A resolution is a fraction: 1920×1080 simplifies by dividing both numbers by their greatest common divisor, 120, and gives 16:9. The same idea finds a missing height, compares two screens, and shows why 2560×1080 is sold as “21:9” without being 21:9.
| Resolution | GCD | Ratio | Width ÷ height |
|---|---|---|---|
| 1920×1080 | 120 | 16:9 | ≈ 1.7778 |
| 3840×2160 | 240 | 16:9 | ≈ 1.7778 |
| 1024×768 | 256 | 4:3 | ≈ 1.3333 |
| 1920×1200 | 240 | 8:5 | 1.6 |
| 1366×768 | 2 | 683:384 | ≈ 1.7786 |
| 2560×1080 | 40 | 64:27 | ≈ 2.3704 |
| 3440×1440 | 80 | 43:18 | ≈ 2.3889 |
| 1080×1920 | 120 | 9:16 | 0.5625 |
Two resolutions have the same shape when their lowest-terms ratios match. The quotient column lets you sort them from squarest to widest, and the ≈ sign means the decimal is rounded to four places. The vertical video in the last row is the same 16:9 turned on its side, which is why its ratio is the reverse.
How to get from a resolution to its ratio
Worked example: the ratio of 1920×1080
A monitor is 1920 pixels wide and 1080 pixels tall. What is its aspect ratio?
- Write the width first. The ratio is 1920:1080.
- Find the greatest common divisor. Euclid: 1920 = 1 × 1080 + 840, 1080 = 1 × 840 + 240, 840 = 3 × 240 + 120, and 240 = 2 × 120. The last nonzero remainder is 120.
- Divide both terms by 120. 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9.
Result: 16:9.
Check: 16 × 120 = 1920 and 9 × 120 = 1080, and 16 and 9 share no factor.
This is the same operation as simplifying a fraction, and the simplify ratios tool does it with integers, decimals, or fractions. To see how the divisor is found, the greatest common divisor page lists the Euclidean algorithm, and the guide on GCD and LCM explains why the divisor, not the multiple, is what you need here.
How to find the missing width or height
Once you know the ratio, one side determines the other through a proportion. For a 16:9 image that is 1600 px wide, 1600/h = 16/9. Cross-multiplying, 16 × h = 1600 × 9 = 14,400, so h = 900 px. That is the same as multiplying the width by 9/16.
Going the other way, multiply the height by 16/9. A 16:9 frame that is 540 px tall is 540 × 16/9 = 960 px wide. For a 4:3 thumbnail that is 480 px wide, the height is 480 × 3/4 = 360 px. If the result is not a whole number, round to the nearest pixel and keep in mind that the shape changes slightly.
The proportions calculator solves for the missing term, and ratio to fraction turns the width:height ratio into the fraction width/height when you need to calculate with it.
A 16:9 image on a 4:3 screen
If you fit the image to the width of a 4:3 screen, two bars are left, one above and one below. With width W, the screen is 3/4 of W tall and the image is 9/16 of W. The fraction of the height used is (9/16) ÷ (3/4) = 3/4.
The image covers three quarters of the height, and the remaining quarter is split into two bars of 1/8 each. With numbers: on a 1600×1200 screen, a 1600×900 image leaves (1200 − 900) ÷ 2 = 150 px above and another 150 px below.
The opposite case gives the same fraction. A 4:3 video on a 16:9 screen takes up 3/4 of the width, and the side bars add up to the remaining quarter. On a 1920×1080 screen, the video is 1440×1080 and leaves 240 px on each side. Cropping avoids the bars, at the cost of losing part of the picture.
Why 2560×1080 is not exactly 21:9
Spanish Wikipedia notes that the 64:27 format is sold under the name 21:9. Other “ultrawide” screens have ratios of their own: 3440×1440 is 43:18 and 3840×1600 is 12:5, exactly 2.4. Before comparing two devices, simplify their resolutions instead of trusting the label.
What the ratio does not tell you
The ratio describes shape, not size: 1280×720 and 3840×2160 are both 16:9, yet one has nine times as many pixels as the other. Nor is everything that looks like 16:9 actually 16:9. 1366×768 gives 683:384, and its cross products give it away: 1366 × 9 = 12,294 versus 768 × 16 = 12,288. The guide on fractions and scales applies the same ratio logic to plans and maps, and the one on ratio, proportion, and fraction sorts out the three ideas.
Practice with three resolutions
- Simplify 1440×1080.
- Find the height of an 8:5 image that is 1280 px wide.
- Is 2048×1080 a 16:9 resolution?
Show the answers
- The GCD is 360 and the ratio is 4:3.
- 1280 × 5/8 = 800 px.
- No. The GCD is 8 and the ratio is 256:135, because 2048 × 9 = 18,432 and 1080 × 16 = 17,280.
There are more everyday uses of ratios and fractions on the blog.
Sources
- Spanish Wikipedia, Relación de aspecto, in Spanish: defines aspect ratio as the proportion between width and height, and places 4:3 as the traditional television standard and 16:9 as the standard for high definition and computers (accessed October 17, 2026).