because their formats, content and compression levels differ. Ratio alone does not tell you how much storage the image uses.
Why aspect ratio matters when resizing
When you preserve the ratio, one dimension determines the other. If a source is 16:9 and you set the width to 1280, the proportional height is 720. If you independently choose 1280 × 800, the image shape changes unless you crop or pad.
Why aspect ratio matters when cropping
When a destination ratio differs from the source, cropping can remove part of the image to fit the new shape without stretching. Position the crop around the most important content before resizing to the final dimensions.
When a destination specifies exact dimensions
Check whether the destination requires exact pixels, an aspect ratio, or both. If it requires 1200 × 628, meeting only the approximate ratio may still fail. Follow the actual specification.
Common mistakes
- Confusing aspect ratio with pixel dimensions.
- Changing width and height independently.
- Stretching an image to match a different ratio.
- Assuming the ratio determines file size.
- Ignoring exact destination dimensions.
Quick checklist
- □ I know the source width and height.
- □ I simplified the ratio correctly.
- □ I know whether the destination requires ratio, exact dimensions, or both.
- □ I used proportional resize or cropping instead of stretching.
- □ I checked the final output.
Quick answer
Calculate aspect ratio by simplifying the image width and height with their greatest common divisor. Use the ratio to preserve shape when resizing and to decide when cropping is necessary for a destination with a different shape.