About the Image Resize Calculator
Almost every image that ends up on a website, in an email, on a slide or in a print layout has been resized from the size the camera or design tool produced. A modern phone takes photos of 12 megapixels or more, far larger than most screens can show at once, so images are routinely scaled down to a set width, a percentage of their size or a box they must fit inside. Doing that by hand means keeping the proportions right, or the picture ends up stretched or squashed.
This image resize calculator works out the new pixel dimensions of an image while keeping its aspect ratio. You can resize by a percentage, to a width, to a height or to fit inside a box. Alongside the new size it shows the scale used, the megapixels before and after, the uncompressed memory the image needs, and a note on quality — whether you are reducing, which keeps an image sharp, or enlarging, which cannot add detail.
How to Use the Image Resize Calculator
Choose how you want to resize: by percentage, to a width, to a height or fit in a box.
Enter the original width and original height of the image in pixels. Most image viewers and file properties show these.
Then enter the target: the percentage, the new width, the new height, or the box width and box height the image must fit inside.
The calculator gives the new size rounded to whole pixels, since images cannot have fractions of a pixel.
How Resizing Is Worked Out
by percentage: scale = percent ÷ 100
to a width: scale = new width ÷ original width
to a height: scale = new height ÷ original height
fit in a box: scale = the smaller of (box width ÷ width) and (box height ÷ height)
new width = original width × scale
new height = original height × scaleUsing one scale for both sides is what keeps the proportions. If the width and height were scaled by different amounts, circles would become ovals and faces would look stretched.
Step-by-Step Example
A 4000 × 3000 pixel photo resized to 50 percent.
Scale: 50% = 0.5
New width: 4000 × 0.5 = 2,000 pixels
New height: 3000 × 0.5 = 1,500 pixels
Megapixels: 12 MP → 3 MP
Memory: 36 MB → 9 MB uncompressed at 24-bit colourHalving both sides leaves a quarter of the pixels, which is why the megapixels fall from 12 to 3 rather than to 6.
Resizing the same photo to a width of 1200 pixels gives a scale of 1200 ÷ 4000 = 0.3, so the new size is 1,200 × 900 pixels, still 4:3. Resizing it to a height of 900 pixels gives exactly the same result.
Fitting an Image Inside a Box
Fitting is common when an image must fill a screen, a slot on a web page or a frame without being cropped. Work out the scale for each side and take the smaller one, so neither side spills over.
The 4000 × 3000 photo fitted inside 1920 × 1080.
Width scale: 1920 ÷ 4000 = 0.48
Height scale: 1080 ÷ 3000 = 0.36
Use 0.36: 4000 × 0.36 = 1,440; 3000 × 0.36 = 1,080The result is 1,440 × 1,080 pixels. The height fills the box exactly, and there is space left at the sides because a 4:3 photo is narrower than a 16:9 box. To fill the box completely instead, you would use the larger scale and crop the overflow.
Reducing and Enlarging
Reducing an image throws away pixels, but the remaining ones still carry real detail, so a reduced image looks sharp. After a large reduction, a small amount of sharpening can bring back crispness lost in the averaging.
Enlarging is different. A 1000 × 800 image at 200 percent becomes 2,000 × 1,600 pixels, but the extra pixels are invented by blending the ones around them. The image has the same detail as before spread over a larger area, so it looks softer the more it is scaled up. Modern upscaling tools can guess plausible detail, but they cannot recover what the camera never captured.
Megapixels and Memory
Megapixels are width × height ÷ 1,000,000. A 4000 × 3000 photo is 12 megapixels. Uncompressed, each pixel at 24-bit colour takes 3 bytes, one each for red, green and blue, so the image needs about 36 MB of memory while it is open in an editor.
Saved files are much smaller, because formats such as JPEG, PNG and WebP compress the data. A 12 megapixel JPEG is usually 3 to 6 MB, and a web-sized copy of 1,200 pixels wide might be 150 to 400 KB depending on the quality setting and the content.
Sizes for Websites and Screens
For the web, an image rarely needs to be wider than the space it is shown in, allowing up to twice that width for high-density screens such as phones and modern laptops. A full-width banner might be 1,920 to 2,400 pixels wide, an image in a blog column 800 to 1,200 pixels, and a thumbnail 300 to 400 pixels. Serving images far larger than they appear slows pages down without making them look any better.
Understanding Your Result
The headline is the new size in pixels, width first, rounded to whole pixels.
The scale shows the percentage of the original and whether the image is being reduced or enlarged.
The megapixels compare the pixel count before and after.
The memory line shows the uncompressed size at 24-bit colour, useful for understanding how much work an editor or device has to do.
The quality note warns when enlarging will soften the image.
When Should You Use This Calculator?
Use it when preparing photos for a website, blog or online shop, so they load quickly and look right.
Use it to fit images to a screen, slideshow or digital frame without cropping.
Use it when a form or platform asks for images of a maximum width or height.
Use it to check how much an image will shrink before exporting a batch of files.
Common Mistakes
Changing width and height separately. Scaling only one side, or both by different amounts, distorts the image. Keep the aspect ratio locked.
Enlarging a small copy. Always resize from the original, full-size file rather than scaling up a thumbnail or a previous export.
Forgetting high-density screens. An image exactly as wide as its slot can look soft on phones and high-resolution laptops. Allow up to twice the display width.
Confusing pixels with file size. Halving the width and height quarters the pixels, but the saved file size also depends on the format and quality setting.
Expecting the box to be filled. Fitting keeps the whole image visible, so a photo of a different shape leaves space on two sides.