Quick answer
Pixels are an image’s width and height. Inches or centimeters together with DPI describe how big it prints: pixels = inches × DPI, or cm ÷ 2.54 × DPI. A 4×6 inch print at 300 DPI needs 1200 × 1800 pixels. File size in KB or MB is a separate thing: it depends on the pixels, the format and the quality setting.
Inches to pixels at 300 DPI
300 DPI (dots per inch) is the usual standard for sharp photo prints. Multiply each side in inches by 300 to get the pixels.
| Print size | 300 DPI | 200 DPI |
|---|---|---|
| 2 × 2 in (passport) | 600 × 600 | 400 × 400 |
| 4 × 6 in | 1200 × 1800 | 800 × 1200 |
| 5 × 7 in | 1500 × 2100 | 1000 × 1400 |
| 8 × 10 in | 2400 × 3000 | 1600 × 2000 |
| 11 × 14 in | 3300 × 4200 | 2200 × 2800 |
A typical 12-megapixel phone photo (4032 × 3024) prints sharply up to about 10 × 13 inches at 300 DPI.
Centimeters to pixels
pixels = cm ÷ 2.54 × DPI
So a 3.5 × 4.5 cm photo at 300 DPI needs 413 × 531 pixels, and at 200 DPI it needs 276 × 354 pixels. Going the other way, inches = pixels ÷ DPI, and cm = pixels ÷ DPI × 2.54.
| Size | 300 DPI | 200 DPI |
|---|---|---|
| 35 × 45 mm (passport, many countries) | 413 × 531 | 276 × 354 |
| 50 × 70 mm (Canadian passport) | 591 × 827 | 394 × 551 |
| 10 × 15 cm (postcard print) | 1181 × 1772 | 787 × 1181 |
| A4 page (210 × 297 mm) | 2480 × 3508 | 1654 × 2339 |
What DPI really changes (300 DPI vs 72 DPI)
JPG and PNG files carry a DPI value as a label inside the file. Changing that label, say from 72 to 300, doesn’t add or remove a single pixel, doesn’t change how the photo looks on screen, and doesn’t change its file size. It only tells printers, and some upload forms, what physical size you intend.
What matters for print quality is having enough pixels for the size: a 600 × 600 image prints sharply at 2 × 2 inches, whatever its DPI label says. If a form or print shop asks for “300 DPI”, set the label and make sure the pixels match the table above.
File size: KB and MB
- 1 MB is about 1,000 KB. Most computers count 1 KB as 1,024 bytes, while some websites count in thousands; stay a little inside any limit.
- The same pixels can make very different file sizes. It depends on the format, the quality setting and how much detail the picture holds.
- To make a file smaller, reduce the pixels first, then the quality; see how to make a photo smaller.
The order that works
- Crop to the shape you need, such as 2:3 for a 4×6 print or square for 2×2.
- Resize to the exact pixels.
- Set the DPI label if a form or print shop asks for one.
- Save in the right format, usually JPG. Renaming a file from .png to .jpg doesn’t convert it.
- Compress last, if there’s a file size limit.
A worked example
Say you want a 6 × 4 inch landscape print from a 4032 × 3024 phone photo. The print is a 3 : 2 shape and the photo is 4 : 3, so crop it first: keeping the full width of 4032 pixels, the height becomes 4032 ÷ 3 × 2 = 2688 pixels. Then resize the 4032 × 2688 crop to 1800 × 1200, set 300 DPI, and save as a JPG. Without the crop, the print shop would either cut off part of the photo or leave white bars.
Questions people ask
How many pixels is 300 DPI?
DPI isn’t a number of pixels by itself; it’s pixels per inch. At 300 DPI, every inch of print needs 300 pixels, so a 4 × 6 inch print needs 1200 × 1800 pixels.
How many pixels is a 2 × 2 inch photo?
600 × 600 pixels at 300 DPI.
Does changing DPI reduce file size?
No. The DPI label doesn’t change the pixels or the file size; only resizing or compressing does.
How do I convert cm to pixels?
Divide the centimeters by 2.54, then multiply by the DPI. For example, 5 cm at 300 DPI is 5 ÷ 2.54 × 300 = 591 pixels.