What is aspect ratio, and how do you calculate it?
Published 17 October 2026 · SepoooTools blog
You upload a photo and the website cuts off half of someone's head. Or a print comes back with a slice missing at the edge. In almost every case the reason is the same: the aspect ratio of your photo did not match the shape that was needed. Once you understand this one idea, you can fix the problem yourself in a minute.
What aspect ratio means
Aspect ratio is the shape of a picture. It compares the width to the height. It does not tell you how big the picture is. It only tells you how wide it is compared to how tall.
It is written as two numbers with a colon: width : height. So 16:9 means "16 units wide for every 9 units tall". A small 16:9 image and a huge 16:9 image have exactly the same shape.
| Ratio | Shape | Where you see it |
|---|---|---|
| 1:1 | Square | Profile pictures, many social media posts |
| 4:3 | A bit wider than tall | Default photo on most phones, older screens |
| 3:2 | Wider | Many cameras, 10x15 cm and 4x6 inch prints |
| 16:9 | Wide | TVs, laptops, YouTube videos |
| 4:5 | A bit taller than wide | Portrait posts on social media |
| 7:9 | Tall | 35 x 45 mm passport photos |
If the first number is bigger, the picture is landscape (lying down). If the second is bigger, it is portrait (standing up). 3:2 and 2:3 are the same shape, just turned.
How to calculate the aspect ratio of a photo
You need the width and height in pixels. On a computer, right-click the file and look at the properties or info. On a phone, open the photo details. Then:
- Write the two numbers: width and height.
- Find the largest number that divides both of them evenly. (This is called the greatest common divisor.)
- Divide both numbers by it.
Example 1: 1920 x 1080
The largest number that fits into both is 120. 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9. So the ratio is 16:9.
Example 2: a phone photo, 4032 x 3024
Both divide by 1008. 4032 ÷ 1008 = 4 and 3024 ÷ 1008 = 3. The ratio is 4:3.
Example 3: 1080 x 1350
Both divide by 270. 1080 ÷ 270 = 4 and 1350 ÷ 270 = 5. The ratio is 4:5, a portrait shape.
The quick way: divide width by height
Finding the common divisor can be slow. A faster trick is to divide the width by the height and compare the result:
- 1.00 = 1:1
- 1.33 = 4:3
- 1.50 = 3:2
- 1.78 = 16:9
So 1920 ÷ 1080 = 1.78, which tells you straight away that it is 16:9. Small differences, like 1.77 or 1.79, usually come from rounding.
How to find a missing width or height
Often you know the ratio and one side, and you need the other side. Use these two formulas:
- Height = width × (second number ÷ first number)
- Width = height × (first number ÷ second number)
Example: you need a 16:9 image that is 1200 pixels wide. Height = 1200 × 9 ÷ 16 = 675 pixels.
Example: you need a 4:3 image that is 800 pixels tall. Width = 800 × 4 ÷ 3 = 1066.7, so round to 1067 pixels.
What happens when the ratio does not match
A photo can only change its shape in two ways. Both have a cost:
- Cropping cuts away some of the edges. The picture stays sharp and natural, but you lose part of it.
- Stretching squeezes the picture into the new shape. Nothing is cut, but faces look too wide or too thin.
Some websites also add empty bars (often black or white) at the sides instead. That is called letterboxing.
Cropping is almost always the better choice. The trick is to decide yourself what gets cut, instead of letting a website or print shop decide for you.
How much do you lose? A real example
You have a 4:3 phone photo of 4032 x 3024 pixels and want a 16:9 version. Keep the full width. The new height is 4032 × 9 ÷ 16 = 2268 pixels. You cut 3024 − 2268 = 756 pixels, which is 25% of the height.
The same photo printed as 10x15 (3:2) loses less. Keep the width: 4032 ÷ 1.5 = 2688 pixels high. You cut 336 pixels, about 11%. That is why prints from phone photos often lose a thin strip at the top or bottom.
A simple routine that avoids surprises
- Check which ratio the website, form or print shop expects.
- Crop the photo to that ratio yourself, and place the important part (a face, a document) in the middle.
- Only then resize it to the exact pixel size, if one is asked for.
- Keep the original, so you can crop again for a different shape later.
Our free Crop Image tool has fixed ratios you can pick, and the Image Resizer sets the final pixel size. For round profile pictures, use Circle Crop. For passport photos, the Passport Photo Maker already knows the right shape. All of them run in your browser, so your photo is never uploaded.
Questions people ask
Is 1920x1080 16:9?
Yes. Divide both numbers by 120 and you get 16 and 9.
What aspect ratio is a phone photo?
Most phone cameras take photos in 4:3 by default, for example 4032 x 3024 pixels. Many phones also offer 16:9 and 1:1 in the camera settings.
Does changing the aspect ratio reduce quality?
Cropping removes pixels from the edges, so the image gets smaller, but the part you keep stays just as sharp. Stretching an image to a new ratio without cropping makes it look distorted.
Printing at home? Read 10x15 cm in pixels for the exact print sizes, and one photo, many sizes for passport, school and form photos.