Resolution is the most misunderstood word in digital images. People use it to mean three different things, and two of them don't matter.

Let me untangle it.

Pixels: the thing that actually matters

Resolution, properly, is pixel dimensions. A 4000 by 3000 image has 12 million pixels. More pixels means more detail, and more detail means you can print bigger or crop harder without the image falling apart.

For screens, pixels are the whole story. A 800-pixel-wide image on an 800-pixel-wide slot looks perfect. The same image stretched across a 4K monitor looks soft. Nothing else about "resolution" changes this.

DPI: mostly a label

DPI (dots per inch) is a printing instruction. It tells the printer how densely to pack the pixels. A 3000-pixel image at 300 DPI prints at 10 inches wide. The same image at 150 DPI prints at 20 inches.

Here's the part that trips people up: changing the DPI number without changing the pixels changes nothing about the image. It's a note attached to the file. The pixels are the image.

For print, the rule of thumb is 300 DPI for anything viewed up close (brochures, photos) and 150 DPI for things viewed at arm's length or more (posters). Below 100 DPI, up-close viewing gets crunchy.

Megapixels: marketing with a kernel of truth

Megapixels are just pixel count with a fancy name. A 12-megapixel camera makes 4000 by 3000 images. More megapixels help when you crop aggressively or print large.

For posting online, anything above 8 megapixels is overkill. Your screen can't show the difference, and the extra pixels just slow the upload.

Practical tip: before printing, do the division yourself. Take the pixel width, divide by the print width in inches. If the answer is above 200, it'll look good. A 4000-pixel image printed 13 inches wide gives you about 300. Math you can do on your phone.

The one-sentence version

Pixels are the image. DPI is a printing note. Megapixels are pixel count wearing a tuxedo.

Match your pixels to the job (screen size for screens, print size times 300 for print), and resolution stops being confusing. Jiggle X Wiggle's free converter shows you the dimensions of any image you upload, so you can check before you commit to a print.