Pixel dimensions vs file size

Halving an image's width and height leaves a quarter of the pixels. How pixel dimensions, megapixels and file size relate, and the order that shrinks files most.

Reviewed: 2026-10-06 · Sources checked: 2026-10-06

How much smaller does a file get when I reduce the image size?

Halving both the width and the height keeps only a quarter of the pixels, so the file usually gets much smaller. The exact size still depends on the picture's content, the format and the quality setting, so the only reliable number is the size of a file you actually create. Sajinbaro shows the real size of the file it made, not an estimate.

1. Dimensions and pixel count

Pixel count is width × height. A 4032×3024 phone photo has about 12.2 million pixels (12 MP). Reducing the width while keeping the aspect ratio shrinks both sides, so the pixel count falls with the square of the scale.

Pixel count when reducing the width (example values)
Original (px)	New width	Result (px)	Pixels kept
4032×3024	2016	2016×1512	about 25%
4032×3024	1600	1600×1200	about 15.7%
1920×1080	1280	1280×720	about 44.4%

2. The order that shrinks files most

Resize to the pixel dimensions you actually need first — this usually has the biggest effect.
If the file is still too big, lower the JPEG or WebP quality.
Saving a photo as PNG often makes it larger, because PNG is lossless.
If a form says “under 200 KB”, use a target-size tool that searches for the highest quality that fits.

3. Why the same size can be shown differently

Some programs count 1 KB as 1,000 bytes and others as 1,024 bytes (for example Windows File Explorer). A 200,000-byte file is “200 KB” in one and “195.3 KB” in the other. When a limit matters, compare the exact byte count.

Sources and rules

Calculation: The size examples in this guide are generated by the same code as the tools on this site.
Pixel count: Pixel count = width in px × height in px. Halving both sides leaves one quarter of the pixels.
HTMLCanvasElement.toBlob() (MDN): How a browser re-saves an image with a chosen format and quality (checked 2026-10-06).

Tools for this guide

ResizeChange the pixel width and height of a JPEG, PNG or WebP image while keeping its aspect ratio, and see the real output file size before you download. The image is processed in your browser and never uploaded.
CompressRe-save an image as JPEG or WebP at the quality you choose and see the real output size and how much smaller it got. If the result is larger, you are told. Nothing is uploaded — it all happens in your browser.
Target file sizeGet an image under a size limit such as 100 KB, 200 KB or 500 KB. The highest quality that fits is found automatically, and the image is scaled down only if quality alone is not enough. Processed in your browser, never uploaded.