A print shop says your photo is 72 DPI and they need 300. Or a form rejects your signature because it "must be 200 DPI". You open the photo, it looks perfectly sharp, and you have no idea what to change.
It's one of those problems that sounds technical and is actually very simple, so I built a separate tool for it. Here's what DPI really is, why the same photo shows different numbers in different apps, and how to change it without making the photo worse.
What DPI actually means
A digital photo is a grid of pixels, say 1200 wide and 1800 tall. That grid has no size in inches or centimetres until something decides how many pixels to squeeze into each inch. That number is the DPI (dots per inch), and it's saved in the file's header next to things like the camera model.
So the same 1200 × 1800 photo prints at:
- 16.7 × 25 inches at 72 DPI
- 8 × 12 inches at 150 DPI
- 4 × 6 inches at 300 DPI
Same file, same pixels, same sharpness on your screen. Only the print size changes. Your phone writes 72 into almost every photo by default, and it doesn't mean anything. Nobody measured it.
Strictly speaking, the number in a file is PPI (pixels per inch) and DPI is about ink dots on paper. But printers, forms and Photoshop all say DPI, so I'll stick with that.
How many pixels you need for 300 DPI
Multiply the print size in inches by 300. For centimetres, divide by 2.54 first. These are the sizes that come up most:
| Print size | Pixels at 300 DPI | Pixels at 200 DPI |
|---|---|---|
| 3.5 × 4.5 cm (Indian passport size) | 413 × 531 | 276 × 354 |
| 2 × 2 in (US passport) | 600 × 600 | 400 × 400 |
| 4 × 6 in | 1200 × 1800 | 800 × 1200 |
| 5 × 7 in | 1500 × 2100 | 1000 × 1400 |
| A4 (21 × 29.7 cm) | 2480 × 3508 | 1654 × 2339 |
If your photo has at least that many pixels, all you need to change is the DPI number. A normal 12 MP phone photo (4000 × 3000) is enough for about a 10 × 13 inch print at 300 DPI.
Why Windows says 72 and Photoshop says 300
This is the confusing part. A JPG doesn't keep its DPI in one place. It can keep it in up to three:
- The JFIF header, which is the basic JPG header. Many websites and simple apps only read this.
- EXIF, the block where the camera saves the date, the phone model and so on. Windows reads the DPI from here first.
- Photoshop's own resolution block, if the file was ever saved from Photoshop. Photoshop trusts this one.
If one tool changed only one of them, the file now has two different answers, and each app shows the one it reads. That's how you get a photo that's "300 DPI" in one place and "72 DPI" in another. A PNG is simpler: it has one field, called pHYs, which stores the value in pixels per metre instead of per inch.
When I wrote the DPI tool, I made it read all of them, show you each one, and write the same value into every copy that exists. If a JPG has no JFIF header at all, which is common with photos straight from a phone, it adds one.
Changing DPI without losing quality
Here's the part most people don't realise. Many online "DPI converters" open the photo, draw it, and save it as a new JPG with the DPI you asked for. Every time a JPG is saved like that, it's compressed again and loses a little detail. You asked for a header change and got a slightly worse photo.
There's no need for that. The DPI is a few bytes in the header, so you can change those bytes and leave the image data alone. That's what our tool does, and I checked it the boring way while testing:
- I decoded the original and the changed file and compared the pixels. They were identical, every single one, for JPGs and PNGs.
- The file size didn't change at all when the file already had a DPI field. When a JPG had no JFIF header, it grew by exactly 18 bytes. A PNG with no DPI field grew by 21 bytes.
- 300 DPI in a PNG is saved as 11,811 pixels per metre, and some apps convert that back as 299.9994. That's only rounding, and Windows shows it as 300.
How to change a photo to 300 DPI
- Open the Change Image DPI tool and drop in your JPG or PNG. You can add several at once.
- Look at the "Now" value. It shows the DPI the file has and where it's stored. If it says "Not set", apps will guess 72 or 96.
- Pick 300, or 200 if your form asks for that. If the form gives a size instead, like 3.5 cm wide, switch to print width and type it. The tool works out the DPI for you.
- Check the "Prints at" size, then download. Files are named like
photo-300dpi.jpgso you don't mix them up with the originals.
To check it worked on Windows, right-click the new file, open Properties, then the Details tab, and look at Horizontal resolution and Vertical resolution.
When changing DPI isn't enough
Say you have an old 800 × 600 photo and want a 4 × 6 print. At 300 DPI, 800 pixels is only 2.7 inches wide (6.8 cm). You can set the DPI to 300 and the print shop will be happy, but the print will come out tiny, or the shop will stretch it and it'll look soft.
That photo needs more pixels, not a different number. The photo enlarger for print shows you what DPI your photo will actually print at for a chosen size, and enlarges it with AI when it's small. It saves the DPI in the file too, so you don't need a second step.
For exam and job forms: do the DPI last
Some application forms ask for a photo at a set pixel size, under a KB limit, and at 200 or 300 DPI, all at once. The order matters, because resizing or compressing saves a new file, and most tools reset the DPI when they do that.
- First get the pixel size and KB right with the exam photo and signature resizer or the compress to 50 KB tool.
- Then set the DPI. Since that only changes the header, the size in KB stays the same.
- Upload that final file, and don't open and re-save it in another app before you do.
If a website still rejects it, check whether it wants JPG or PNG. WebP and HEIC photos don't really have a DPI field that apps respect, so convert them with the JPG converter first.
