Someone asked me last month why their "300 DPI" logo still looked blurry on their website, and the honest answer is that DPI mostly doesn't matter for screens at all. It's a print concept that got dragged into web conversations where it doesn't really apply the same way, and that's caused a fair amount of confusion over the years.
What DPI is actually measuring
DPI stands for dots per inch. It describes how many printed dots a printer packs into each inch of paper, which makes it a print-specific measurement because it only means anything relative to a physical, inch-based surface. 300 DPI is the standard for sharp, professional print output; drop below roughly 150 DPI and things start to look visibly soft once they're on paper.
Why it doesn't carry over to screens
A screen doesn't really have "inches" in any meaningful sense here — it has pixels, and how large those pixels appear depends on the device's own pixel density plus your browser's zoom level. An image that's technically "300 DPI" but only 300 pixels wide will still look tiny, and possibly blurry, on a modern high-resolution display, because DPI was never actually the thing controlling sharpness on screen. Pixel dimensions were doing that job the whole time.
For web use, it's honestly easiest to forget DPI exists entirely and just think in pixels. A 1200-pixel-wide image looks identical whether its file metadata claims 72 DPI or 300 DPI.
What actually matters, for each case
For print: multiply your target size in inches by 300 to get the pixel dimensions you need. An 8×10 inch print at 300 DPI needs a source image roughly 2400 × 3000 pixels. Go lower and you'll see softness on paper, especially under good lighting.
For web: match your pixel dimensions to how large the image actually displays, keeping in mind that a lot of modern screens pack more physical pixels into the same space (often called "retina" or high-DPI displays). A decent rule of thumb is exporting at roughly 1.5–2× the display size to stay crisp on those screens, without going so large you're wasting bandwidth on detail nobody can perceive anyway.
If a product image displays at 600px wide on your site, exporting at 1200px wide covers high-density screens nicely while still compressing down small — usually well under 200 KB for a typical product photo at a reasonable JPEG quality.
Prepping something for a printer? Care about DPI and physical inches. Prepping something for a website, app, or social platform? Think entirely in pixels and let the DPI metadata sit there unused — it isn't doing anything for you in that context anyway. ImgReduce's dimension mode works in pixels for exactly this reason.