Aspect ratio is the proportion of width to height, written as width:height in lowest terms. To calculate it, divide both dimensions by their greatest common divisor (GCD). A 1920 × 1080 screen has a GCD of 120, so its aspect ratio is 1920 ÷ 120 : 1080 ÷ 120 = 16:9. As a decimal, that is 1920 ÷ 1080 ≈ 1.78:1.
The formula
This is the same as simplifying a fraction. If the GCD is hard to see, Euclid’s algorithm finds it quickly; see how to find the GCF and LCM, or use the GCF calculator.
GCD(1920, 1080): 1920 = 1 × 1080 + 840; 1080 = 1 × 840 + 240; 840 = 3 × 240 + 120; 240 = 2 × 120 + 0 → 120
1920 ÷ 120 = 16 · 1080 ÷ 120 = 9 → 16:9
Common resolutions and their ratios
| Resolution | GCD | Exact ratio | Decimal | Usually called |
|---|---|---|---|---|
| 1920 × 1080 | 120 | 16:9 | 1.78 | Full HD, 16:9 |
| 3840 × 2160 | 240 | 16:9 | 1.78 | 4K UHD |
| 1440 × 900 | 180 | 8:5 | 1.60 | 16:10 |
| 2560 × 1600 | 320 | 8:5 | 1.60 | 16:10 |
| 1280 × 1024 | 256 | 5:4 | 1.25 | 5:4 |
| 4032 × 3024 | 1008 | 4:3 | 1.33 | 4:3 photo |
| 6000 × 4000 | 2000 | 3:2 | 1.50 | 3:2 camera sensor |
| 2560 × 1080 | 40 | 64:27 | 2.37 | “21:9” ultrawide |
| 3440 × 1440 | 80 | 43:18 | 2.39 | “21:9” ultrawide |
| 1080 × 1350 | 270 | 4:5 | 0.80 | Portrait feed post |
| 1080 × 1920 | 120 | 9:16 | 0.56 | Vertical video |
Note that 16:10 reduces to 8:5. It is conventionally written 16:10 to make comparison with 16:9 easy. The aspect ratio calculator reduces any pair of dimensions and solves for a missing one.
Resizing without distortion
To keep an image from stretching, scale width and height by the same factor.
A 1920 × 1080 image resized to 1280 px wide
New height = 1280 × 1080 ÷ 1920 = 720 px → 1280 × 720, still 16:9
A 1080 × 1350 portrait image needed 800 px wide
New height = 800 × 1350 ÷ 1080 = 1000 px
If the result is not a whole number, round to the nearest pixel; the distortion from one pixel of rounding is invisible. The image resize calculator also scales by percentage.
Cropping to a new aspect ratio
Changing the ratio without distortion means cutting something off. Keep the dimension that already fits and shorten the other:
Crop a 6000 × 4500 photo (4:3) to 16:9 for a slideshow
Keep the full width: new height = 6000 × 9 ÷ 16 = 3375 px
Result: 6000 × 3375, trimming 1,125 px from the top and bottom combined
If the target is taller than the source, keep the height and trim the width instead. Going from 16:9 to a vertical 9:16 discards about two-thirds of the frame, which is why vertical video is usually shot vertically rather than cropped later.
Aspect ratios of print sizes
Standard photo print sizes have their own ratios, and most do not match a camera’s 3:2 or 4:3 frame.
| Print size (inches) | Ratio | Matches |
|---|---|---|
| 4 × 6 | 2:3 | 3:2 camera sensors |
| 5 × 7 | 5:7 | Nothing common; always cropped |
| 8 × 10 | 4:5 | Nothing common; always cropped |
| 8.5 × 11 (letter) | 17:22 | Close to 4:3 but not exact |
| 11 × 14 | 11:14 | Close to 4:5 |
Printing the 6000 × 4000 (3:2) photo as a landscape 10 × 8 print means a 5:4 crop: keep the full 4000-pixel height and narrow the width to 4000 × 5 ÷ 4 = 5000 pixels, trimming 1,000 pixels from the sides. Framing the crop yourself before ordering avoids losing heads and feet to the lab’s automatic center crop.
Screen size from diagonal and ratio
Screens are sold by their diagonal. With the ratio w:h and diagonal d, the Pythagorean theorem gives the width and height:
27-inch 16:9 monitor: √(16² + 9²) = √337 ≈ 18.36
Width = 27 × 16 ÷ 18.36 ≈ 23.5 in · Height = 27 × 9 ÷ 18.36 ≈ 13.2 in
That is why a 27-inch ultrawide is much shorter than a 27-inch 16:9 screen; the same diagonal is spread over a wider rectangle. The screen size calculator compares screens, the PPI calculator adds pixel density, and how to use the Pythagorean theorem explains the geometry.
Letterboxing and pillarboxing
When content and screen have different ratios, something gives: bars appear, or the picture is cropped.
Letterboxing (bars top and bottom): a 2.39:1 film on a 16:9 TV fills (16 ÷ 9) ÷ 2.39 ≈ 74% of the screen height. On a 1080-pixel-tall screen, the picture is about 803 px tall with bars of about 138 px above and below.
Pillarboxing (bars at the sides): 4:3 video on a 16:9 screen fills (4 ÷ 3) ÷ (16 ÷ 9) = 75% of the width, leaving 240-pixel bars on each side of a 1920-pixel screen.
| Content | On a 16:9 screen |
|---|---|
| 4:3 (old TV, many photos) | Bars on the sides |
| 16:9 (HDTV, streaming) | Fills the screen |
| 1.85:1 (many films) | Thin bars top and bottom |
| 2.39:1 (widescreen films) | Thick bars top and bottom |
Common mistakes
- Not reducing fully. 1440:900 is correct but unhelpful; 8:5 is the reduced form.
- Changing one dimension only, which stretches or squashes the image.
- Trusting marketing labels, such as “21:9” for monitors that are really 64:27 or 43:18.
- Confusing ratio with resolution. A 16:9 screen can be 1280 × 720 or 3840 × 2160; ratio says nothing about sharpness.
Aspect ratios are simply ratios, so the general techniques in the ratio simplifier apply to any of these numbers.
Frequently asked questions
How do you calculate aspect ratio?
Divide the width and height by their greatest common divisor. For 1920 × 1080, the GCD is 120, so the ratio is 1920 ÷ 120 : 1080 ÷ 120 = 16:9. You can also express it as a decimal: 1920 ÷ 1080 ≈ 1.78.
How do I resize an image without changing the aspect ratio?
Scale both dimensions by the same factor. If you know the new width, the new height is new width × original height ÷ original width. A 1920 × 1080 image resized to 1280 pixels wide becomes 1280 × 1080 ÷ 1920 = 720 pixels tall.
Is 21:9 really 21:9?
Not exactly. The term is a marketing label. Common ultrawide resolutions are 2560 × 1080, which reduces to 64:27 (about 2.37:1), and 3440 × 1440, which reduces to 43:18 (about 2.39:1). Both are close to 21:9, which is 2.33:1.
What is the difference between 16:9 and 16:10?
16:10 (1.6:1) is slightly taller than 16:9 (1.78:1). A 16:10 monitor at 1920 × 1200 has 120 more rows of pixels than a 16:9 monitor at 1920 × 1080, which is useful for documents and code.
What aspect ratio is a phone photo?
Most phone cameras capture 4:3 by default, such as 4032 × 3024 pixels, while their screens are much taller, around 19.5:9 or 20:9. Vertical video for social apps is usually 9:16.