Almost every mower has a lever marked with numbers, usually one to five or one to seven, and almost everyone assumes a higher number means a taller cut. That part is true, but the numbers themselves tell you nothing useful about how short your grass actually ends up. A “3” on one mower can leave the lawn at a completely different height than a “3” on another, because the numbers are simply lever positions chosen by the manufacturer, not a measurement. If you want to cut at the right height for the season, and avoid the scalping that turns a lawn brown, you need to know the real cutting height in millimetres, and the only reliable way to get it is to measure.
The Number Is a Position, Not a Measurement
The height lever on a mower moves the deck, or the wheels relative to the deck, through a series of fixed notches. The numbers printed beside those notches are just labels for each position, and there is no industry standard linking a given number to a given height. One brand may space its settings between 20 and 70 millimetres, another between 25 and 80, and the gaps between positions are not always even. So the moment you switch from one mower to another, the number you trusted becomes meaningless. Set your new mower to the position you always used on the old one and you can easily end up cutting far shorter, or far longer, than you intended.
Some manufacturers do publish the actual heights, and it is worth finding yours. The Bosch Rotak 34, for example, offers eleven settings between 20 and 70 millimetres, the smaller Rotak 32 runs from 20 to 60 millimetres, and the basic Rotak 320 has just three positions, with setting one at 20 millimetres, two at 40 and three at 60. Notice that even within one family of mowers the same number means different things. If your manual lists the heights, write them on a label and stick it to the mower. If it does not, you measure them yourself. The figure that counts is the height of cut, defined as the distance from the tip of the blade down to the ground with the engine switched off.
How to Measure Your Real Cutting Height
Measuring takes five minutes and turns the guesswork into a chart you can rely on. First, make the mower safe. Park it on a hard, flat surface such as a patio or a path, switch it off, and on a petrol machine pull the spark plug cap so it cannot start, or remove the battery and the safety key on a cordless one. Never put your hand near the blade until you have done this. Then set the lever to its lowest position and use a tape measure or steel rule to measure from the ground up to the lowest point of the cutting blade. Write down the number, move the lever up one notch, and measure again, working through every position.
You now have your own height chart: position one equals so many millimetres, position two so many, and so on. On a rotary mower the grass will actually finish a few millimetres taller than the blade height, because the blades spring back up after the cut, but the measured figure is close enough to work from. One detail catches a lot of people out. On some mowers each wheel or each axle is adjusted separately rather than by a single central lever, and if the front and back, or left and right, are set to different positions, the deck sits at an angle and the mower scalps on one side. Always confirm that every wheel is on the same setting before you start.
What Height to Actually Choose
Most mowers cover a range of roughly 25 to 100 millimetres (1 to 4 inches), and where you sit within that depends on the lawn and the season. A hard-wearing family or utility lawn is happiest cut at 25 to 40 millimetres (1 to 1.5 inches). A fine ornamental lawn can go shorter, down to 12 to 25 millimetres, but only if you are prepared to mow it often and feed it well. The change that catches most people out is summer. As the weather warms, you should raise the cut to around 40 to 65 millimetres (1.5 to 2.5 inches), and higher still in shade.
The reason is biology. Grass produces its energy through photosynthesis in the leaf, so a taller blade means more leaf area, which fuels a deeper, stronger root system. Deep roots reach moisture far below the surface, which is what keeps a lawn green through a dry spell. Taller grass also shades the soil, keeping it cooler and slowing evaporation, and it casts enough shade to suppress weed seeds that need light to germinate. Cut short in the heat and you do the opposite: you remove the leaf the plant needs, expose the soil to baking sun, and force the lawn to brown. Our guide to how high to cut your lawn in summer to keep it green through a drought goes into the seasonal detail.
The One-Third Rule and Common Mistakes
Whatever number you settle on, one rule overrides it: never remove more than a third of the grass’s height in a single cut. If the lawn has grown long while you were away, do not drop straight to your usual setting, because shearing off half or more of each blade in one pass forces the plant to pull on its root reserves to regrow, which weakens it, thins the sward and opens gaps for weeds. Instead, cut on a high setting first, wait a few days, then lower the lever a notch and cut again, stepping the lawn down over two or three mowings until it is back to the height you want.
Cylinder mowers, the type that shear grass against a fixed bottom blade, set their height differently again. Rather than a notched lever, they are usually adjusted by raising or lowering a front roller, and the height of cut is the gap between the bottom blade and the ground once that roller is set. The same principle still applies: the markings are a guide, and the only certainty comes from measuring the finished gap on a flat surface. Whatever the machine, it helps to keep a short written chart of which position gives which height taped inside the shed door, with a note of the height you use in each season. That way you are not re-measuring every spring, and anyone else who mows cannot quietly reset it to the wrong number. A cheap habit that saves a lot of brown grass is to walk the lawn before the first cut after a holiday and pick the setting from how long the grass is now, not from wherever the lever happened to be left last time.
The mistakes that cause the most damage all trace back to the numbers. Trusting the same number across two different mowers leads to accidental scalping the first time you use a new machine. Leaving one wheel or axle on the wrong setting tilts the deck and shaves a stripe to the soil. Cranking the lever to its lowest position for a tidy, close finish feels satisfying but scalps every high point on an uneven lawn, leaving the pale, bare patches that moss and weeds quickly colonise, as our guide to what causes a mower to scalp the lawn explains. And forgetting to raise the height for summer is the single most common reason a lawn that looked fine in spring turns brown by July. Mowing permanently too high has its own downside, encouraging a soft, floppy sward and a build-up of thatch, so the goal is not simply high or low but the right height for the season, set by measurement rather than by a number you assumed you understood.
