Image Resize Calculator: Keep the Aspect Ratio
Find the new height or width of an image when you resize it and keep the aspect ratio. See the reduced ratio, scale percent and megapixels before and after. No file needed.
- New size
- 1,280 × 720 px
- Aspect ratio
- 16:9
- Scale
- 66.67%
- Megapixels before
- 2.07 MP
- Megapixels after
- 0.92 MP
- Pixels after
- 921,600
About the Image Resize Calculator
This calculator tells you the other side of an image when you change one side and keep the aspect ratio. Type the original width and height, then type either a new width or a new height. The other side fills in at once, together with the reduced aspect ratio, the scale as a percentage and the megapixel count before and after. It works on numbers only: there is no file to choose, nothing is read from your computer and nothing is sent anywhere.
Keeping the ratio means the width and the height are scaled by the same factor. If the original is and you choose a new width , the new height is:
Choosing a new height works the same way with the roles swapped, . Because pixels are whole, the result is rounded half up, and a side never drops below one pixel. The scale is the new side divided by the old one, for example 1280 / 1920 = 66.67 percent.
How to use it
Steps
- Enter the Original width (px) and Original height (px) of your image. The page opens with 1920 × 1080.
- Type a New width (px) or a New height (px), or press one of the target width buttons such as 1024 or 1920.
- Read the new size, ratio, scale and megapixels in the result grid.
Common ratios
The ratio buttons 1:1, 4:3, 3:2, 16:9, 21:9 and 9:16 set the original height from the original width. Use them when you are planning a layout, for example to find the height of a 16:9 banner that is 1920 pixels wide.
Worked examples
Example 1: Full HD to 1280 wide
1920 × 1080 resized to a width of 1280 gives a height of 1080 × 1280 / 1920 = 720. The scale is 66.67 percent, and the image drops from 2.07 to 0.92 megapixels.
Example 2: An odd ratio
1000 × 667 resized to a width of 500 gives 667 × 500 / 1000 = 333.5, which rounds half up to 334. The ratio does not reduce, so it stays 1000:667.
Example 3: Upscaling
800 × 600 resized to a width of 1600 gives a height of 1200 and a scale of 200 percent. Doubling a side quadruples the pixel count, from 0.48 to 1.92 megapixels.
Why keep the aspect ratio
Scaling the width and height by different amounts stretches or squashes the picture, so faces look wide and circles turn into ovals. Keeping the ratio avoids that. When a design needs a different shape, crop the image first and resize afterwards. The calculator also shows megapixels, a quick guide to file size: halving both sides leaves a quarter of the pixels, so the file is often around a quarter of the size too.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device. The page keeps no history and needs no account.
Related tools
To change a real picture rather than the numbers, use the Image Resizer. The Pixels to PPI Calculator turns a resolution into pixel density, and the Golden Ratio Calculator finds pleasing proportions.
Frequently asked questions
How do I resize an image and keep the aspect ratio?
Multiply the original height by the new width and divide by the original width. For a 1920 × 1080 image resized to 1280 wide, the height is 1080 × 1280 / 1920 = 720. This calculator does the sum for you as you type.
How is the aspect ratio worked out?
Divide both sides by their greatest common divisor. For 1920 × 1080 the divisor is 120, which leaves 16:9. Sizes that share no common factor, such as 1000 × 667, stay as they are and show as 1000:667.
Why is my new height one pixel off?
Pixels are whole numbers, so the height is rounded half up. A 1000 × 667 image resized to 500 wide would be 333.5 tall, which becomes 334. The ratio is kept as closely as whole pixels allow.
Can I type a new height instead of a new width?
Yes. Type in New height and the New width follows the original ratio. The field you edit last is the one the calculator treats as your target, and the other field is worked out from it.
What do the ratio and width buttons do?
The ratio buttons set the original height from the original width, so pressing 4:3 with a width of 1920 gives 1440. The width buttons set the new width to a common value such as 1024 or 1920.
Does this tool resize my picture file?
No. It only works out numbers and never reads or uploads a file. To change the pixels of a real picture, use the Image Resizer, then check the target size here first.
Why do megapixels matter?
Megapixels are width times height divided by one million, a quick guide to file size and print quality. Halving both sides leaves a quarter of the megapixels, so smaller images load faster and use less data.