Aspect Ratio Calculator
Enter a width and height to reduce them to their simplest aspect ratio, such as 1920 × 1080 to 16:9. Switch to Resize to choose a preset or custom ratio, enter a new width or height, and calculate the matching dimension without changing the proportions.
How to Use the Aspect Ratio Calculator
Use Find ratio when you already know the width and height. Enter both dimensions and the calculator reduces them to the simplest width:height ratio, shows the decimal ratio, and identifies the orientation.
Use Resize when you know the ratio you want to preserve. Choose a common ratio or enter your own, select whether you know the new width or height, and enter that dimension. The matching side is calculated automatically.
Width and height must use the same unit when finding a ratio. Pixels are common for digital media, but the same calculation also works with centimetres, inches, millimetres, or another shared unit.
| Dimensions | Ratio | Orientation | Typical use |
|---|---|---|---|
| 1920 × 1080 | 16:9 | Landscape | Widescreen video and displays |
| 3840 × 2160 | 16:9 | Landscape | 4K widescreen video |
| 1080 × 1920 | 9:16 | Portrait | Vertical video |
| 1080 × 1080 | 1:1 | Square | Square images and layouts |
| 1200 × 1500 | 4:5 | Portrait | Portrait images |
| 600 × 400 | 3:2 | Landscape | Photography and prints |
How to Calculate an Aspect Ratio
An aspect ratio describes the proportional relationship between width and height. It is normally written as width:height.
For whole-number dimensions, reduce the two values by their greatest common divisor. The greatest common divisor of 1920 and 1080 is 120, so dividing both dimensions by 120 gives 16:9.
The decimal form comes from dividing width by height. For 1920 × 1080, 1920 ÷ 1080 is about 1.7778, so the same shape can also be described as approximately 1.7778:1.
Resize Without Changing the Aspect Ratio
To preserve an aspect ratio, width and height must change by the same proportion. If the ratio is 16:9 and the new width is 1280 pixels, the matching height is 1280 × 9 ÷ 16 = 720 pixels.
The calculation also works in reverse. If a 16:9 frame must be 720 pixels high, its matching width is 720 × 16 ÷ 9 = 1280 pixels.
Keeping this relationship fixed prevents the dimensions themselves from introducing horizontal or vertical stretching. The calculator only provides the dimensions; it does not modify the source image or video.
Aspect Ratio Versus Resolution
Aspect ratio and resolution describe different things. Aspect ratio describes shape, while resolution gives the actual width and height.
For example, 1280 × 720, 1920 × 1080, and 3840 × 2160 are different resolutions, but all three simplify to the same 16:9 aspect ratio.
Two files can therefore have the same aspect ratio while containing very different numbers of pixels.
Landscape, Portrait, and Square Ratios
A ratio is landscape when its width is greater than its height, portrait when its height is greater than its width, and square when both dimensions are equal.
For example, 16:9 is landscape, 9:16 is portrait, and 1:1 is square. Swapping width and height changes the orientation while reversing the ratio.
Decimal Dimensions and Pixel Rounding
The calculator accepts decimal dimensions as well as whole numbers. A width of 1.5 and height of 1, for example, simplifies to 3:2.
Digital image and video dimensions normally need whole pixels. If Resize produces a fractional pixel value, round to the whole-pixel size required by your workflow. Rounding can make the final ratio differ very slightly from the exact mathematical proportion.
Why an Ultrawide Resolution May Not Be Exactly 21:9
Names such as 21:9 are often used as convenient format labels rather than exact reduced ratios for every display sold under that description.
For example, 2560 × 1080 simplifies exactly to 64:27, not 21:9. When exact proportions matter, calculate the ratio from the actual pixel dimensions instead of relying only on a format label.
Aspect Ratio Formulas
Use the width and height together to find the ratio, or use a known ratio to calculate the missing dimension.
- Width
- Height
- Width divided by height
- Width part of the aspect ratio
- Height part of the aspect ratio
Examples
Find the ratio of 1920 × 1080
Width 1920; height 1080.
16:9, approximately 1.7778:1, landscape.
Find a portrait aspect ratio
Width 1080; height 1920.
9:16, portrait.
Find the height for a 4:5 image
Ratio 4:5; width 1080 px.
Height 1350 px, giving dimensions of 1080 × 1350.
Find the width for a 16:9 frame
Ratio 16:9; height 720 px.
Width 1280 px, giving dimensions of 1280 × 720.
Simplify decimal dimensions
Width 1.5; height 1.
3:2.
Frequently Asked Questions
How do I calculate aspect ratio from width and height?
Reduce the width and height by their greatest common divisor. For example, 1920 and 1080 are both divisible by 120, giving the simplified ratio 16:9.
What aspect ratio is 1920 × 1080?
1920 × 1080 is exactly 16:9. Its decimal width-to-height ratio is approximately 1.7778:1.
What aspect ratio is 1080 × 1920?
1080 × 1920 simplifies to 9:16. It is the portrait orientation of a 16:9 frame.
How do I resize without changing the aspect ratio?
Keep the same width-to-height proportion. Enter the ratio and the new width or height in Resize mode, and the calculator finds the matching dimension.
How do I calculate a missing height from an aspect ratio?
Multiply the known width by the ratio height and divide by the ratio width. For a 16:9 ratio at 1280 pixels wide, 1280 × 9 ÷ 16 gives a height of 720 pixels.
How do I calculate a missing width from an aspect ratio?
Multiply the known height by the ratio width and divide by the ratio height. For a 16:9 ratio at 720 pixels high, 720 × 16 ÷ 9 gives a width of 1280 pixels.
Is aspect ratio the same as resolution?
No. Resolution is the actual width and height, while aspect ratio is the proportion between them. 1280 × 720 and 3840 × 2160 are different resolutions but both have a 16:9 aspect ratio.
Does aspect ratio have a unit?
No, provided width and height use the same unit. The units cancel when the two dimensions are compared, so the same ratio can describe pixels, inches, centimetres, or another shared unit.
Is 2560 × 1080 exactly 21:9?
No. 2560 × 1080 simplifies exactly to 64:27. The term 21:9 is often used as a convenient label for a family of ultrawide displays rather than an exact ratio for every resolution.
What should I do if the calculated size contains a fraction of a pixel?
Round to the whole-pixel dimension required by your image or video workflow. The rounded dimensions may differ very slightly from the exact mathematical ratio.
Can I use decimal dimensions?
Yes. The calculator accepts positive dimensions with up to six decimal places. This is useful when working with physical measurements as well as pixel dimensions.
Are my dimensions uploaded anywhere?
No. The aspect-ratio calculation runs in your browser from the numbers you enter. No image upload is required for this calculator.