Knowledge · Images and files
What is an aspect ratio?
Short answer
An aspect ratio is the proportion between an image's width and its height, written as two numbers such as 16:9 or 4:3. It describes shape, not size: 1920 × 1080 and 1280 × 720 are both 16:9. To keep the ratio when resizing, compute the new height as new width × height ÷ width; for 16:9 at 1200 pixels wide, that is 675 pixels high.
Open Aspect ratioCommon aspect ratios and where they are used
| Ratio | Decimal | Example size (px) | Typical use |
|---|---|---|---|
| 1:1 | 1.00 | 1080 × 1080 | Square social posts, profile pictures |
| 4:5 | 0.80 | 1080 × 1350 | Portrait social feed posts |
| 4:3 | 1.33 | 1600 × 1200 | Phone camera photos, older TVs, tablets |
| 3:2 | 1.50 | 6000 × 4000 | DSLR and mirrorless photos, 4 × 6 prints |
| 16:9 | 1.78 | 1920 × 1080 | HD and 4K video, monitors, presentations |
| 9:16 | 0.56 | 1080 × 1920 | Vertical video, stories, short-form clips |
| 21:9 (marketed) | about 2.37 | 2560 × 1080 | Ultrawide monitors |
| 2.39:1 | 2.39 | 4096 × 1716 | Widescreen cinema |
| 1:√2 | 0.71 | 2480 × 3508 | ISO paper sizes such as A4 |
The 16:9 ratio became the HDTV standard through ITU-R BT.709, which defines 1920 × 1080 and 1280 × 720 pictures. 3:2 comes from the 36 × 24 mm frame of 35 mm film.
How to find the ratio of any image
Divide width and height by their greatest common divisor (GCD).
- Take the pixel size, for example 1920 × 1080.
- Find the largest number that divides both: 120.
- Divide: 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, so the ratio is 16:9.
Some sizes only approximate a named ratio. A 1366 × 768 laptop screen reduces to 683:384, which is 1.779 and is sold as 16:9. A 2560 × 1080 "21:9" monitor is exactly 64:27. Comparing decimals (width ÷ height) is often easier than comparing fractions.
Calculating a new size without distortion
Keep the ratio by scaling both sides by the same factor. Two formulas cover every case:
new height = new width × original height ÷ original widthnew width = new height × original width ÷ original height
Example: a 4032 × 3024 photo (4:3) resized to 1600 pixels wide becomes 1600 × 3024 ÷ 4032 = 1200 pixels high. For a 9:16 video at 720 pixels wide, the height is 720 × 16 ÷ 9 = 1280.
When the result is not a whole number, round to the nearest pixel. Video encoders usually require even dimensions, so round to an even number for video.
Changing the ratio: crop, pad or stretch
Turning a 4:3 photo into a 16:9 frame or a 1:1 post cannot be done by resizing alone. There are three choices:
| Method | What happens | When to use it |
|---|---|---|
| Crop | Cuts away part of the image | Most photos; keep the subject inside the frame |
| Pad (letterbox or pillarbox) | Adds bars or a blurred background | When nothing may be cut, such as documents or art |
| Stretch | Scales width and height unequally | Almost never; faces and circles look distorted |
Cropping a 4000 × 3000 image to 16:9 keeps the full width and cuts the height to 4000 × 9 ÷ 16 = 2250 pixels, discarding 25% of the picture. Cropping the same image to 1:1 keeps 3000 × 3000.
Aspect ratio on the web
CSS has an aspect-ratio property, for example aspect-ratio: 16 / 9, which reserves the right amount of space before an image or video loads. Setting it, or giving images width and height attributes, prevents the page from jumping as media arrives, which improves the Cumulative Layout Shift score.
FAQ
What aspect ratio is 1920 × 1080?
16:9. Both numbers divide by 120, giving 16 and 9.
Is 4K 16:9?
Consumer 4K UHD is 3840 × 2160, exactly 16:9. Cinema 4K (DCI) is 4096 × 2160, about 1.90:1.
What is the aspect ratio of a phone photo?
Most phone cameras shoot 4:3 by default, such as 4032 × 3024. Many also offer 16:9 and 1:1 modes, which crop the sensor image.
Does changing the aspect ratio reduce quality?
Cropping removes pixels but does not blur the rest. Stretching distorts shapes, and enlarging after a heavy crop can make the image look soft.