What DPI means for an image
DPI is the most misunderstood number attached to a picture. It adds no detail and removes none. It is an instruction about printing, and one line of arithmetic explains everything a form or a print shop means by it.
DPI is a note in the file
A digital picture is a grid of pixels: so many across, so many down. That grid is all the detail the picture has. DPI, dots per inch, is a separate number stored beside it, and it says how large the picture should be when it is printed: how many of those pixels go into each inch of paper.
Two files can hold the same 1200 × 1800 pixels, one marked 72 DPI and the other 300. On a screen they are identical, because a screen shows pixels and ignores the note. Printed at their stated sizes, the first is a poster more than 40 cm wide and the second a 4 × 6 inch photo. Nothing about the picture differs. Only the instruction does.
You will also see PPI, pixels per inch. For a file it is the same idea, and the two names are used for the same field. DPI is the word forms and print shops use, so it is the word used here.
The one sum: pixels are inches times DPI
Everything follows from one line. The number of pixels along a side is the length of that side in inches multiplied by the DPI. For centimetres, divide by 2.54 first.
So a photo that must print 3.5 cm wide at 300 DPI needs 3.5 ÷ 2.54 × 300, which is 413 pixels. Turn the sum round and you get the print size of a file you already have: 2480 pixels at 300 DPI is 8.27 inches, the width of an A4 sheet.
| Size | At 200 DPI | At 300 DPI | At 600 DPI |
|---|---|---|---|
| 3.5 × 4.5 cm passport photo | 276 × 354 | 413 × 531 | 827 × 1063 |
| 6 × 2 cm signature | 472 × 157 | 709 × 236 | 1417 × 472 |
| 4 × 6 inch print | 800 × 1200 | 1200 × 1800 | 2400 × 3600 |
| A4 sheet, 210 × 297 mm | 1654 × 2339 | 2480 × 3508 | 4961 × 7016 |
The Image Dimension Calculator does these sums in either direction.
When a form asks for a DPI
Some application forms state a DPI, and it is worth knowing what they can and cannot check. The instructions for an online PAN application ask for images scanned at 200 DPI. Railway recruitment notices ask for a signature at 100 DPI or more.
What such a form really needs is a picture with enough pixels for the size it names, and the DPI is how the notice says so. A 3.5 × 2.5 cm photograph at 200 DPI means 276 × 197 pixels. Make the picture that many pixels and write 200 into the file, and you have met the instruction in both senses.
Many forms give a size in centimetres and no DPI at all. Then no particular pixel size is being asked for, and what the upload screen checks is the file size in kilobytes. Any sensible DPI will do; 200 is the usual setting for scanning a document.
Changing the DPI, with and without changing the picture
There are two different things people mean by "change the DPI", and mixing them up is where the trouble starts.
Changing the note. The number in the file is rewritten and every pixel stays as it was. The picture is no sharper and no softer, and the file is the same size to within a few bytes. This is what a form that says "must be 300 DPI" is usually satisfied by, and it is what the DPI Converter does. Where the file allows it, the image data is not even encoded again.
Changing the pixels. To print a small picture larger at the same DPI you need more pixels than you have. A program can make them by enlarging, which spreads the existing pixels over more space. That is ordinary interpolation: the result is bigger and softer, and no detail is created. Raising a 500-pixel photo to "300 DPI at A4" gives a file with A4's pixel count and a 500-pixel photo's detail.
The honest summary is that DPI cannot be used to improve a picture. If a print looks soft, the picture needs more real pixels, which means going back to a larger original.
Which formats can hold a DPI
JPG and PNG both have a field for resolution, and a DPI written there is read by print programs and by the forms that check it. WebP has no such field, so a WebP file cannot carry a DPI at all. That is why the tools on this site that set a physical size or a DPI save JPG or PNG only.
A JPG can state its resolution in more than one place: in its own header and again in its EXIF data. Programs disagree about which to believe, so a file whose two fields differ can be reported as 72 DPI by one program and 300 by another. A tool that sets the DPI should write both.
How to check a file's DPI
The DPI Checker lists every resolution field a file holds and what print size each gives, which is the quickest way to see a disagreement of the kind just described. On a computer, the file's properties show a resolution too, though usually only one of the fields.
If a file reports no DPI at all, nothing is wrong with it. Many pictures, including most that have been through a messaging app or saved from a website, have had the field removed or never had one. Programs then assume 72 or 96. Set the value you need and the file will say so from then on.
Frequently asked questions
How many pixels is 300 DPI?
It depends on the size. DPI is pixels per inch, so 300 DPI means 300 pixels for every inch of print: 1200 × 1800 pixels for a 4 × 6 inch photo, 413 × 531 for a 3.5 × 4.5 cm one, and 2480 × 3508 for an A4 sheet.
Does increasing the DPI improve image quality?
No. Changing the DPI number changes how large the picture prints, not how much detail it has. Enlarging the picture to match a higher DPI adds pixels by interpolation, which makes it bigger and softer and creates no detail.
What DPI should I use for a form?
The one the notice states. If it states none, any sensible value will do, because the upload can only check the pixel size and the file size; 200 is the usual setting for a scan.
Why does my image show as 72 or 96 DPI?
Either the file was saved with that value, or it has no resolution field and the program showing it is assuming one. Neither affects the picture. If you need a different value, the DPI Converter writes it into the file.