Nearly every garden calculation starts with one number: how much ground there is. Seed rates, mulch depths, fertiliser doses, turf quantities and edging lengths are all quoted per square metre, and every one of them is wrong if the area underneath is wrong. This draws the shape as you describe it, works the area out with the measurements on the plan, and then separates what you can actually plant from what has to stay as path.
The ground you have
Pick the shape that matches your plot and the measurements it asks for change.
Fine tuning
Drag this up if the plot has permanent paths, a shed base or wide borders.
Result
Most plots are not rectangles, and pretending they are costs money
The standard advice — measure the length, measure the width, multiply — works on about a third of real gardens. The rest have a return along one side, a bay window projecting into them, a corner taken by a garage, or a boundary that follows a fence line nobody has surveyed since 1974. On those, multiplying the two longest sides overstates the area, sometimes by a quarter, and every quantity you then buy is a quarter too much.
It is worth saying what that costs. Over-ordering topsoil, mulch or turf is not a rounding error: it is a delivery you have to store, shift and dispose of, and on bark mulch in particular the surplus goes slimy before you find a use for it. Under-ordering is worse, because the second delivery carries the same haulage charge as the first and arrives after you have already started. The area figure is the one input every downstream calculation depends on, so it is the one worth getting properly.
Pick the shape first, then measure
Deciding which shape you are dealing with is the actual work; the arithmetic afterwards is trivial. What you are doing is choosing the decomposition that wastes the least ground, which usually means the one with the fewest pieces rather than the one that looks closest.
Four square corners
- Two measurements only: width and length.
- Check the diagonals are equal before trusting it.
- Most suburban plots are this, or close enough.
One corner missing
- Measure the full outline, then the notch that is cut out of it.
- Area is the outline rectangle minus the notch.
- The boundary length is unchanged by the notch, which surprises people.
Round bed or lawn
- Take two diameters at right angles and use the smaller.
- Area is overestimated by using a radius measured to the outside of an edging.
An L shape has exactly the same perimeter as the rectangle it was cut from. Cutting a notch removes the two outer edges but adds the two inner ones, and they are the same total length. So edging, fencing and skirting quantities stay at the outer-rectangle figure even though the area drops.
This is the single most useful thing to know when ordering edging for an awkward bed: measure the outline, not the outline minus the corners.
How to measure ground that fights back
A tape measure across a flower bed in June is not a measurement, it is a negotiation with whatever is growing. What you want is the distance between two fixed points, taken on a line that clears the planting, and the honest way to get that is with string.
- Put in the fixed points firstDrive a cane at each corner of the area you are measuring. Measure cane to cane, never plant to plant. Canes do not move; foliage does.
- Run string along each edgeTaut string between canes gives you a straight reference and lets you measure in clear air above the plants rather than through them.
- Check for squareMeasure both diagonals of a rectangle. If they match within a couple of centimetres, the corners are square and length times width holds. If they do not, one corner is out and you should treat the plot as a taper or an L instead.
- Take the height square to the baseOn triangles and tapered plots the height must be measured at right angles to the base, not along a sloping side. Run the tape from the apex and swing it until the reading is smallest; that shortest reading is the perpendicular height.
- Write down the raw numbersDo not round as you go. Four measurements each rounded up by 5 cm compound into a noticeable error by the time they are multiplied.
Do
- Measure to fixed points: canes, corners of buildings, manhole covers, fence posts.
- Take each measurement twice and use the smaller, because a sagging tape reads long.
- Use the smaller of two diameters on anything claiming to be a circle.
- Work in metres with decimals rather than mixing metres and centimetres.
Avoid
- Measuring around the outside of an edging and calling it the growing area.
- Averaging two sides of a taper without checking they are the parallel ones.
- Using a pacing estimate — a pace varies by 15% between a wet lawn and a dry path.
- Assuming a plot is square because the house is.
What the shape does to the materials bill
Area on its own is an abstract figure. What you actually buy comes from it, and the relationship is not always linear, which is where people end up with a half-used pallet.
| Material | Per m² | Note |
|---|---|---|
| Mulch at 75 mm | 0.075 m³ | About 13 m² from a cubic metre, less once it settles |
| Topsoil at 150 mm | 0.15 m³ | A tonne covers roughly 3.5 m² at that depth |
| Turf | 1 m² | Order 5 to 10% over for cutting and shaping |
| Lawn seed, new lawn | 35 g | Half that rate for overseeding an existing sward |
| Granular fertiliser | 35 g | Typical NPK application, not a seasonal total |
| Paving slab, 600 mm | 2.8 units | Before cutting waste, which adds 10 to 15% |
| Gravel at 50 mm | 0.05 m³ | Roughly 90 kg per m² when compacted |
Once the area is known, the mulch volume calculator turns it into cubic metres and bags, and the raised bed soil calculator does the same for imported soil where the depth is greater than a border would take. Where the plot is being sown rather than planted, the seed quantity calculator uses the area directly, and the plant spacing calculator takes over when it is plugs going in rather than seed.
Paths, edges and the ground you cannot plant
The percentage you set aside for access is the difference between a plan that works and one that looks fine on paper. A plot with no path in it is a plot where you walk on the soil, which compacts it, which is the one soil problem that is genuinely hard to undo.
