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Geometry · print
Resize an image in centimetres
An image has no physical size of its own. It has pixels, and a density tag that says how many of them go into an inch. Give this tool both and it does the conversion in the right direction, then writes the density into the file so the printer agrees with you.
Result
The arithmetic, in case you want to check it
Pixels needed equals physical size in inches multiplied by DPI. A 10 cm wide photo at 300 DPI is 10 ÷ 2.54 × 300, which comes to 1181 pixels. Work it the other way and a photo that is 1181 pixels wide will print at 10 cm if the printer is told 300 DPI, at 20 cm if it is told 150, and at 41 cm if it is told 72. Same pixels every time.
That is why the DPI field matters more than it looks. This tool writes the value into the JFIF header of a JPEG or the pHYs chunk of a PNG, which is what page layout software, Word and most print shops read when they decide how large to place your picture.
Which DPI to ask for
- 300 is the standard for anything held in the hand: photo prints, brochures, book pages.
- 150 is fine for posters and banners, which are viewed from further away. Doubling the viewing distance halves the resolution the eye can resolve.
- 72 is a screen convention with no real meaning on modern displays, but some software still expects it.
If the tool warns that it had to enlarge your image to reach the size you asked for, the honest answer is that the print will be soft. Either accept a smaller print or start from a higher resolution original.
Common questions
How many pixels is 4 x 6 inches at 300 DPI?
1200 × 1800 pixels. Multiply each dimension in inches by the DPI. For centimetres, divide by 2.54 first.
Why does my printed photo come out smaller than I asked?
Almost always because the printing software ignored the density tag and used its own default, or because "fit to page" was switched on. Check the placement settings before blaming the file. You can confirm what the file claims with the DPI tool.
Can I increase DPI without losing quality?
You can change the tag without touching a pixel, which changes the print size, not the quality. What you cannot do is add real detail. Raising DPI while holding the physical size means enlarging, and enlarging invents pixels.
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