How PPI Is Calculated
PPI (pixels per inch) is the number of pixels along one inch of the screen. It’s calculated from the resolution and the diagonal size:
PPI = √(width² + height²) ÷ diagonal in inches
For a 27-inch 2560 × 1440 monitor: √(2560² + 1440²) = 2937 pixels across the diagonal, ÷ 27 = 108.8 PPI.
Dot pitch is the reverse — the distance from one pixel to the next, in millimetres. Smaller is sharper.
PPI of Common Screens
| Screen | Resolution | PPI |
|---|---|---|
| 24” 1080p monitor | 1920 × 1080 | 92 |
| 27” 1440p monitor | 2560 × 1440 | 109 |
| 34” ultrawide | 3440 × 1440 | 110 |
| 27” 4K monitor | 3840 × 2160 | 163 |
| 32” 4K monitor | 3840 × 2160 | 138 |
| 15.6” 1080p laptop | 1920 × 1080 | 141 |
| Typical modern phone | ~1080 × 2400 | 400–500 |
Viewing Distance and Visible Pixels
Whether you can see individual pixels depends on PPI and how far away you sit. A person with 20/20 vision can resolve detail about 1 arcminute in size. The pixels invisible beyond result shows the distance past which you can’t make out separate pixels — what Apple calls a “Retina” display.
That’s why a 400 PPI phone held at 30 cm and a 110 PPI monitor at 80 cm can both look perfectly sharp.
Choosing a Monitor Size and Resolution
- 24 inches: 1080p is fine; 1440p is noticeably sharper.
- 27 inches: 1440p is the sweet spot for gaming; 4K is excellent for text and photo work.
- 32 inches: 4K is recommended; 1440p looks soft up close.
- Ultrawide 34 inches: 3440 × 1440 gives similar sharpness to a 27-inch 1440p.
Higher PPI makes text sharper but needs more graphics power in games and often needs scaling (125–150%) in your operating system so text isn’t tiny.
Check the Screen Itself
Once you’ve chosen a monitor, check it for defects with the dead pixel test and confirm it runs at full speed with the refresh rate test.
Frequently Asked Questions
What Is a Good PPI for a Monitor?
For a desktop monitor at about 60–70 cm, around 90–110 PPI looks sharp with normal scaling, and 140+ PPI looks very crisp. That's why 27-inch monitors are popular at 1440p (109 PPI) and 4K (163 PPI).
What Is the Difference Between PPI and DPI?
PPI (pixels per inch) describes screens. DPI (dots per inch) originally describes printers, and is also used for mouse sensitivity. People often say DPI for screens, but PPI is the correct term.
Is Higher PPI Always Better?
Only up to the point your eyes can see. Beyond the "pixels invisible beyond" distance shown above, extra pixels don't make the image look sharper — though they still give you more workspace or need more scaling.
How Do I Find My Screen's Resolution?
Press the "Use my screen's resolution" button. It reads your display's native resolution from the browser. Then enter the diagonal size, usually printed in the model name or specs.
Why Doesn't 3440 × 1440 Show as 21:9?
21:9 is a marketing label. The exact ratio of 3440 × 1440 reduces to 43:18, which is about 2.39:1 — close to 21:9 (2.33:1), but not identical.
Related Tests
- Dead Pixel TestFill your screen with solid colours one at a time to spot dead, stuck or hot pixels.
- Monitor Refresh Rate TestFind out how many times per second your screen is really refreshing — and whether your high-refresh monitor is stuck at 60 Hz.
- Monitor Ghosting TestWatch blocks move across three backgrounds and look for trails, blur or bright halos behind them.
Last updated · Part of our display tests.