The 28% figure that comes out of a standard plot is not waste. It is the standing space, the barrow route, the mowing strip and the few centimetres along every edge where the soil is too dry to grow anything well. Skip it in the plan and you will reclaim it anyway, by walking a track across the bed you meant to keep intact.
A worked example, start to finish
Here is an L-shaped plot measured properly, from the first tape reading to the order.
- Outline firstCanes give an overall width of 10.4 m and an overall depth of 8.6 m. The outline rectangle is 89.4 m².
- Then the notchA garage takes the corner: 3.6 m across by 2.8 m deep, which is 10.1 m².
- Subtract89.4 − 10.1 = 79.3 m². Measuring the two longest sides instead would have given 89.4 m² — twelve square metres of ground that is under a garage.
- Take off accessAt 15% for paths and edges, 67.4 m² is plantable. That is roughly 23 beds of 1.2 × 2.4 m.
- Check the boundaryThe perimeter is 2 × (10.4 + 8.6) = 38 m, unchanged by the notch. That is the edging length, not 38 m minus anything.
Nearly every shortcut overstates area. Measuring to the outside of an edging, using the longest two sides, pacing instead of taping, rounding each dimension up — all of them push the answer the same way, and none of them push it down.
The practical consequence is that if you are going to be careless, be careless in the knowledge that you will over-order, and order the smaller delivery first. Topping up is cheaper than disposing.
Slopes, levels and why the plan area is not the surface area
Everything here works on plan area: the area you would see looking straight down from above. On sloping ground the real surface is larger, by the cosine of the slope angle. A 1 in 5 slope adds about 2% to the surface; a 1 in 3 slope adds about 5%. That matters for turf and for mulch, which drape over the actual surface, and it does not matter for seed rates or fertiliser, which are quoted per unit of plan area because rain and roots do not care about the extra few percent.
For anything steeper than about 1 in 8, measure the slope distance and multiply by the cosine to get the plan distance before you use it. Most people skip this, and most people are within 3%, which is inside the tolerance of everything else in the calculation.
Where the numbers stop being true
The five shapes cover the overwhelming majority of plots, but they are models. A boundary that curves, a plot with five sides, or ground where the levels change within it will not map cleanly onto any of them. In those cases the honest approach is to decompose: split the plot into pieces that each match one of the shapes, work them out separately, and add them. Three rectangles and a triangle is a perfectly good description of a real garden.
The path percentage is a judgement, not a measurement. If you have not decided the layout yet, treat the figure as provisional and re-run it once you have, because the access share is the number most likely to change.
Before you order anything off this number
- Check the diagonals on any rectangle; unequal diagonals mean it is a taper or an L.
- Measure to canes or fixed points rather than to plants or edging.
- Use the smaller of two diameters on a circle.
- Decide the access percentage from what has to travel through the plot, not from what looks tidy.
- If the plot decomposes into two shapes, work them separately and add.
- Sanity check against the boundary length: 40 m of edge on a plot you think is 20 m² means something is wrong.
How the arithmetic works
Rectangle: width × length. L shape: (overall width × overall depth) − (notch width × notch depth), with the boundary at 2 × (width + depth) because the notch adds back what it removes. Circle: π × (diameter ÷ 2)², boundary π × diameter. Triangle, taken as isosceles with the apex over the middle of the base: half × base × height, boundary base + twice the square root of (half base squared + height squared). Taper: (short side + long side) ÷ 2 × the perpendicular distance between them, with the sloping sides worked out the same way as the triangle.
Plantable area is total area × (1 − access share ÷ 100). Square feet are square metres × 10.7639. Beds are the plantable area divided by 2.88 m², which is a 1.2 × 2.4 m bed, rounded down because a part bed is still a bed you have to build.
Questions people ask
My plot has five sides. What do I do?
Split it. Nearly every five-sided plot is a rectangle with a triangle on one end, or an L with a corner chamfered. Work the pieces, add them, and check the result against a rough bounding rectangle: it should be smaller, but not dramatically smaller.
Should I measure to the inside or outside of the edging?
Inside for planting quantities, outside for edging quantities. They differ by the edging width all the way round, which on a 40 m boundary with 50 mm edging is about 2 m² — small, but it is the difference between a border that is fully mulched and one with a bare strip.
How accurate does this need to be?
Within 5% is plenty, because delivery quantities come in fixed units anyway: whole bags, whole rolls, whole tonnes. The purpose of the calculation is to pick the right number of units, not to produce a precise figure.
Why does the L shape not reduce the boundary length?
Because cutting a notch out of a corner removes two outer edges and creates two inner ones of exactly the same total length. The perimeter is identical to the full rectangle, which is why edging for an L-shaped bed costs the same as for the rectangle it came from.
Can I use this for a lawn, or is that different?
The geometry is identical. The lawn area calculator adds the two things a lawn specifically needs — an allowance for beds and obstacles inside it, and a mowing time — but the area underneath is worked out the same way.
What about a plot that changes shape between seasons?
Measure the permanent outline. Seasonal beds inside it are handled by the access share or by subtracting them, not by changing the shape. If a third of the plot is a summer border and a winter bare patch, it is still part of the area.