Aspect Ratio Calculator
Find a ratio from two known dimensions, or calculate a missing width or height from any positive ratio. Use the same units on both sides; the tool returns measurements, not an edited file.
Find the ratio
Use pixels, inches or any consistent unit.Ratio preview
Visual guideThe frame updates as you change the dimensions.
Calculate a missing dimension
Choose a target ratio and enter one known dimension. If both are filled, width takes priority.
Common sizes
| Width | Height | Ratio |
|---|
Choose the calculation that matches your input
Know both dimensions? Find the simplified ratio above. Know a ratio and one side? Use “Calculate a missing dimension” to keep that shape at a new size. For example, a 4000 × 3000 image has a 4:3 ratio; a 1200-pixel-wide version is 1200 × 900.
If you only have a file, the image checker reads its dimensions for you. If you need a different shape rather than a different size, use the canvas and crop comparison. Video presets and editing guidance are available in the video dimensions calculator.
Resize a 4:3 image to 1200 pixels wide
Task: prepare a smaller copy of a 4000 × 3000 image without changing its shape. Keep the original file; this calculator supplies dimensions, not an edited image.
- In “Find the ratio”, enter Width 4000 and Height 3000. The result is 4:3.
- In “Calculate a missing dimension”, set Width to 1200, leave Height blank and enter Target ratio 4:3. Select “Calculate dimensions”.
- The matching size is 1200 × 900. In your image editor, enable its linked-proportions or aspect-lock setting and resize to that width and height.
- Export a new copy. Open it in the image checker to confirm 1200 × 900 and 4:3 before delivery.
If it does not fit: a destination requiring 1200 × 630 has a different shape. Do not force only the height to 630; follow the 1200 × 630 crop example or compare a padded canvas. Upscaling an image does not recover missing detail.
The same steps work with another positive ratio and one known side. If only total pixel count is specified, use the megapixel calculator instead.
Aspect ratio formulas
For integer pixel dimensions, simplify width and height by dividing both by their greatest common divisor (GCD).
simplified ratio = (width ÷ GCD) : (height ÷ GCD)1920 and 1080 have a GCD of 120, so 1920:1080 becomes 16:9.
Find a missing dimension
height = width × ratio height ÷ ratio widthwidth = height × ratio width ÷ ratio heightWorked examples
| Known values | Calculation | Result |
|---|---|---|
| 1920 × 1080 | Divide both by 120 | 16:9 |
| 1080 px wide at 4:5 | 1080 × 5 ÷ 4 | 1080 × 1350 |
| 1280 px high at 9:16 | 1280 × 9 ÷ 16 | 720 × 1280 |
| Square, 2048 px wide | 2048 × 1 ÷ 1 | 2048 × 2048 |
Aspect ratio, resolution and image quality
A ratio describes shape, while resolution describes pixel count. A 16:9 file can be 640 × 360 or 3840 × 2160. Both fit the same frame, but the higher resolution contains more detail and usually produces a larger file.
When a formula returns a fraction, most bitmap and video exports need a whole pixel. Round once at the final step. For encoded video, even dimensions are often the safest practical choice because many codecs process pixel blocks.
Explore exact ratio guides
Use the dedicated guides for 1:1, 4:3, 4:5, 16:9, 9:16 and 21:9 to see formulas, examples and ratio-specific caveats.
Frequently asked questions
What is an aspect ratio?
It is the proportional relationship between width and height. It describes a frame's shape independently from its resolution.
How do I calculate 16:9 height?
Multiply width by 9 and divide by 16. A width of 1920 pixels produces a height of 1080 pixels.
Why did the calculator simplify my dimensions?
Dividing both dimensions by their greatest common divisor produces the smallest equivalent whole-number ratio. The original resolution is not changed.
Can two resolutions have the same aspect ratio?
Yes. 1280 × 720, 1920 × 1080 and 3840 × 2160 are all 16:9.
Does this tool resize my file?
No. It calculates ratios and dimensions. Use the numbers in an image or video editor to perform the actual resize or crop.