Lawn mower height settings and the one-third rule

The one-third rule

The RHS advice is to remove no more than one third of the grass height at any one cut. Cutting more than that stresses the grass, exposes yellow stems and slows recovery. If the lawn has grown long after a holiday or a wet spell, bring it down in stages over several cuts a few days apart.

Example: grass at 60 mm should be cut to no lower than 40 mm. At the next cut, a third of 40 mm takes it to about 27 mm. Our one-third rule tool works out the stages for your own heights in mm or inches.

Typical cutting heights through the year

These are general ranges for UK lawns, in line with widely published RHS guidance. Adjust for your grass, your garden and the weather.

• Raise the cut for the first mows of spring and lower it gradually.
• In drought, longer grass shades the soil and copes better, so raise the cut and mow less often.
• Grass in shade grows thinner, so leave it longer.
• Do not cut frozen or waterlogged grass.

What the height lever positions mean

Mowers show height in different ways. Some print the millimetre value by each notch. Others number the positions 1 to 5 or 1 to 6, with 1 usually the lowest. The only way to know what position 3 means on your mower is the manual or the listing, because the range differs between models.

For example, the Ryobi OLM1833B lists five cutting heights from 25 to 65 mm, the Greenworks 24V 33 cm mower lists five heights from 25 to 70 mm on a single lever, and the Husqvarna Aspire LC34 lists 30 to 70 mm. On a five step lever with a 25 to 65 mm range, each step is roughly 10 mm, but check the actual values in your manual.

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Measuring your actual cut

The number on the lever is measured from a flat hard surface. On a soft lawn, wheels sink a little and the real cut can be lower. To check, mow a strip, then stand a ruler in the grass and measure from the soil surface to the top of the cut blades. Adjust up or down a notch to hit your target.

If your lawn has bumps, a very low setting will scalp the high spots. Raise the cut until it clears them, and fill the hollows with a top dressing over time. Keep the blade sharp too: a dull blade tears grass at any height. See how to sharpen or replace mower blades.

1. 1

   Set the lever to your target and mow a test strip.

2. 2

   Measure from soil to cut tips in a few places.

3. 3

   Adjust one position at a time.

4. 4

   Note the setting that works for each season.

Note:Before you touch a blade, remove the battery or safety key and wait for the blade to stop completely. Wear sturdy gloves and eye protection, and keep children and pets away while you work. If a blade is bent, cracked or you are unsure, take it to a dealer or sharpening service.

Robot mower cutting height

Robot mowers cut little and often, so they remove only a few millimetres at a time and the one-third rule takes care of itself once the lawn is at height. Many models adjust height with a dial on the mower, and some through an app. Husqvarna lists the Automower 315 at 20 to 50 mm.

When you first install a robot, mow the lawn down to the robot's working range with a push mower in stages, then let the robot take over. Start at a high setting and lower it a step every week or so. In a hot dry spell, raise it again. Our robot lawn mower guide compares models, including their cutting ranges, and the cordless lawn mower guide covers push mowers.

Frequently asked questions

What is the best lawn mower height setting? For most family lawns, about 25 to 40 mm in summer and around 10 mm higher in spring, autumn, drought and shade. Fine ornamental lawns can be cut shorter, at 13 to 25 mm in summer.

Which number is the highest cut on my mower? On most mowers the highest number is the highest cut, but not on all. Check the manual or the markings by the lever.

What cutting height should I use on a Husqvarna Automower? Start high after installation and lower gradually to your target. Husqvarna lists the Automower 315 at 20 to 50 mm; other models have their own range in the manual.

How short should the first cut of spring be? Keep it high. Take off only the top of the grass, no more than a third, and lower the setting over the following weeks as growth picks up.