Aspect Ratio Explained: Resize Without Stretching or Cropping
Aspect ratio is an image's shape: width divided by height. Keep it the same and you can resize freely without distortion, using one formula: new height = new width × original height ÷ original width. Changing the shape needs a crop or padding, never a stretch.
What aspect ratio means
An image's aspect ratio is its width divided by its height, written as two numbers such as 16:9 or 4:5. A 1920 × 1080 image and a 1280 × 720 image are different sizes but the same shape, because both are 16:9. The ratio describes the shape; the pixel dimensions describe the size.
When you resize an image and keep the ratio, the picture gets bigger or smaller without distortion. When you change the ratio, something has to give: the picture is stretched, squashed, cropped or padded with bars.
The one formula you need
If you know the original size and one new side, the other side is:
- New height = new width × original height ÷ original width
- New width = new height × original width ÷ original height
This is exactly what the Keep aspect ratio option in the Image Resizer does: type one side and the other is filled in for you.
Worked example 1: shrink a phone photo for a blog
A photo is 4032 × 3024 and the blog column is 1200 px wide. New height = 1200 × 3024 ÷ 4032 = 900 px. Result: 1200 × 900, still 4:3.
Worked example 2: a banner that must be 400 px tall
A 1920 × 1080 image needs to be 400 px tall. New width = 400 × 1920 ÷ 1080 = 711.1, so 711 px. Round to the nearest whole pixel; the tiny error is invisible.
Worked example 3: a portrait photo to a fixed width
A 3000 × 4000 portrait needs to be 800 px wide. New height = 800 × 4000 ÷ 3000 = 1067 px.
Common ratios and where they're used
| Ratio | Decimal | Typical sizes (px) | Used for |
|---|---|---|---|
| 1:1 | 1.00 | 1080 × 1080 | Profile photos, square posts |
| 4:5 | 0.80 | 1080 × 1350 | Portrait social posts |
| 2:3 | 0.67 | 1000 × 1500 | Pinterest pins, 4×6 in prints (portrait) |
| 3:4 | 0.75 | 3024 × 4032 | Phone photos held upright |
| 4:3 | 1.33 | 4032 × 3024 | Phone photos, tablets, older screens |
| 3:2 | 1.50 | 6000 × 4000 | DSLR and mirrorless cameras, 6×4 in prints |
| 16:9 | 1.78 | 1920 × 1080 | Video, YouTube thumbnails, presentations |
| 1.91:1 | 1.91 | 1200 × 630 | Link previews (Open Graph) |
| 9:16 | 0.56 | 1080 × 1920 | Stories, Reels, Shorts, TikTok |
For exact sizes per platform, see Image Sizes for Every Platform.
How to find an image's ratio
Divide width by height and compare the decimal with the table. For 1600 × 900: 1600 ÷ 900 = 1.78, so 16:9. To write it as whole numbers, divide both sides by their greatest common divisor: for 1600 and 900 it's 100, giving 16:9. For 1080 × 1350 it's 270, giving 4:5.
Changing the ratio without stretching
If you need a different shape, you have three honest options:
- Crop. Cut away part of the image so the rest has the new shape. Best when the subject is in the middle. To turn a 4:3 photo (4032 × 3024) into 16:9, keep the full width and crop the height to 4032 × 9 ÷ 16 = 2268 px, trimming 378 px from the top and bottom.
- Pad. Place the image on a larger canvas with bars or a blurred background. Nothing is cut off, but the picture appears smaller.
- Recompose. Use a different photo that was shot in the right orientation. Often the best result, and worth planning when you take the picture.
What you should not do is type a new width and height with the aspect ratio lock turned off. The Image Resizer allows it for cases where you've already cropped, but on an uncropped photo it squashes faces and makes circles into ovals.
Crop first, then resize
Order matters. Crop to the ratio you need at full resolution, then resize down to the pixel size. If you shrink first and crop afterwards, you throw away pixels you'd already paid for, and may end up below the size the platform needs.
Why a platform crops your image anyway
Even with the right ratio, some places show a different crop in previews: a 4:5 post appears 3:4 on an Instagram profile grid, and a wide header is trimmed on phones. Keep important content away from the edges, roughly in the central 80% of the frame.
Frequently asked questions
Is 1920 × 1080 the same as 16:9?
Yes. 1920 ÷ 1080 = 1.78, which is 16 ÷ 9. So are 1280 × 720, 2560 × 1440 and 3840 × 2160.
What aspect ratio is a phone photo?
Most phone cameras shoot 4:3 by default (4032 × 3024 on many iPhones). Some offer 16:9 or 1:1 modes, which crop the sensor image.
Why is my resized image slightly off by one pixel?
Many ratios don't divide into whole pixels, so the result is rounded. A one-pixel difference is invisible and won't cause cropping.
Does DPI change the aspect ratio?
No. DPI only affects print size. The ratio depends only on the pixel width and height.
Related
- Image Resizer
- Image Sizes for Every Platform: Social Media, Email and Websites
- How to Reduce a Photo to Under 100 KB or 200 KB