Compare Two Images Online

Comparing two images sounds simple until you actually try to do it well. Flipping between two browser tabs loses the detail. Stacking screenshots in a slide deck is clumsy. And spotting whether two photos are really the same — after one was resized or re-saved — is almost impossible by eye. This guide covers three precise ways to compare images, and a free tool that does all three right in your browser.

The three kinds of image comparison

There isn’t one “compare two images” task — there are three, and each answers a different question:

1. Before / after — “Show me the change so a person can see it.” A draggable slider that wipes between two images is the most human, shareable way to reveal an edit, a retouch, or a redesign.
2. Difference — “Tell me exactly which pixels changed.” A pixel-level diff paints every changed pixel and reports the percentage that moved, so nothing slips past.
3. Duplicates — “Are these the same image, even after editing?” A perceptual fingerprint groups images that look alike, catching copies that exact file matching would miss.

Pick the mode that matches your question and the rest of the tool gets out of your way.

How to use the before/after slider

A before/after slider overlays one image on top of the other and lets you drag a divider to reveal each side. It’s ideal for editing reveals, photo retouching, restorations, room makeovers, and design iterations.

1. Choose the Before/after mode at the top of the tool.
2. Add two images — drop the “before” into slot A and the “after” into slot B. You can drag and drop, or click to choose a file.
3. Drag the divider left and right to compare. On a phone, the handle is large enough to drag with a finger.
4. Export it. Pick a slider snapshot (a single frozen frame with a divider line) or a side-by-side composite (both images in full), then download a PNG or JPG.

The composite is built on a canvas sized to the larger of the two images, so neither one is cropped even if they have different dimensions.

How to find the difference between two images

An image difference compares two pictures pixel by pixel and shows you precisely where they disagree — the exact counterpart to the eyeball test. It’s the fastest way to catch a small edit, verify an export, or check a screenshot for visual regressions.

1. Switch to the Difference mode and load both images. For an exact diff they should be the same size.
2. Set the threshold. This is the per-channel tolerance before a pixel counts as “changed.” Raise it to ignore the faint noise that JPG compression sprinkles everywhere; lower it to catch the subtlest change.
3. Adjust the background dimming so unchanged areas fade toward grey and the highlighted differences pop.
4. Click Find differences. Changed pixels light up in colour, and the tool reports the percentage of the image that changed.
5. Download the overlay if you want to keep or share the highlighted result.

This mode is also the engine behind a classic spot-the-difference puzzle: load the two near-identical pictures and let the tool find every change for you, instead of hunting by eye.

How to find duplicate images

Exact-match tools only catch files that are byte-for-byte identical, which misses the most common duplicates: the same photo saved at two sizes, exported as both a JPG and a PNG, or re-compressed when it passed through a chat app. The duplicate finder solves this by comparing how images look rather than their raw bytes.

1. Switch to the Duplicates mode and drop a whole set of images — or pick several files at once.
2. Let it fingerprint. Each image is reduced to a small perceptual hash (an average hash plus a difference hash) locally in your browser as it loads.
3. Read the groups. Images whose fingerprints are close are grouped together and badged as duplicates, so the look-alikes are obvious at a glance.
4. Tune the sensitivity. A low tolerance groups only near-identical copies — perfect for cleaning out true duplicates. A higher tolerance also surfaces visually similar shots, like burst photos or lightly edited versions, which helps when you want to pick the best of a set.

Because the perceptual hash is robust to scaling and re-compression, a large JPG and a small PNG of the same photo are still grouped. The tool highlights duplicates so you decide what to keep — it never deletes anything for you.

How perceptual hashing works (in plain terms)

A perceptual hash is a tiny “visual fingerprint.” To build one, the image is shrunk to a few pixels, converted to grayscale, and turned into a short string of bits that captures its overall light-and-dark structure. Two images are compared by counting how many of those bits differ — the Hamming distance. A small distance means the images look alike, even if their files are completely different. This is why a resized or re-saved copy still matches the original: shrinking the image to a handful of pixels throws away exactly the details that compression and resizing change, and keeps the broad structure that makes the picture recognisable.

Tips for accurate comparisons

• Align before you diff. A pixel difference assumes the two images line up. If one is shifted or scaled, the offset itself shows up as a change. Crop or resize them to match first.
• Use the slider for people, the diff for precision. A slider is the clearest way to show a change to someone else; a difference overlay is the clearest way to measure it.
• Match the duplicate tolerance to the job. Stricter to dedupe a library; looser to cluster similar shots.
• Keep originals. When you export a composite, you’re creating a new image — your source files are untouched.

Privacy

The comparison happens in your browser, so the slider snapshots, difference overlays and duplicate fingerprints are all created on your own device. How any data associated with the tool is handled is described in our privacy policy. If you’re working with something highly sensitive, an offline desktop application is always the most private choice.

Ready to compare? Scroll back up, pick a mode, and drop in your images.