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DPI / PPI Calculator

Pixel dimensions

2,550 × 3,300 px

Pixels
2,550 × 3,300
Physical
8.5 × 11 in
DPI
300

Estimates for educational purposes — not financial, medical, or legal advice. See terms.

The relationship between pixel dimensions, physical size, and resolution is one simple formula — pixels equals physical size times resolution — applied three different ways depending on which two of the three values you know. This calculator handles all three cases with a mode toggle.

Pick what you want to solve for, enter the two knowns, and the answer appears immediately. Paper-size presets (A4, US Letter, photos, business cards) fill in the physical dimensions without typing; DPI presets (72 for web, 300 for photo print, 600 for fine art) fill in standard resolutions.

The formula

pixels=inches×DPI\text{pixels} = \text{inches} \times \text{DPI}

Rearrange to solve for any of the three:

inches=pixelsDPIDPI=pixelsinches\text{inches} = \frac{\text{pixels}}{\text{DPI}} \qquad \text{DPI} = \frac{\text{pixels}}{\text{inches}}

That is the entire thing. The tool supports inches, centimeters, and millimeters as input and output units, converting everything to inches internally (1 inch = 25.4 mm exactly) so the math stays consistent regardless of which unit you pick.

Example: sizing an image for a photo print

You are ordering a 4 by 6 inch photo print and want to know how large the source image needs to be. Set the mode to “pixels”, enter 4 and 6 inches for the physical size, and pick the 300 DPI preset. The calculator returns 1200 by 1800 pixels. That is the minimum size your source file needs to be for a print that will look sharp at normal viewing distance; anything below that and you will start seeing individual pixels on close inspection.

For an A4 print at the same resolution, pick the A4 preset (which switches the unit to mm and fills 210 by 297). At 300 DPI, you need roughly 2480 by 3508 pixels — that is 8.7 megapixels, well within the range of any modern camera.

Example: computing the physical size of an existing image

Working the other direction — you have a 1920 by 1080 pixel image and want to know how large it will print. Set the mode to “physical size”, enter the pixel dimensions, and pick a DPI. At 300 DPI it comes out 6.4 by 3.6 inches; at 150 DPI it is 12.8 by 7.2 inches. Lower DPI means more inches but worse visible quality — the pixels get spread further apart on the paper. 150 is acceptable for draft work, 300 for anything you’ll actually hang on a wall.

Example: computing DPI from pixels and physical size

The third mode is the one you reach for when you are trying to figure out whether an image is high-enough resolution for a given print. You have a 2400 by 3000 pixel photo and want to print it 8 by 10 inches. Mode: “DPI”, plug in the numbers, and the answer is 300 DPI on both axes — perfect for photo printing. If the same source had to go 16 by 20 inches, the DPI would drop to 150, which is still acceptable for anything viewed from a couple of feet away but starts looking soft up close.

Screen DPI vs print DPI

These are related but measure different things:

  • Print DPI is ink dots per inch of paper. 300 is the standard for glossy photos; 600 and above are for high-end fine-art prints.
  • Screen PPI is image pixels per inch of screen. A 27-inch 4K monitor is roughly 163 PPI; a standard 1080p laptop is around 100; a phone is 300 to 500.

The web platform has a convention that 1 CSS pixel = 1/96 of an inch at typical viewing distance. That is where the common “96 DPI for web images” figure comes from — but it is a layout convention, not a measurement of any real screen. If you want an image to look sharp on retina displays, source it at 2x the nominal size and let CSS scale it down — the image dimension calculator gives you the exact dimensions for the 2× source.

Why does my image show different DPI on each axis?

If the pixel aspect ratio does not match the physical aspect ratio — for instance, a 1200 by 1600 image printed 4 by 6 inches — the calculator reports split values on the two axes. One axis comes out 300 DPI, the other comes out roughly 267 DPI. The mismatch usually means someone has cropped or scaled one dimension independently of the other. Print drivers typically ignore it and use whichever axis matches the paper orientation, but the tool shows both so you know what is going on.

What this tool does not do

It does not resize or rescale actual image files — it is purely a calculator for the numbers. To change the pixel dimensions of a file you need an image editor, a browser canvas, or a command-line tool like ImageMagick. For aspect-preserving math on arbitrary pairs (not just against a physical print), the aspect ratio calculator is the simpler tool. And it does not model printer-specific behaviour (halftone screens, dot gain, or the difference between addressable and perceptual resolution), which matters for high-end prepress work but not for anything a calculator can cover in a page of math.

Frequently asked questions

What is the difference between DPI and PPI?

DPI (dots per inch) is a printing term — the number of ink dots a printer lays down per inch of paper. PPI (pixels per inch) is a display term — the number of image pixels per inch of screen or target surface. They mean different things physically but the math is identical, so any DPI calculator is also a PPI calculator. In practice everyone uses the terms interchangeably and this tool treats them as the same thing.

What is a good DPI for photo printing?

300 DPI is the traditional standard for glossy photo prints, which is what most consumer and prosumer photo printers target. Fine art prints sometimes go to 600 DPI for extra sharpness on detailed work, but the visible difference is small at normal viewing distances. For draft prints and things you will not look at closely, 150 DPI is fine. Below 150 the individual pixels start becoming visible to the naked eye.

Why is my screen only 96 DPI?

Because that is a historical Windows default that most of the web still assumes. Actual modern screens vary wildly — a phone might be 400 PPI, a standard desktop monitor around 100, a retina laptop around 220, and a 4K monitor around 140 to 160 depending on size. The 96 DPI figure is a convention for computing layout sizes in CSS, not a real measurement of any particular screen. Use the actual pixel dimensions of your screen for real calculations.

How do I know how many pixels I need for a given print size?

Multiply the physical size by the target DPI. A 4 by 6 inch photo at 300 DPI needs 1200 by 1800 pixels. An A4 poster at 150 DPI needs roughly 1240 by 1754 pixels. This calculator does the math for you — pick 'solve for pixels' and enter the physical size and DPI.

My image has different DPI on each axis. Is that normal?

It means the aspect ratio of the pixel grid does not match the aspect ratio of the physical size. This happens when someone has cropped or scaled one dimension independently — either deliberately to fit a specific slot, or by accident when resizing. Most photo printers assume the two are the same and will just use whichever one matches the paper orientation. The calculator reports the split DPI separately so you can see the mismatch.