What this calculator covers
- Three ways round: the DPI of an image at a print size, the print size of an image at a DPI, and the pixels a print size needs at a DPI.
- Print sizes in inches, millimeters or centimeters, and resolution in dots per inch, pixels per inch or pixels per centimeter.
- Paper and photo presets: ISO A0 to A7, US Letter and Tabloid, the US business card, 4 × 6, 5 × 7 and 8 × 10 inch prints, 18 × 24 and 24 × 36 inch posters, and 10 × 15 to 20 × 30 cm prints. A preset is turned to match the orientation of the image.
- An image you drop or choose is decoded in your browser to read its width and height in pixels. It is never uploaded, and nothing is sent to a server.
- Pixel counts, print sizes and resolutions greater than 0 and up to 1,000,000 in the chosen unit.
- Everything runs in your browser. No sign-up, no login, no pop-ups.
- This page is about print. For the pixel density of a screen, use the PPI calculator.
How it is calculated
DPI, print size and pixels
DPI = pixels ÷ print size in inches, worked out for the width and for the height. 3000 px across 10 in is 3000 ÷ 10 = 300 dpi.
Print size in inches = pixels ÷ DPI. 3000 px at 300 dpi is 3000 ÷ 300 = 10 in.
Pixels = print size in inches × DPI. 8 in at 300 dpi is 8 × 300 = 2,400 px. Pixel counts are rounded to the nearest whole pixel, and the unrounded value is shown too.
Millimeters are divided by 25.4 and centimeters by 2.54 to get inches. Pixels per centimeter are multiplied by 2.54 to get pixels per inch.
The effective resolution is the lower of the width and height values. When the print and the image have different shapes the two values differ, and the image would be stretched unless it is cropped or fitted.
Pixels, ratio and file size
Total pixels = width × height, and megapixels = total pixels ÷ 1,000,000.
Aspect ratio is the width and height divided by their greatest common divisor. The common ratio is the closest of 1:1, 5:4, 4:3, 7:5, √2:1 (the ratio of ISO A paper), 3:2, 16:10 and 16:9, with the difference in percent. The actual ratio is width ÷ height.
Pixel pitch on paper = 25,400 ÷ DPI in micrometers, and pixel area = width pitch × height pitch.
TIFF size = width × height × 3 bytes: an uncompressed 8-bit RGB image, not counting the file header. 1 MB here is 1,000,000 bytes.
JPEG size depends on what is in the picture, so it is not estimated from the pixel count. When you drop an image, your browser encodes it as a JPEG at quality 0.8 and the page shows the size of that file.
Largest print and viewing distance
The largest print at 300, 200 and 150 dpi is the pixel count divided by that DPI.
20/20 vision is defined as resolving two points one minute of arc (1′) apart. Seen from a distance d, one printed pixel spans 1′ at 1 ÷ (d × tan 1′) dots per inch, which is about 3,437.75 ÷ d with d in inches. At 30 cm that is 291.06 dpi. The page compares the effective resolution with that number at 30 cm, 60 cm, 1 m, 2 m and 5 m, and gives the distance from which the pixels blend: 1 ÷ (DPI × tan 1′) inches.
Rounding
DPI, print sizes, megapixels, pixel pitch, pixel area and file sizes are shown to 2 decimal places, distances and the ratio difference to 1, and pixel counts as whole numbers. Halves are rounded away from zero. Every result is calculated from unrounded numbers and the unrounded result is always shown, so the same inputs always give the same answer.
FAQ
How do I calculate the DPI of an image?
Choose "Find DPI", drop the image or type its size in pixels, and enter the print size. A 3000 × 2400 px image printed at 10 × 8 in is 300 dpi.
What print size can I get from my image?
Choose "Find print size" and enter the resolution you want to print at. The results also list the largest print at 300, 200 and 150 dpi.
How many pixels do I need for an 8 × 10 print?
Choose "Find pixels", pick the 8 × 10 in preset and enter the resolution. At 300 dpi it is 2,400 × 3,000 px for a portrait print, or 3,000 × 2,400 px for a landscape one.
Is DPI the same as PPI here?
For an image file the number counts pixels per inch of the print, whichever name is used, and you can label it either way. A printer's own DPI setting counts ink dots and is a different number. To convert a single length between pixels, inches, centimeters and millimeters, use the pixels to inches converter.
Why are there two DPI values?
DPI is worked out across the width and down the height. They match when the print has the same shape as the image; when they differ, the lower one is the effective resolution.
Is my image uploaded?
No. Your browser opens the file to read its width and height and, for the JPEG size, encodes it locally. Nothing leaves your device.