How to print a photo at an exact size
A photo has no physical size until you give it one. The pixels decide how much detail is available; you decide the centimetres — and the two only meet at the moment you print.
Pixels are not a size
DPI is not a property of your file. It is a ratio between two things you already have: pixels, and the size you want on paper. A 3000-pixel-wide photo is 25 cm across at 300 DPI and 38 cm across at 200 DPI, and nothing about the file changed in between.
Files do carry a stored DPI number, and it is a suggestion nothing is obliged to honour. What actually determines the size of your print is the size you set in the print dialog, which is why setting it explicitly is the only reliable way to get 13 × 18 cm on paper rather than something near it.
Standard sizes and the pixels they need
At 300 DPI, which is the usual standard for something held at reading distance. Sizes are grouped by unit, not by area, because the metric and imperial families are not interchangeable — 10 × 15 cm and 4 × 6 in are the same idea in two systems, and they are not the same size.
| Size | In the other unit | Pixels at 300 DPI | Proportion | What to know |
|---|---|---|---|---|
| 10 × 15 cm | 3.9 × 5.9 in | 1181 × 1772 px | 3:2 | The standard metric print. Matches a camera sensor exactly, so nothing is cropped. |
| 13 × 18 cm | 5.1 × 7.1 in | 1535 × 2126 px | 1.38:1 | Close to 5 × 7 in without being it. Worth checking which one a lab means. |
| 15 × 20 cm | 5.9 × 7.9 in | 1772 × 2362 px | 4:3 | Matches a phone photo exactly, which no imperial size does. |
| 20 × 30 cm | 7.9 × 11.8 in | 2362 × 3543 px | 3:2 | The large metric print, still 3:2, so still no cropping from a camera. |
| A4 | 8.3 × 11.7 in | 2480 × 3508 px | 1.41:1 | A paper size, not a photo size. A 3:2 photo loses about 6% of its length. |
| 4 × 6 in | 10.2 × 15.2 cm | 1200 × 1800 px | 3:2 | The imperial standard, and very slightly larger than 10 × 15 cm. |
| 5 × 7 in | 12.7 × 17.8 cm | 1500 × 2100 px | 1.4:1 | A 3:2 photo loses about 7% of its length. |
| 8 × 10 in | 20.3 × 25.4 cm | 2400 × 3000 px | 4:5 | A 3:2 photo loses a sixth of its length here. Crop it yourself or the lab will. |
| 11 × 14 in | 27.9 × 35.6 cm | 3300 × 4200 px | 1.27:1 | The common framed size, and awkward for every camera proportion there is. |
| 16 × 20 in | 40.6 × 50.8 cm | 4800 × 6000 px | 4:5 | Needs 29 megapixels at 300 DPI. At the distance it will hang, far fewer will do. |
The proportion is what crops your photo
The pixel count tells you whether a print can be sharp. The proportion tells you whether it can be the picture you took, and it is the one people are surprised by.
Cameras shoot 3:2. Phones shoot 4:3. Print sizes were standardised for neither: 8 × 10 in and 16 × 20 in are 4:5, 5 × 7 in is 1.4:1, A4 is 1.41:1. Whenever those do not match, something is cut off — and if you do not decide what, the lab or the print dialog decides for you, usually by taking equal slices off both ends of the long side.
The numbers are worth knowing before you commit. A 3:2 photo printed at 8 × 10 loses a sixth of its length. At 5 × 7 it loses about 7%, at A4 about 6%. A 4:3 phone photo at 8 × 10 loses about 6%. If the composition has anything near the top or bottom edge — a head, a horizon, a signature — crop it deliberately first.
How many pixels you actually need
300 DPI is the standard because it is roughly where a normal eye stops resolving individual dots at reading distance. It is a ceiling, not a threshold, and treating it as a minimum is what makes people think their photos are unprintable.
At arm's length, 200 DPI is difficult to tell apart from 300 on most subjects, which turns a 16 × 20 in print from a 29-megapixel demand into 3200 × 4000 px, or 13 megapixels. Below about 150 DPI it starts to show as softness, and below 100 it is visible as structure.
The rule that actually works: divide the pixels you have by the inches you want and see what DPI falls out. Above 240, print it. Between 150 and 240, print it and look at it from where it will hang. Below 150, either print it smaller or upscale it first.
Getting the exact size onto paper
Set the size explicitly rather than letting anything fit it for you. Then, in the print dialog, the same rule that governs every other kind of exact-size printing: scaling at 100%, Actual size, or None — never Fit to page, which will resize your carefully specified 13 × 18 cm to whatever clears the printer margin.
Choose the paper size the printer is really holding, set the paper type to match what is in the tray, and check the result against a ruler on the first print rather than the fifth. The full walkthrough is in the guide to printing at actual size.
Borderless printing, and why it moves your size
Borderless printing works by enlarging the image slightly — typically two or three per cent — so that it runs past the edge of the paper on all sides and no white line can appear if the sheet feeds a fraction crooked. That enlargement is a scaling you did not ask for, and it is applied on top of whatever size you set.
For a photo it is usually invisible and entirely fine. If the exact dimension is the point — a print that has to fit a mount with a fixed opening, or a set that has to match — print with a border on a slightly larger sheet and trim it down. Trimming is the only way to get an exact edge out of a printer that cannot reach its own paper edge.