What the extra 10 KB buys over 30 KB
40 KB is a third more room than 30 KB. You can spend it on higher quality at the same pixel size, or on more pixels at about the same quality. Here's the difference for a JPG photo at common sizes, from a busier picture (the lower figure) to a plain-background one (the higher figure), based on our own test encodes:
| Pixel size | Quality at 30 KB | Quality at 40 KB |
|---|---|---|
| 413 × 531 px | About 75–90% | About 85–92% |
| 480 × 640 px | About 60–85% | About 75–90% |
| 600 × 600 px | About 45–80% | About 65–87% |
| 800 × 600 px | About 35–70% | About 50–80% |
| 600 × 800 px | About 35–75% | About 55–85% |
At around 75% and above, a photo looks like the original at normal viewing size; below about 50%, smudged textures and faint halos around edges start to appear. That makes 40 KB the point where a 480 × 640 px portrait or a 600 × 600 px square becomes comfortable, while 600 × 800 px is still a compromise.
Good uses for a 40 KB image
40 KB is less common on forms than 20, 30 or 50 KB, but it does turn up, and it's a sensible target for your own uploads too:
- Application photos on portals that allow a larger image than the usual 20–30 KB, sometimes as a range with a minimum.
- Square profile pictures for staff directories, alumni sites and learning platforms, where 600 × 600 px is plenty.
- Card-sized scans such as the front of an ID card, a fee receipt or an admit card, when only a small file is allowed.
- Listing and email images, such as a landscape product or property photo that should load quickly.
Where a form sets the rule, read its instructions for pixel dimensions, file type and any minimum as well as the 40 KB maximum.
Card-sized scans and documents at 40 KB
Text behaves differently from faces. Crisp black-on-white edges are expensive for JPG, so a document needs more data per pixel than a photo to stay readable.
- A card-sized scan (an ID card, a receipt slip, an admit card) usually stays readable at 40 KB at about 600–800 px wide. Crop away the table or scanner lid around it first, because every pixel of background costs data.
- Part of a page, such as a single paragraph or a stamp and signature block, fits easily once it's cropped.
- A full A4 page of printed text is a poor fit. To reach 40 KB it has to shrink to roughly 500 px wide, where small print gets hard to read. Compress image to 100KB or Compress PDF are better choices if the site allows them.
Whatever you upload, open the result and zoom in on the smallest text. ID numbers, dates and names are what the person checking your application needs to read.
Choosing the output format
The Output format setting matters more at small sizes than most people expect. Here's what each choice does:
- Same keeps JPG as JPG, PNG as PNG and WebP as WebP. iPhone HEIC photos become JPG, and GIF or BMP files become PNG.
- JPG is right for photos and for almost every form, since it's the most widely accepted format. Transparent areas are filled with white.
- PNG suits logos, screenshots and simple graphics. To shrink it, the tool reduces the number of colours, down to as few as 16, before it touches the dimensions, and transparency is kept. A photo saved as PNG looks far worse at 40 KB than the same photo as JPG.
- WebP gets a little more quality out of each KB for websites, but many upload portals don't accept it. Some browsers, including Safari, can't create WebP files; the tool then uses JPG and tells you so.
If a portal asks for JPEG and you have a PNG or a screenshot, choosing JPG here converts and compresses in one step. The PNG to JPG converter does the conversion alone.
Worked example: a 600 × 600 px profile photo
Say a staff portal wants a square profile photo of 600 × 600 px, no larger than 40 KB, and your photo is a 4 MB portrait from your phone.
- In the Image Resizer, enter 600 × 600 px and choose Crop to fill exact size. The crop is taken from the centre, trimming the top and bottom of the portrait, so keep your face near the middle of the frame.
- Add the square photo here. The 40 KB target is already set.
- The result line should keep 600 × 600 px and show a quality somewhere around 65–87%, depending on how busy the background is.
- If it says the dimensions were reduced instead, the photo didn't fit even at 35% quality. Low-light grain or a cluttered background is the usual cause: retake it against a plain wall in daylight rather than giving up the 600 px the portal asked for.
Prefer one step? The Image Resizer's own Limit file size option keeps exactly 600 × 600 px and lowers the quality as far as it needs to, so the dimensions can never change.
40 KB, or 30 KB or 50 KB?
- 30 KB is enough for a passport-size photo up to 413 × 531 px. If the form caps you at 30, don't fight it with a big image: use compress image to 30KB.
- 40 KB suits 480 × 640 px portraits, 600 × 600 px squares and cropped card scans.
- 50 KB adds another quarter, which helps 600 × 800 px photos and busier pictures look natural. See compress image to 50KB.
If the form sets a minimum as well as the 40 KB maximum, check the result against both numbers. This tool makes files smaller but never larger, while the Image Resizer can pad a file up to a minimum without changing the picture.