Pixels, DPI, PPI, megapixels — resolution numbers are quoted everywhere and understood almost nowhere. Learn what they actually mean for screens, prints, and file sizes.
By Ernzi Tools team · November 7, 2026
Resolution is one of the most quoted and least understood numbers in photography. A camera boasts 48 megapixels, a print shop asks for 300 DPI, a website wants images "at least 1200 pixels wide" — and these numbers all describe different things. The confusion comes from mixing up three separate ideas: how many pixels an image has, how densely those pixels are packed, and how big the image appears. Separate them, and every resolution number becomes straightforward.
A digital image is a grid of pixels, and its resolution is simply the size of that grid — for example, 4000 × 3000 pixels, or 12 million pixels in total. A "megapixel" is just a million pixels, so a 12-megapixel photo is 12 million pixels. This number is real and absolute: it does not depend on the screen, the printer, or any setting. It decides how much detail the image holds and how large it can be displayed or printed before it starts to look soft. More pixels means more detail, but also a bigger file — a 12-megapixel photo carries roughly four times the data of a 3-megapixel one.
DPI (dots per inch) and PPI (pixels per inch) describe density: how many pixels are squeezed into one inch of physical space. The terms are often used interchangeably, though strictly DPI belongs to printers and PPI to screens. Here is the key point: DPI is not a property of the image itself, it is an instruction about how big to print it. A 3000 × 2000 pixel image printed at 300 DPI comes out 10 × 6.7 inches; the same image printed at 150 DPI comes out 20 × 13.3 inches, with the same pixels spread over twice the area. The image has not changed — only the print size has.
This is why changing the DPI number in an image file without changing the pixel count does nothing to the image. It only changes the default print size recorded in the file's metadata. An image that is 1000 × 1000 pixels at 72 DPI and the same image at 300 DPI contain exactly the same pixels and look identical on screen. The DPI value only matters at the moment of printing.
A useful formula: the pixels you need equal the print size in inches multiplied by the DPI. An 8 × 10 inch print at 300 DPI needs 2400 × 3000 pixels. If your image has fewer pixels than that, you have two honest choices: print smaller, or accept a lower DPI. Upscaling the image to invent pixels does not add real detail.
For the web, the only resolution number that matters is pixel dimensions. A 4000-pixel-wide photo displayed at 800 pixels wide is carrying five times more data than anyone will see, and it slows the page down for no benefit. Resize images to the size they will actually be shown — typically no more than 1600–2000 pixels wide for a full-width web image, and 800–1200 for a content image — and the file shrinks dramatically. Retina and high-density screens complicate this slightly: they pack two or four physical pixels into each display pixel, so an image shown at 800 display pixels wide looks sharpest at 1600 actual pixels. A good compromise is to serve images at 1.5–2× the display size, and no larger.
The most common mistake is judging an image by its DPI number instead of its pixel dimensions — a "300 DPI" image that is only 300 × 300 pixels is still a tiny image, and it will print at one inch wide. The second is upscaling a small image to a large print size and expecting it to look sharp; it will look soft, because the detail was never captured. The third is saving everything at maximum resolution "just in case": it wastes storage, slows uploads, and makes web pages sluggish, all for pixels nobody will ever see.
Resolution is just a count of pixels, DPI is just a print instruction, and the right size is the size the image will actually be used at. Keep those three ideas separate, and the numbers stop being confusing.
All tools are free and run entirely in your browser — your files never leave your device.