How to compress images without visible quality loss

A photo straight from a phone is often 3–5 MB. That is too big for many upload forms, slow on a website and wasteful in an email. The good news is that most of that size is detail your eyes cannot see at normal viewing sizes.

How lossy compression works

JPEG and WebP are lossy formats: when they save an image they throw away the detail human vision is least sensitive to, mostly subtle variation in colour. That is why a photo can lose 70% of its file size and look essentially the same.

PNG is the opposite. It is lossless: every pixel is stored exactly, which is why it is the right choice for screenshots and logos and why its files are large. There is no quality setting to turn down on a PNG, which matters later in this guide.

Compress an image step by step

1. Add one or more images to Compress Image.
2. Drag the quality slider — the estimated size updates as you move it.
3. Compare the original and compressed previews side by side, zoomed in.
4. Download each image, or all of them at once as a ZIP.

A quality of 75–85 is the sweet spot for photographs. Below about 60, blocky artefacts start to appear around sharp edges and in smooth areas like sky.

Resize first — it saves the most

Pixel dimensions matter more than quality. A 4,000-pixel-wide photo shown 1,200 pixels wide on a website carries more than ten times the data it needs. If you know where the image will be used, set the width in Resize Image (or in the compressor's resize option) before worrying about quality.

Some sensible widths to aim for:

• A full-width picture on a web page: 1,200–1,600 pixels.
• A thumbnail or product tile: rarely more than 400 pixels.
• A photo in an email or a document: about 1,200–1,600 pixels is plenty for reading on a screen.
• A print: work from the paper size instead — about 300 pixels for every inch, so 1,800 × 1,200 pixels for a 6 × 4 inch print.

A worked example

Take a 12-megapixel phone photo: 4,000 × 3,000 pixels and about 4 MB. You want to put it on a web page where it will be shown 1,600 pixels wide.

1. Resize to 1,600 pixels wide. The height follows to 1,200. The picture now has 1.92 million pixels instead of 12 million — 16% of the original. File size scales roughly with pixel count, so most of the saving is already made.
2. Set quality to about 80. This removes detail the eye does not notice. A photograph of this size typically lands at a few hundred kilobytes.
3. Compare at full zoom. Look at hair, text on signs and the edges of objects against sky. If it looks the same, you are done.

The result is usually more than ten times smaller than the original and indistinguishable on the page.

Pick the right format

• Photos: JPEG or WebP. WebP is usually smaller at the same quality.
• Screenshots, logos, text and sharp lines: PNG, or WebP. JPEG smears sharp edges and often makes screenshots bigger, not smaller.
• PNG files that are too big: quality settings do not apply to PNG, because it is lossless. Resize it, or convert it with PNG to WebP.
• Anything with a transparent background: PNG or WebP. JPEG has no transparency, so converting a cut-out logo to JPEG fills the clear areas with a solid colour.

WebP is displayed by every current browser, so for a website it is the safe default. JPEG is still the format to choose when a file must open anywhere — an old program, a printer kiosk, an upload form that lists only JPG and PNG. The Image Converter switches a whole batch between the three formats in one pass.

How to get under a specific limit

Upload forms often say something like “maximum 200 KB” or “no larger than 2 MB”. Work towards the number in this order:

1. Resize to the largest dimensions the form actually needs, if it states any.
2. Lower the quality in steps of five, watching the estimated size, until it is under the limit.
3. If you reach about 60 and are still over, stop lowering quality and resize smaller instead. A smaller sharp image looks better than a larger blocky one.

How to spot over-compression

Zoom in to 100% and look for three things:

• Blocks — faint squares in smooth areas such as sky, skin or a plain wall.
• Halos — a smudgy fringe around text and other hard edges.
• Banding — a gradient, like a sunset, breaking into visible stripes.

If you can see any of them at the size the image will be viewed, raise the quality a step.

Keep your originals

Lossy compression is one-way, and it adds up. Every time a JPEG is opened, edited and saved again, a little more detail is thrown away. Keep the original photo somewhere safe, make compressed copies from it for each use, and never compress a copy that has already been compressed.

How much smaller will it get?

Typically 50–80% for photographs at quality 80. A detailed landscape compresses less than a portrait against a plain background. The compressor shows the real before-and-after size, so you can decide with numbers rather than guesses — and the images are processed on your own device, not uploaded. Images up to 50 MB each are accepted; on a phone, work through very large ones in smaller batches.