Calculator
What the Thumbnail Generator does
Thumbnail Generator: resize to an exact width and height (with or without keeping the aspect ratio), to a longest-side preset, or by a percentage, then export PNG/JPEG/WebP. Everything runs in your browser and the file is never uploaded.
The Thumbnail Generator generates the file you choose without uploading it anywhere. Processing is local, so speed depends on your machine rather than someone else’s server load.
It is mostly used by someone converting between formats. Sending a photo to a server just to resize it is more exposure than the task deserves.
Nothing is transmitted: the file is decoded and re-encoded entirely in the browser.
Further down the page: what each field expects, how the result is derived, and one example worked from the defaults.
What do the Thumbnail Generator fields mean?
The Thumbnail Generator uses 9 inputs. The form opens pre-filled, so the fastest way in is usually to overwrite the one field you actually care about.
| Field | What to enter | Default |
|---|---|---|
| Image file | stays on your device | none |
| Resize by | choose from Width and height (pixels), Longest side (preset) or Percentage of original | Width and height (pixels) |
| Width | measured in px; accepts 1 to 10000 | 800 |
| Height | measured in px; accepts 1 to 10000 | 600 |
| Aspect ratio | choose from Keep ratio: fit inside width x height, Keep ratio: match width, auto height, Keep ratio: match height, auto width or Ignore ratio: stretch to exact size | Keep ratio: fit inside width x height |
| Longest side | choose from 64px, 128px, 256px, 512px, 1024px or 1536px | 512px |
| Scale | measured in %; accepts 1 to 400 | 50 |
| Output format | choose from PNG, JPEG or WebP | PNG |
| Quality % | accepts 10 to 100 | 90 |
How does the Thumbnail Generator work?
Each output is derived from the inputs above in a single pass; there is no hidden state carried between runs, so the same inputs always give the same generated output.
Validation is deliberately strict: silently coercing a bad value is how wrong numbers get trusted.
What a typical run looks like
You provide: Image file, Resize by, Width, Height, Aspect ratio, Longest side, Scale, Output format and Quality %
What happens: the file is read into memory by your browser, processed locally, and offered back as a download. It is never uploaded.
You get back: the result as a file you can save, with a one-line summary beside it.
Nothing queues behind a server, so the wait is your device working, and the size limit is set by memory rather than by an upload cap.
Thumbnails sizes
A thumbnail has one job: stay legible at a fraction of its size. That constraint, rather than the source resolution, should drive every decision.
| Use | Size | Ratio |
|---|---|---|
| YouTube | 1280 × 720 | 16:9 |
| Blog / article card | 1200 × 630 | 1.91:1 |
| Product grid | 800 × 800 | 1:1 |
| Video player preview | 640 × 360 | 16:9 |
| List view icon | 150 × 150 | 1:1 |
Worth knowing
- Design at full size, then check it at 15% zoom. If you cannot tell what it is, neither can anyone scrolling.
- High contrast and a single focal point beat detail. Fine detail disappears entirely at thumbnail scale.
- Three or four words is the practical text limit at typical display sizes.
- Generate thumbnails from the original, not from an already-compressed copy. Recompressing a JPEG degrades it again.
Platforms revise their published specifications from time to time. Where a ratio and a pixel size conflict, match the ratio — every platform downscales an oversized image, but none of them will fix a wrong shape except by cropping it.
Read next: JPEG, PNG, WebP or AVIF: Choosing an Image Format
How to use it
- Choose your file under Image file, or drag it onto the drop area.
- Pick a value for Resize by: the options are Width and height (pixels), Longest side (preset) or Percentage of original.
- Fill in Width in px (1 to 10000).
- Type Height in px (1 to 10000).
- Pick a value for Aspect ratio: the options are Keep ratio: fit inside width x height, Keep ratio: match width, auto height, Keep ratio: match height, auto width or Ignore ratio: stretch to exact size.
- Pick a value for Longest side: the options are 64px, 128px, 256px, 512px, 1024px or 1536px.
- Put Scale in % (1 to 400).
- Pick a value for Output format: the options are PNG, JPEG or WebP.
- Fill in Quality % (10 to 100).
- Press Resize.
- The results panel then shows Status.
- Both Copy Link and Print capture the current state, so pick whichever suits how you file things.
What this tool does not do
- EXIF metadata may be dropped during processing, which is usually desirable but worth knowing.
- Re-encoding a lossy format such as JPEG always loses some detail, even at maximum quality.
- Assumptions built into the method are stated above, but they are still assumptions.
- Check that each figure is in the unit the label asks for; that is the most common source of a wrong result.
- Because everything runs locally, processing speed depends on your device rather than our servers.
Frequently asked questions
Enter Width in px, Height in px and Scale in %. Units are printed on the form itself. Match them and no manual conversion is needed.
Width accepts 1 to 10000 px. Values outside that range are rejected with a message naming the field, rather than producing something that looks valid but is not.
Your file stays on your machine. Nothing is transmitted, stored or queued for processing elsewhere.
It costs nothing, and there is no limit on repeat use.
It matches what you would get working it out by hand, with fewer opportunities to slip.