A lawn is quoted by the square metre and bought by the roll, the bag or the pallet, and both ends of that transaction go wrong when the area is wrong. This draws the lawn as you describe it, takes off whatever is inside it that is not grass — beds, tree pits, a path across the corner — and turns what is left into the two numbers you actually plan around: how much to order, and how long it will take to mow every week from April to October.
The grass
Choose the shape, then tell it what is inside the lawn that is not grass.
Fine tuning
Guess at first, then measure them properly once the layout is decided.
Result
Two numbers matter, and only one of them is on the quote
Suppliers sell turf by the roll and seed by the kilogram, and both are priced off an area you have to supply. Get it 10% high and you pay for grass you will be carting to the tip by the weekend, because turf is a perishable product with a shelf life measured in days. Get it 10% low and you patch, and a patched lawn shows its seams for a season.
The second number is the one nobody quotes and everybody lives with: how long the lawn takes to cut. It is the single largest recurring cost of owning grass, and it scales directly with area. A 100 m² lawn cut weekly from April to October is roughly thirty cuts a year; at ten minutes a cut that is five hours, which is a fair trade. At forty minutes a cut it is twenty hours, which is a different decision, and it is a decision best made before the turf goes down rather than after.
Take the shape off the ground, not off the plan
Garden plans lie. They are drawn from a survey that was itself a simplification, and they show the lawn as a clean rectangle with a bed at one end. The ground rarely agrees: the bed has grown, the corner the previous owner paved is bigger than the plan says, and the boundary fence bows. Measure what is there.
- Walk it with canesPut a cane at each change of direction. Four canes for a rectangle, six for an L, and as many as it takes for a lawn that wanders.
- Measure cane to caneTaut tape, read twice, take the smaller. A tape bowed around a border reads long by more than you would think.
- Check the diagonals on any rectangleEqual within a couple of centimetres means the corners are square. If they are not, treat it as an L or measure each triangle separately.
- Measure what is inside, not what is aroundBeds, tree pits, a manhole, the corner of a shed. Either subtract them individually or put them in as a percentage, which is quicker and good enough at this stage.
- Note the edge run separatelyEdging, strimming and the mowing strip are bought by the metre. That figure is the perimeter, not the area, and it does not shrink when a corner is taken out.
Why the L shape is the one that catches people out
An L-shaped lawn is the most common awkward case, and the error is always in the same direction. People measure the two longest sides because that is what fits in the notebook, multiply them, and order for the rectangle. The missing corner then gets turfed over, because turf is delivered in a quantity and the temptation is to use it.
Cutting a rectangle out of the corner of a lawn removes two outer edges and creates two inner ones of exactly the same total length. So the edging run for an L shape is the perimeter of the full outline, 2 × (width + depth), not that figure minus anything.
Practical consequence: order edging for the outline, order turf for the area. They are worked out from the same two measurements and they are not the same number.
| Deduction | Typical share | How to handle it |
|---|---|---|
| Beds cut into the lawn | 5 to 15% | Measure them individually if there are fewer than four |
| Tree pits and shrub circles | 2 to 5% | Radius squared times pi, one per tree |
| Path across a corner | 5 to 20% | Treat as a notch and use the L shape |
| Shed, greenhouse, play equipment | 3 to 10% | Footprint, measured to the outside |
| Steep bank left unmown | 5 to 15% | Subtract it; nobody mows a 1 in 3 slope weekly |
What the mowing time is really telling you
The mowing figure is not a stopwatch prediction; it is a comparator. It says what this lawn costs you per week relative to the alternatives, and that is how to use it. The formula is simple: the effective rate is the cut width times a realistic walking speed times an efficiency factor for turning, overlap and emptying the box.
The interesting thing about that table is how little the mower matters compared with the area. Going from an 18 in push mower to a 48 in tractor halves the mowing time; halving the lawn halves it too, and costs nothing. Most people who buy a bigger mower would have been better off shrinking the lawn and putting the difference into a border.
Do
- Measure the lawn as it will be, not as it is, if you are about to change the layout.
- Subtract obstacles properly rather than hoping the cutting waste covers them.
- Use the mowing time to compare layouts, not to schedule your Sunday.
- Keep the edge run in metres; it is a different purchase from the turf.
Avoid
- Ordering turf for the gross area when a sixth of it is beds.
- Assuming a ride-on mows a small lawn faster; turning time dominates under about 300 m².
- Forgetting that obstacles cost more time than area, because every bed means a turn.
- Measuring a bank as flat ground, then discovering it cannot be mown at all.
From area to what you buy
Once the net figure is settled, everything else follows. Turf is sold by the roll, typically covering about 1 m², and needs 5 to 10% over for cutting around edges and obstacles. Seed is sold by weight at a rate per square metre, and needs no cutting allowance but does need a realistic view of how much of it birds will eat.
The sod roll calculator turns this area into rolls and pallets including the cutting margin, and the fertiliser calculator works out the seasonal feed from the same figure. Where the lawn is being watered rather than left to the sky, the irrigation run time figure and the drip line length both start from square metres as well. And if the shape you are dealing with is a border rather than a lawn, the garden area calculator handles the tapered and triangular cases this one leaves out.
A worked example
A rectangle with a patio taken out of one corner, which is the most common real lawn there is.
