The short answer
Higher than most people do. Around 4cm for an ordinary family lawn, and never remove more than a third of the blade height in one cut.
- Set the mower higher than feels right. Roughly 4cm for a normal lawn, 2.5cm only for a fine ornamental one that is fed and watered.
- Cut little and often in the growing season, weekly or more, rather than occasionally and hard.
- Never take more than a third of the blade in one pass. If it has got long, cut it in two or three stages over a fortnight.
- Raise the height further in hot dry spells and in shade.
- Vary the direction each time, which stops the grass leaning and stops ruts forming.
What will ruin it
Scalping a lawn, cutting it very short in one go, is what turns a tired lawn into a brown one full of weeds. The bare soil is an invitation to every weed seed in the neighbourhood.
Why it works
Grass photosynthesises with its leaves, and root depth roughly tracks leaf height. A taller cut means deeper roots, better drought tolerance and enough shade at soil level to suppress weed germination.
Tipking is general household information, not professional advice. Test anything on a hidden patch first. Full disclaimer.