How to Reduce Image File Size

A photo will not attach to an email, a form rejects it, a page loads slowly. The usual reaction is to reach for a quality slider and drag it down until the number is small enough. That works, and it is the wrong first move.

Why resizing wins

File size follows the number of pixels, and pixel count follows the square of the dimensions. Halve the width and the height and you have removed three quarters of the data before any compression is involved.

A current phone photograph is around 4000 pixels wide. In an email, a document or a web page it will be displayed at perhaps 1200. The other 2800 pixels across are carried, transmitted and then thrown away by whatever displays it. Removing them costs nothing visible, because nothing was showing them.

Compression is different: every bit of size it saves is paid for in detail. Use it after the free saving has been taken, not instead of it.

The order that works

1. Decide the display size. Roughly what will this be viewed at? Double it if it must stay sharp on a high-density screen.
2. Resize to that with Resize image, keeping the aspect ratio locked.
3. Compress what is left with Compress image, starting at 80% and moving down only if you must.
4. Check the result at the size it will actually be seen, not zoomed to 100%.

Compressing a 4000-pixel photo straight down to a small file makes it look bad because the encoder has an impossible job. Resize first and the same target size looks clean.

Dimensions worth knowing

• 1200–1600 px long edge — general web use, including high-density displays.
• 1080 × 1080 — square social posts; 1080 × 1350 portrait.
• 1200 × 630 — link preview images.
• Under 500 px — thumbnails and avatars.
• Print — different rules entirely; keep the original.

What the quality number means

JPEG discards detail the eye is poor at noticing. The quality setting decides how aggressively:

• 90–100% — near-archival, little saving.
• 75–85% — the normal choice. Large saving, no visible change on a photograph.
• 60–75% — noticeably smaller, artefacts appearing on flat areas and edges.
• Below 60% — only under a hard size limit.

Guessing is unnecessary: the compressor re-encodes as you move the slider and shows the real file size, not an estimate.

Photographs and screenshots are different problems

Everything above assumes a photograph — gradients, texture, skin, sky. JPEG was built for exactly that.

A screenshot, a logo or a diagram is flat colour and hard edges, which is what JPEG handles worst. It smears the edges, and the file frequently comes out larger than the PNG you started with. Keep those as PNG, or convert to WebP with the image converter — WebP handles both kinds well and is typically 25–35% smaller than the equivalent JPEG or PNG.

Fitting a hard limit

• Email attachments — usually 20–25MB in total, so a couple of resized photos is rarely a problem. It is the ten-photo attachment that fails.
• Web forms — often 2MB or 5MB. A 1600-pixel JPEG at 80% lands well inside that.
• Page performance — aim under 200KB for anything above the fold.

Compress once, from the original

Each lossy pass throws away more detail permanently, and repeated passes accumulate. If a result is too big, go back to the original and redo it at different settings rather than compressing the compressed copy.

One useful side effect: resizing and compressing both redraw the image, so EXIF metadata including GPS does not survive. If that is the point rather than a bonus, Remove EXIF does it directly.

Related guides

• JPEG vs PNG vs WebP — picking the right format first
• Removing EXIF data — metadata and privacy
• Reducing MP3 file size — the same trade-off in audio

Summary

Resize to the size it will be displayed at, then compress at around 80%. Resizing is the free saving and compression is the paid one, so take them in that order. Keep screenshots and logos as PNG or WebP, and always work from the original rather than from an already-compressed copy.