Image resize calculator
Work out what dimensions you will actually get, what the ratio becomes, and whether the image is being cropped or enlarged — before you process anything.
The only formula you need
Resizing proportionally is one division and one multiplication. Divide the new width by the old width to get a scale factor, then multiply the height by the same factor.
new height = old height × (new width ÷ old width)
A 4000 × 3000 photograph resized to 1600 wide has a scale factor of 0.4, so the height becomes 1200. Every proportional resize is that calculation, whichever edge you start from.
Why pixel count falls faster than you expect
Halving both dimensions leaves a quarter of the pixels, not half, because area is the product of two edges. This is the single most useful fact about image optimisation: cutting an oversized image down to its display size typically removes 70 to 90 percent of the file weight before you touch a quality setting.
When the ratio changes
If the target ratio differs from the source, no resize can satisfy both edges without a compromise. The calculator flags this, because it is the point where a batch quietly goes wrong — a set of portraits centre-cropped to square loses heads, and nobody notices until the grid is published.
Fill crops to the box and lets you choose what survives with a focal point. Pad keeps the whole image and fills the remainder with a background colour. Stretch distorts and is almost never what you want for photographs.
Enlarging
Scaling above 100 percent has to invent information. Interpolation does this smoothly, which is why enlarged images look soft rather than blocky, but the detail is not there to recover. If a print or a banner needs more pixels than you have, printing smaller or reshooting are the honest options.
Questions
- How do I keep the aspect ratio when resizing?
- Set only one dimension and let the other follow. Divide the new width by the old width to get the scale factor, then multiply the height by it. A 4000 × 3000 image resized to 1600 wide becomes 1600 × 1200.
- What happens if I set both width and height?
- Unless the target matches the source ratio exactly, something has to give. Fit keeps the whole image and lands inside the box; Fill covers the box and crops the overflow; Pad adds background; Stretch distorts.
- Is it bad to enlarge an image?
- It cannot add detail that was never captured, so enlarged images look soft. Modern upscalers invent plausible detail, which works reasonably for texture and poorly for faces and text. Printing smaller is usually the better answer.
- Why does halving the dimensions remove three quarters of the pixels?
- Pixel count is width multiplied by height, so both edges halving means a quarter of the area remains. That quadratic relationship is why resizing reduces file size so much more effectively than compression does.