To calculate aspect ratio, divide the width by the height and simplify the result to its smallest whole numbers. A 1920 × 1080 image divides down to 16:9, and a 4000 × 3000 image divides down to 4:3. Once you know the ratio, you can resize, crop or fit any image without stretching it.
What is aspect ratio?
The aspect ratio of an image is the ratio of its width to its height, written as two numbers separated by a colon: width:height. A 16:9 image is 16 units wide for every 9 units tall. The units are not pixels or inches, only a proportion, so a small and a large image can share the same ratio.
This is why two images can have very different sizes and still look the same shape. A 1280 × 720 image and a 3840 × 2160 image are both 16:9.
How do you calculate aspect ratio step by step?
- Write down the width and height in pixels.
- Find the largest whole number that divides both of them evenly (the greatest common divisor).
- Divide the width and the height by that number.
- Write the two results as width:height.
Example 1: 1920 × 1080. The largest number dividing both is 120. 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, so the ratio is 16:9.
Example 2: 4000 × 3000. The largest number dividing both is 1000. 4000 ÷ 1000 = 4 and 3000 ÷ 1000 = 3, so the ratio is 4:3.
Example 3: 6000 × 4000. The largest number dividing both is 2000, which gives 3:2.
You can also divide width by height to get a decimal. 1920 ÷ 1080 = 1.7778, which is the same as 16:9 written as about 1.78:1.
Common aspect ratios and where they are used
| Ratio | Decimal | Where it is commonly used | Example pixel size |
|---|---|---|---|
| 1:1 | 1.00 | Square images, often seen on social media | 1080 × 1080 |
| 4:3 | 1.33 | Standard fullscreen TV; the main video format of the 20th century | 1024 × 768 |
| 3:2 | 1.50 | Still photography (35 mm film cameras and DSLRs) | 6000 × 4000 |
| 16:10 (8:5) | 1.60 | Computer displays and tablets | 1600 × 1000 |
| 16:9 | 1.78 | High-definition television | 1920 × 1080 |
The usage column follows the description on Wikipedia's aspect ratio page. Real devices and platforms change over time, so check the current requirement of wherever you will upload or print the image.
How do you resize an image without stretching it?
Keep the ratio fixed. Decide on one new side, then work out the other from the original proportions.
New height = new width × (original height ÷ original width)
| Original | New width | New height |
|---|---|---|
| 4000 × 3000 | 1200 | 1200 × 3000 ÷ 4000 = 900 |
| 1920 × 1080 | 800 | 800 × 1080 ÷ 1920 = 450 |
| 6000 × 4000 | 1000 | 1000 × 4000 ÷ 6000 = 666.67, round to 667 |
When the result is not a whole number, round to the nearest pixel. The ratio then differs from the original by a very small amount.
What if the image and the frame have different ratios?
There are three ways to fit an image into a frame with another ratio. Cropping and adding bars keep the shapes in the picture correct. Stretching does not: the same page notes that stretching a 4:3 image to fill a 16:9 screen makes subjects look short and fat.
| Method | What happens | Example with a 4000 × 3000 (4:3) photo |
|---|---|---|
| Crop | Cut away the extra edges | To 16:9: 4000 × 2250 (750 pixels of height removed). To 1:1: 3000 × 3000 (1000 pixels of width removed). |
| Add bars | Keep the whole image and fill the gap with bars | A 4:3 image on a 1920 × 1080 screen is 1440 × 1080, with bars of 240 pixels on each side (pillarboxing). |
| Stretch | Force the image to fill the frame | Changes the shape of everything in the picture; avoid. |
Cropping removes pixels, so check the size after the crop if you need a minimum resolution, for example for a print.
Common mistakes and a quick checklist
- Resizing width and height separately. Typing two unrelated numbers changes the ratio and distorts the image.
- Forgetting that rounding exists. Round to the nearest whole pixel; images cannot have fractions of a pixel.
- Mixing up width:height order. 16:9 is wide, 9:16 is tall. The first number is always the width.
- Cropping first, checking later. Every crop changes the pixel size.
Before you export:
- Note the original width and height in pixels.
- Work out the ratio, or the target ratio of the frame.
- Change one side only and calculate the other, or crop to the target ratio.
- Round to whole pixels and view the result at 100%.
Frequently asked questions
What is the aspect ratio of 1920 × 1080? 16:9. Both sides divide by 120, giving 16 and 9.
How do I find the aspect ratio of 4000 × 3000? Divide both numbers by 1000 to get 4:3.
What is the difference between aspect ratio and resolution? Resolution is the number of pixels, such as 1920 × 1080. Aspect ratio is only the proportion between width and height, so many different resolutions share one ratio.
How do I resize a 16:9 image to 800 pixels wide? Multiply 800 by 9 ÷ 16, which gives a height of 450 pixels.
Does changing the aspect ratio change the image quality? It changes the shape or the amount of the scene that fits. Cropping also removes pixels, and stretching distorts the picture, so keep the original ratio when you only need a different size.
Check your own numbers
Doing this by hand is fine for one image, but it gets tedious across many sizes. The free Image Aspect Ratio Calculator on BytePriva lets you check ratios and new sizes without working out the divisions yourself. Whichever way you calculate, confirm the final size with the platform or printer that will use the image.
Disclaimer
This guide is general information. Ratio and size requirements differ between devices, platforms and printers and can change, so follow the current specification of the service you are using. Written by the BytePriva Team. Last reviewed: 23 October 2026.