How to use this calculator
- Pixels → print size. You have a file and want to know how big you can print it. Enter the pixel dimensions (right-click the file → Properties or Get Info) and the resolution you intend to print at. The result is the physical size, plus a verdict on quality.
- Print size → pixels. You know the finished size you need and want to know whether your camera or scan is up to it. Enter the size, the unit, and your target DPI. The result is the minimum pixel dimensions required.
- What DPI am I getting? You already have both the file and the intended size, and want to know the effective resolution. This is the mode to use when a printer says "your file is too low-res" and you want to see the actual number.
The formula, in full
There is only one equation here, rearranged three ways. All three modes are the same relationship:
Print size (in) = pixels ÷ DPI
Pixels = print size (in) × DPI
DPI = pixels ÷ print size (in)
Metric inputs are converted first: 1 inch = 2.54 cm = 25.4 mm. Megapixels are simply width × height ÷ 1,000,000.
Notice what is absent: nothing about file size in megabytes, colour depth, or format. A 12 MB JPEG and a 90 MB TIFF with the same pixel dimensions print at exactly the same size and exactly the same quality. File weight tells you about compression and colour data, not about resolution. This is why "my file is 20 MB, it must be high quality" is a claim that means nothing to a printer.
Why DPI is only a label until you print
An image file stores a DPI value in its metadata, and design software displays it, but that number does not affect the pixels at all. It is a note attached to the file saying "when placed, default to this scale." Changing it from 72 to 300 in Photoshop's Image Size dialog — with resampling switched off — does not add a single pixel. It just shrinks the default placement size by a factor of about four.
What this means practically: stop worrying about the DPI field and start counting pixels. A 6000 × 4000 image is a 6000 × 4000 image whether it is labelled 72 or 300. What matters is the arithmetic between that pixel count and the size you want on paper.
A worked example
Suppose you have a photo from a 24-megapixel camera at 6000 × 4000 pixels and you want to know whether you can print it at A2 poster size.
- A2 is 420 × 594 mm, which is 16.54 × 23.39 inches.
- Effective DPI across the width: 6000 ÷ 23.39 = 256 DPI (using the long edge against the long edge).
- Effective DPI across the height: 4000 ÷ 16.54 = 242 DPI.
The two figures differ because a 3:2 camera frame does not match A2's proportions — you would be cropping or leaving a margin either way. Take the lower number, 242 DPI, as your honest answer. That is comfortably above the 200 DPI floor for a poster viewed at arm's length, so the print will look excellent. If you tried the same file at A1, the figure drops to about 171 DPI, which is still perfectly fine for something on a wall but would look soft in a photo book.
Working the other direction
Now suppose a client asks for a full-bleed A4 magazine spread — 297 × 420 mm across the two pages, which is 11.69 × 16.54 inches — at 300 DPI. Multiply through: 11.69 × 300 = 3507 pixels tall, 16.54 × 300 = 4961 pixels wide. That is about 17.4 megapixels. Any modern full-frame camera clears it; most phone cameras do not once you account for cropping.
What resolution do you actually need?
The 300 DPI convention exists because it is roughly the limit of what the eye resolves at typical reading distance, around 25 to 30 cm. It is not a law of physics, and applying it to a billboard would be absurd. The real rule is that required resolution falls as viewing distance rises.
| Output | Viewing distance | Practical minimum | Comfortable |
|---|---|---|---|
| Business card, book, photo print | 25–40 cm | 250 DPI | 300 DPI |
| Magazine, brochure | 30–50 cm | 240 DPI | 300 DPI |
| A3 poster | 0.5–1 m | 180 DPI | 240 DPI |
| A1/A0 poster | 1–2 m | 120 DPI | 150 DPI |
| Roll-up banner | 1.5–3 m | 100 DPI | 150 DPI |
| Exhibition wall | 3–6 m | 72 DPI | 100 DPI |
| Billboard | 20 m+ | 10 DPI | 20 DPI |
A rough field rule used by large-format printers: divide the viewing distance in inches by roughly 10 to get a usable DPI. At 60 inches (1.5 m) that gives 150 DPI, which matches the table well.
Line art and text are the exception
Everything above concerns continuous-tone images — photographs. Pure black-and-white line art, logos and scanned text behave differently, because there is no gradual tonal transition to disguise a stair-stepped edge. Bitmap line art is typically prepared at 600 to 1200 DPI. Better still, keep logos and type as vector artwork, where resolution stops being a concept at all: an SVG, EPS or vector PDF prints at whatever the imagesetter can resolve, at any size.
Megapixels and standard print sizes
| Print size | Pixels at 300 DPI | Megapixels needed |
|---|---|---|
| 4 × 6 in | 1200 × 1800 | 2.2 MP |
| 5 × 7 in | 1500 × 2100 | 3.2 MP |
| 8 × 10 in | 2400 × 3000 | 7.2 MP |
| A4 (8.27 × 11.69 in) | 2480 × 3508 | 8.7 MP |
| 11 × 14 in | 3300 × 4200 | 13.9 MP |
| A3 (11.69 × 16.54 in) | 3508 × 4961 | 17.4 MP |
| 16 × 20 in | 4800 × 6000 | 28.8 MP |
| A2 (16.54 × 23.39 in) | 4961 × 7016 | 34.8 MP |
Those megapixel figures assume the aspect ratio matches. Cropping a 3:2 camera frame to 4:5 for an 8 × 10 print throws away roughly a fifth of the pixels, so budget accordingly.
Where this calculator can mislead you
A few honest limitations worth knowing:
- It cannot see your image. A 6000-pixel-wide photo that was shot out of focus, cropped from a corner, or saved as a heavily compressed JPEG will not print well no matter what the DPI arithmetic says. Resolution is a ceiling on quality, not a guarantee of it.
- Printer DPI is a different number. An inkjet advertised at "5760 DPI" is describing ink droplet placement, not image resolution. Several droplets combine to make one image pixel. Sending a 5760 DPI file achieves nothing except a very large file.
- Aspect ratio is not checked for you. The tool converts whatever you enter. If your image proportions do not match your print proportions, you will crop or letterbox, and the effective resolution on the cropped result will be lower than calculated.
- Bleed is extra. If the job prints to the edge, you need 3 mm (or 0.125 in) of image beyond the trim on every side. Calculate your pixel requirement from the bleed size, not the finished size.