How to Find Your Screen's True Resolution and Calculate Its Real PPI

If you're designing precise interface elements — a floating screen-overlay utility for Android, a pixel-perfect Tailwind dashboard, anything where physical size matters — you can't rely on the sticker on the back of your monitor. The actual hardware resolution and the one your OS thinks you have can be wildly different once scaling gets involved.

Here's how to find what your machine is actually outputting, on each major platform, and how to turn that into your display's true PPI.

Windows 10/11

Windows scales the UI to make it readable on high-resolution displays. A 4K monitor capable of 3840×2160 might be having its UI rendered as if it were a much lower resolution, purely to make text bigger.

1. Right-click the desktop and choose Display settings.
2. Scroll down to the Display resolution section.
3. Check the Display resolution dropdown — this is the actual resolution Windows is outputting to the panel.
4. Check the Scale percentage just above it. If it's set above 100%, Windows is enlarging UI elements mathematically, and you're not seeing your full native pixel grid.

macOS

Apple has long hidden the true resolution behind its scaling options. By default, the "More Space" setting can make the display behave as if it has a different resolution than its native panel, in exchange for sharper text.

1. Click the Apple logo (top-left corner) and open System Settings (System Preferences on older macOS).
2. Go to Displays.
3. You'll see a row of options — typically Larger Text, Default, More Space, and sometimes a fourth tier depending on your screen.
4. Hold the Option key and click the currently selected option's disclosure control to reveal the raw resolution values behind each tier.

Linux

Whether it's a headless server or a GNOME desktop, the terminal is the most reliable source of truth for hardware specifics.

GUI method

Open Settings → Displays. This shows your current resolution and scaling factor directly.

X11 method

In a terminal, run:

xrandr | grep '\*'

The line marked with an asterisk shows the resolution and refresh rate currently in use.

Wayland method

If your compositor is wlroots-based (Sway and similar), wlr-randr will report display modes directly. Otherwise, you can read the kernel's own mode list:

cat /sys/class/drm/*/modes

This lists the supported modes for every connected output without needing to already know your connector's name.

Calculating True PPI

Your OS will tell you the resolution it's using, but not the true hardware pixel density. For that, you measure your screen's physical diagonal and apply the Pythagorean theorem.

Worked example, for a standard 24-inch monitor at 1920×1080:

1. Square the horizontal and vertical resolution: 1920² = 3,686,400 and 1080² = 1,166,400.
2. Add them together: 3,686,400 + 1,166,400 = 4,852,800.
3. Take the square root to get the diagonal pixel count: √4,852,800 ≈ 2202.9 pixels.
4. Divide by the screen's physical diagonal in inches: 2202.9 ÷ 24 ≈ 91.79 PPI.

That's a raw hardware figure, free of any OS scaling interference — the same 96 PPI-vs-reality gap that makes CSS units unreliable for on-screen measurement in the first place.