What the checker shows
The tool reports several distinct measurements that are easy to confuse. Your screen resolution is the full size of your display in CSS pixels, while the viewport is the smaller area your browser window actually occupies. The device pixel ratio reveals how many physical pixels back each CSS pixel — 2× or 3× on high-density “Retina” screens — which is why a modern phone reports a modest CSS resolution yet renders razor-sharp. It also shows color depth (bits per pixel) and current orientation. Resize your window and hit Refresh to watch the viewport figures change in real time.
Why device pixel ratio matters
Device pixel ratio is the single most misunderstood display number. A phone advertised with a 1170×2532 physical resolution reports about 390×844 CSS pixels because it has a 3× ratio — the browser scales everything up so text and layouts are legible rather than microscopic. For developers, this is essential: it explains why you should supply 2× and 3× images for crisp graphics, and why layout breakpoints are based on CSS pixels, not the marketing megapixel count. Seeing your own ratio makes the concept concrete.
The PPI calculator
PPI (pixels per inch), often called DPI, measures pixel density: how tightly packed the pixels are on a physical screen. The calculator takes a resolution and the diagonal size in inches, applies the Pythagorean theorem to find the diagonal in pixels, and divides by the inches to get PPI. Higher PPI means sharper images and smoother text. As a rule of thumb, around 300 PPI at normal viewing distance is where individual pixels become invisible to most people — the threshold Apple marketed as “Retina” — while typical desktop monitors sit near 90–110 PPI.
Common uses
Web developers and designers use it to test how their site sees a device, to debug responsive layouts, and to confirm the pixel ratio they are designing for. Shoppers compare the sharpness of two monitors or laptops before buying by computing each one's PPI. QA teams record the exact viewport of a device where a bug appears. It is also simply a quick way to answer “what resolution am I running?” without digging through system settings.
Privacy notes
Every value is read directly from your browser's standard properties and displayed on the page. Nothing about your device — not the resolution, not the pixel ratio, not anything — is sent to a server, stored, or logged. The detection is entirely local, so you can check your display specs with complete privacy, and the tool keeps working offline once loaded.