What DPI should I use?

For anything that will only ever be looked at on a screen, the question does not apply: the number is stored in the file and ignored by every browser. For print, 300 is the usual answer for photographs and 600 for line art, but the printer or the specification you are working to is the authority, not a rule of thumb.

I changed the DPI and the picture did not change.

That is the correct behaviour and it is the clearest demonstration of what DPI is. The tag is a note attached to the file saying how big it should be printed. Changing it rearranges nothing and discards nothing; the pixels are identical before and after.

The form wants 3.5 cm × 4.5 cm. What do I type here?

Nothing, until you know the resolution it will be printed at. At 300 pixels per inch, 3.5 cm × 4.5 cm is 413 × 531 pixels; at 600 it is 827 × 1063. If the form has not told you, it usually means the picture is going on a screen and the centimetres were copied from a paper form.

Guide

Pixels and print size

A picture that is 800 pixels wide is not any particular number of centimetres wide. It is 800 pixels wide. The rest is a decision somebody has to make.

A pixel has no size

This is the confusion underneath most of the questions this site gets, and it is worth stating in one sentence: a pixel is a sample, not a length. An 800 × 600 image is a grid of 480,000 samples. How wide that grid is in the physical world depends entirely on how far apart you decide to put the samples when you turn them back into something you can touch.

Put them 300 to the inch and the grid is 2.67 inches across, about 6.8 cm. Put them 96 to the inch and the same file is 8.33 inches, about 21 cm. Nothing in the file changed. The number of samples is the only thing a resizer can alter, and it is the only thing this page alters.

What the DPI number in a file actually is

Most image formats can store a resolution figure — dots per inch, or its metric twin, pixels per centimetre. It is metadata. It is a note attached to the pixels that says how big the person who saved it thought it should be printed. Nothing enforces it and almost nothing on a screen reads it.

Two consequences follow, and they surprise people in opposite directions:

Changing the DPI tag does not change the picture. The file is the same size in kilobytes, the same size in pixels, and looks identical everywhere except in software that lays it out on a page.
Changing the pixel dimensions does change the printed size, if the DPI tag is left alone. Halve the width of a photograph tagged at 300 DPI and it prints half as wide, because the same tag now describes half as many samples.

Physical size	At 150 DPI	At 300 DPI	At 600 DPI
3.5 × 4.5 cm	207 × 266 px	413 × 531 px	827 × 1063 px
10 × 15 cm	591 × 886 px	1181 × 1772 px	2362 × 3543 px
A4, 21 × 29.7 cm	1240 × 1754 px	2480 × 3508 px	4961 × 7016 px

Where to go when the requirement is in centimetres

If the specification you are working to is written in centimetres, inches or millimetres, doing the arithmetic yourself and typing the answer here works, and the table above covers the common cases. But it is arithmetic you should not have to do, and getting it wrong is silent — the file looks fine and comes back rejected.

That job belongs to a tool built around it. RescaleImage is where physical sizing lives: percentages and factors, DPI and PPI, centimetres and inches, and the print-size arithmetic done for you rather than by you. It is also where the choice of resampling filter lives, if that is a decision you need to make. This page owns the other half of the problem, which is the half where you already know the two numbers.

What this page will not tell you

This site never reads or writes the resolution tag. It changes the pixel dimensions, keeps the rest of the metadata block as it found it, and takes no position on how big the result should be printed. If a picture arrives here tagged at 72 DPI it leaves tagged at 72 DPI, whatever its new dimensions are — which means the printed size will have changed even though the tag did not.

That is a deliberate omission rather than an oversight. A resizer that quietly rewrote the resolution tag would be making a print decision on behalf of somebody who came here to type two numbers.

Questions we get asked

What DPI should I use?: For anything that will only ever be looked at on a screen, the question does not apply: the number is stored in the file and ignored by every browser. For print, 300 is the usual answer for photographs and 600 for line art, but the printer or the specification you are working to is the authority, not a rule of thumb.
I changed the DPI and the picture did not change.: That is the correct behaviour and it is the clearest demonstration of what DPI is. The tag is a note attached to the file saying how big it should be printed. Changing it rearranges nothing and discards nothing; the pixels are identical before and after.
The form wants 3.5 cm × 4.5 cm. What do I type here?: Nothing, until you know the resolution it will be printed at. At 300 pixels per inch, 3.5 cm × 4.5 cm is 413 × 531 pixels; at 600 it is 827 × 1063. If the form has not told you, it usually means the picture is going on a screen and the centimetres were copied from a paper form.
Can I just set a big number and print it small?: Yes, and it is often the safe move: more samples than the printer needs are discarded politely, whereas fewer than it needs are invented badly. The cost is a larger file and nothing else.