How to size a monitor
Diagonal is only one number
Monitor diagonal is the screen size, but width and height depend on aspect ratio. A 27″ 16:9 monitor is about 23.5″×13.2″. A 34″ 21:9 ultrawide is about 32″×13.4″ — same height, much wider. Picking by ratio matters more than diagonal for desk fit.
Viewing distance
Optimal monitor distance: arm's length for most desks (about 20–28″). Eyes should sit level with the top quarter of the screen. Curved monitors above 30″ benefit from being closer (a 1500R curve targets ~1.5 m / 5 ft); flat monitors don't care about exact distance within the comfort range.
Resolution pairing
Pixel density (PPI) drives sharpness. 1080p (FHD) is fine up to ~24″; beyond that, individual pixels become visible. 1440p (QHD) is the sweet spot for 27″. 4K (UHD) is needed at 32″+ for the same crispness. Going 4K at 24″ is technically nicer but wastes resolution most users won't see.
Common mistakes
- Sizing by diagonal alone without checking aspect ratio — a 27″ 16:9 has a different width than a 27″ 21:9.
- Ignoring viewing distance — bigger isn't always better at close range.
- Pairing high resolution with too-small a diagonal — 4K under 24″ gets pixel-density returns most users can't see.