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, the table above covers the common cases, and the resizer's Export panel does the rest: type a print width in centimetres or inches and a DPI, and it hands you the pixel width to use. Getting this arithmetic wrong is silent — the file looks fine and comes back rejected — so it is worth letting the page do it. To resize straight to a paper size, choose cm under Size by, or open resize image in cm: type the width, height and DPI and the page resizes to the pixels they stand for.
The same panel also works backwards: for whatever pixel size you are about to save, it prints how large that file will come out at the DPI you type. None of this changes a pixel. It is conversion, and the page says so beside the numbers.
What this page will not tell you
This site never reads or writes the resolution tag. The file you save is written fresh by your browser's own encoder, so the old tag — and the rest of the camera metadata — does not carry over, and print software falls back to its own default. Set the print size in the software that prints, using the calculator's numbers.
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.