When you buy a monitor, TV or laptop, the screen size is usually given as a number of inches. But what do 27", 32" or 15.6" actually mean in centimetres? And why isn't a bigger screen automatically sharper? This guide explains inches, cm, aspect ratio and PPI in plain terms. Check every value yourself with the Screen sizes.
Inches and centimetres: the diagonal
The inch figure always describes the screen diagonal – the distance from one corner to the opposite one. One inch equals exactly 2.54 cm. A 27-inch screen therefore has a diagonal of about 68.6 cm.
Important: the diagonal alone tells you nothing about width and height. How the diagonal splits into width and height depends on the aspect ratio.
Quick mental conversion
- Inches to cm: multiply the inch figure by 2.54
- cm to inches: divide the cm figure by 2.54
- Example: 15.6" × 2.54 = 39.6 cm diagonal (typical laptop)
Aspect ratio: calculating width and height
The aspect ratio states how width relates to height. Common formats are:
| Aspect ratio | Typical use |
|---|---|
| 16:9 | Standard for monitors and TVs |
| 16:10 | Productivity, more height |
| 21:9 | Ultrawide, gaming, multitasking |
| 4:3 | Older devices, presentations |
A 27-inch screen in 16:9 format measures about 59.8 cm wide and 33.6 cm high. With the same diagonal but a 21:9 format, the screen would be noticeably wider and flatter. That is why it pays to work out the real dimensions before buying – will the screen actually fit on your desk or into your furniture?
PPI: how sharp is the image really?
PPI stands for pixels per inch and describes pixel density. The higher the PPI, the finer and sharper the image looks, because the individual pixels are smaller and harder to see. The formula is:
PPI = √(width_px² + height_px²) ÷ diagonal_inches
An example makes the difference clear:
| Screen | Resolution | PPI |
|---|---|---|
| 27" 4K | 3840 × 2160 | approx. 163 |
| 27" Full HD | 1920 × 1080 | approx. 82 |
Both screens are the same size, but the 4K monitor has almost double the pixel density and renders text and edges far more sharply. Work out your own combination of resolution and size with the Pixel DPI calculator.
Viewing distance matters too
Whether you can even see individual pixels depends on the viewing distance. From further away the pixels merge for the eye – so even a lower PPI can look sharp. The often-mentioned "Retina" impression kicks in above a certain PPI depending on distance: at a desk (about 50–70 cm) high PPI values are noticeable, whereas for a TV viewed from several metres a lower pixel density is enough for a sharp image.
Practical buying tips
A large desktop monitor is not automatically sharper. If only the diagonal grows without the resolution increasing, the PPI drops and the image becomes coarser. So always look at the interplay of size, resolution and PPI.
- Check the diagonal in cm: does the screen fit the space you have?
- Choose the aspect ratio: 16:9 as an all-rounder, 21:9 for multitasking and gaming, 16:10 for more vertical working area.
- Compare PPI: for sharp text at a desk, higher values are clearly better.
- Factor in viewing distance: the closer you sit, the more a high pixel density pays off.
Once you understand these four figures, you can buy on purpose instead of by inch count alone – and avoid bad purchases. Use the Screen sizes and the pixel-DPI calculator to run the numbers.