- The outlineCanes give 14 m by 11 m, so the outline rectangle is 154 m².
- The patio cornerIt measures 5 m along the back edge by 4 m deep: 20 m² of paving that is never going to be grass.
- Net shape154 − 20 = 134 m². Measuring the two longest sides and stopping there would have given 154 m², which is fourteen rolls of turf too many.
- What is inside itTwo tree pits and a bed cut into the far corner come to about 10%, leaving 120.6 m² of grass to cut.
- What it costs in timeAt an 18 in cut, 120.6 ÷ 18.9 = 6 minutes. Thirty cuts a year is three hours, which is a lawn worth having.
The edge is the part that takes the time
Ask anyone with a lawn how long it takes and they will give you the mowing figure. Ask them how long the whole job takes and the answer roughly doubles, because the mower does not cut within about 50 mm of anything and everything inside that margin is done by hand with a strimmer or shears.
| Edge treatment | Maintenance | Note |
|---|---|---|
| Mown flush to paving | None | Needs the paving to be level with the soil, or the mower catches |
| Strimmed verge | Weekly in season | Fast, and scalps the grass if you are careless |
| Cut with shears | Weekly, slow | The best finish and the most time |
| Steel or plastic edging | Occasional | One purchase, removes the job almost entirely |
| Brick or setts on edge | Annual | Mow over it and there is nothing to trim |
That table is really an argument for hard edging. Forty metres of steel or plastic edging costs less than a season of strimmer line and removes the single most tedious job in the garden. It is also the one improvement that makes a lawn look deliberate rather than incidental, which is most of what people mean when they say a garden looks tidy.
What the season does to the figure
The mowing time here is a fair-weather number. It assumes dry grass at a height the mower is happy with, and neither of those holds for the whole season. Growth is not linear, and the weeks that matter are the ones at each end of it.
Cold soil, slow growth, and grass that is too long after winter. Take a third off and no more, or you scalp it.
This is the part of the year the area figure really describes: warm, wet, and grass that grows 30 mm a week.
Drought slows growth, which is the one thing a dry summer is good for. Raise the blades and leave the clippings.
Warm soil and autumn rain. Keep cutting while it is growing, because it will not stop until the first real cold.
Higher than the summer setting, leaves cleared, mower serviced and put away.
Multiply the weekly figure by about thirty and you have the season. It is a small enough number on a modest lawn that the whole argument for grass makes sense, and a large enough one on a big one that people start looking at alternatives — which is exactly what the figure is for.
Where the numbers stop being true
The mowing time assumes a clean run at walking pace on a shape that lets you work in parallel passes. Lawns full of obstacles take longer than their area suggests, because each obstacle costs a turn, a reverse and a pass with the strimmer afterwards. On a lawn with more than about eight separate obstacles, add a third to the time rather than trusting the arithmetic.
It also assumes the grass is being cut at a height the mower can cope with. Cutting a week of spring growth at one go halves the effective rate, because the mower clogs and the pass has to be slower. In April and May the honest figure is closer to two thirds of the rate used here.
Before you order
- Measure cane to cane, twice, and take the smaller reading.
- Check diagonals on any rectangle before trusting length times width.
- Decide whether the bank is mown or left; it changes the area more than anything else.
- Subtract obstacles, or put them in as a percentage and revisit once the layout is fixed.
- Order turf on the net area plus 10%, seed on the net area with no margin.
- Keep the edge run in metres as a separate line on the order.
How the arithmetic works
Rectangle: width × length. L shape: (overall width × overall depth) − (notch width × notch depth). Circle: π × (diameter ÷ 2)². Triangle, taken as isosceles: half × base × height. Net grass is the gross area × (1 − obstacle share ÷ 100).
Mowing time is net area ÷ (cut width × 42), where 42 is metres per minute of walking pace times a 70% efficiency factor for turning and overlap. Edging run is the perimeter: 2 × (width + depth) for a rectangle and for an L shape alike, π × diameter for a circle, and base plus twice the sloping side for a triangle.
Questions people ask
Should I order turf for the gross or the net area?
Net, plus 10%. The 10% is cutting waste around edges and obstacles, not a margin for bad measurement. On a lawn with curved beds, go to 15%: curves are cut with a knife and every cut leaves an offcut too small to use elsewhere.
How much difference does the mower really make?
Less than the marketing suggests. Doubling the cut width roughly halves the time on a large open lawn, and does almost nothing on a small one with obstacles, because turning dominates. Under about 300 m² the mower choice is marginal; the shape matters more.
My lawn is not any of these shapes.
Decompose it. Most awkward lawns are two rectangles, or a rectangle with a triangle on the end. Work each piece, add them, and check the total against a bounding rectangle: it should be a bit smaller, not dramatically smaller.
Do I measure to the inside or the outside of the edging?
Inside for grass, outside for edging materials. On a 40 m run with 50 mm edging that is about 2 m² of difference, which is two rolls — worth getting the right way round.
What about a bank or a verge?
Measure the plan distance, not the slope distance, for turf quantities — turf is sold flat and drapes, so it covers slightly more than the plan area. For mowing, assume the bank takes twice as long per square metre, or leave it and mow it twice a year.
How accurate does the obstacle percentage need to be?
Within a few percent. It exists to stop you ordering a sixth too much turf, and once the beds are actually marked out you can replace the guess with real measurements and re-run it.