What you are ordering, and in what units
A turf job is really three orders stacked on top of one another:
- Turf rolls — sold by the roll, and a roll is not a standard unit. The turf calculator defaults to 1 m² per roll.
- Screened topsoil — only needed if you are raising the level, sold by the tonne and in 850 kg bulk bags.
- Starter fertiliser — optional, sized at 35 g per square metre if you tick the box.
The area is the easy part. The two numbers that decide the invoice are the roll size and the cutting allowance, so start there.
Rolls: check the area, not the name
Most UK turf is cut into 1 m² rolls, which is why the calculator's roll size box defaults to 1. But growers also supply 1.2 m² rolls, and a few sell large 2 m² rolls that genuinely need two people to lift and lay. The area printed on the roll is what you divide by — the word "roll" tells you nothing on its own.
That difference is not academic. A lawn of 102.5 m², with the 5% cutting allowance added, is 107.6 m² of turf. At 1 m² a roll that is 108 rolls. At 1.2 m² a roll the same lawn takes 90 rolls. Same grass, same lawn, a very different delivery.
The cutting allowance
Turf covers the rectangle; the lawn is not a rectangle. Every edge gets trimmed, every corner is cut around, and curved or irregular boundaries waste more than straight ones. That is what the cutting allowance is for, and the calculator's default is 5%.
Five per cent is the right figure for a lawn with straight edges and a couple of corners. For a kidney-shaped bed, a lawn with a lot of planting to work around, or an awkward narrow strip, raise it. Dropping it to zero only makes sense if the shape you are covering is already the shape you are laying, which on a real garden is rare.
Worked example: a 12.5 m × 8.2 m lawn
Take a lawn 12.5 m long and 8.2 m wide, laid in 1 m² rolls, with the calculator's standard 5% cutting allowance. Feeding those figures into the turf calculator returns exactly this:
| Item | Value |
|---|---|
| Lawn length | 12.50 m |
| Lawn width | 8.20 m |
| Roll size | 1 m² per roll |
| Waste allowance | 5% |
| Lawn area | 102.5 m² |
| With waste allowance | 107.6 m² |
| Roll size | 1 m² per roll |
| Turf rolls needed | 108 rolls |
The arithmetic is deliberately blunt: 107.6 m² ÷ 1 m² = 107.6, rounded up to 108 because a part roll is a whole roll. Change the roll size box to 1.2 and the same inputs return 90 rolls.
Preparing the ground before the turf arrives
Turf roots into whatever is beneath it, so the surface it lands on decides how it performs. Strip the old turf or vegetation, then dig or rotavate the top 100–150 mm and take out stones, roots and any builders' debris. Firm the whole area by treading it in a grid pattern, then rake it level.
Leave it a few days to settle and give it a final light rake and firm before laying. The one thing not to do is tip loose topsoil and lay turf straight onto it without firming — the turf sinks unevenly, the joins open, and the lawn is lumpy within a month.
Topsoil: the dressing and the settlement allowance
Turf itself needs only a fine, firm surface, so if the ground is already at the right level a 10–20 mm dressing is all that is needed to true it up. Where you are building the level up properly, work on 100–150 mm of good topsoil over the subsoil.
Two calculators touch this, and they agree because they share the same density. The turf calculator converts a dressing at 1.5 tonnes per cubic metre and shows the result in tonnes and 850 kg bulk bags. The topsoil calculator uses the same 1.5 tonnes per cubic metre, but adds a settlement allowance — 10% by default — because loose topsoil is full of air and slumps once it is watered and walked on.
Worked example: a 20 mm dressing over 100 m²
Take a 10 m × 10 m area dressing up 20 mm. Both calculators are given the same job:
| Calculator and inputs | Result |
|---|---|
| Turf calculator — 10 m × 10 m, 1 m² rolls, 5% cutting allowance, topsoil depth 2 cm, starter fertiliser ticked | 105 rolls, 3.00 tonnes of topsoil (4 × 850 kg bulk bags), 3.5 kg of fertiliser |
| Topsoil calculator — 10 m × 10 m, 20 mm, settlement 0% | 3.00 tonnes, 2.00 m³, 4 × 850 kg bulk bags |
| Topsoil calculator — the same job with the default 10% settlement | 3.30 tonnes, 2.20 m³, 4 × 850 kg bulk bags |
The first two rows are the same job seen through two calculators, and they land on the same 3.00 tonnes and four bulk bags — which is what you would expect, because both weight topsoil at 1.5 tonnes per cubic metre. The difference in the third row is the settlement allowance alone: 3.00 tonnes with none, 3.30 tonnes at the topsoil calculator's 10% default. Order for the settled depth you want, not for the depth you tip in.
Laying day
Turf is a living product and it degrades quickly once cut. Order it for the day you can lay it, and get it down within about 24 hours of delivery — sooner in warm weather. Stack the rolls in shade if there is any delay, unroll within a few hours, stagger the joins like brickwork, and butt them tightly without stretching the grass.
Keep off the new lawn for two to three weeks and water it daily for the first fortnight, more often in hot weather. Spring and early autumn are the easiest times to lay; frost and drought are the two conditions to avoid.
Turf and topsoil FAQs
How many rolls of turf do I need for a 12.5 m by 8.2 m lawn?
108 rolls of 1 m² turf. The lawn is 102.5 m², the 5% cutting allowance takes it to 107.6 m², and the order is rounded up to the next whole roll.
What size are turf rolls in the UK?
The turf calculator's default is 1 m² per roll, which is the common UK size. Some suppliers cut 1.2 m² rolls and a few sell large 2 m² rolls, so check the stated area and change the roll size box.
How much waste should I allow when ordering turf?
The turf calculator allows 5% by default, which covers trimming in at the edges and working around corners. Increase it for a curved or irregular lawn; set it to zero only if every edge is dead straight.
How much topsoil do I need for a 20 mm dressing over 100 m²?
3.00 tonnes, which is 2.00 m³ at 1.5 tonnes per cubic metre and about four 850 kg bulk bags. The topsoil calculator then adds its settlement allowance, so at the default 10% the order becomes 3.30 tonnes.
Do I need turf starter fertiliser?
The calculator sizes it at 35 g per square metre if you tick the box, so a 100 m² lawn takes 3.5 kg. It is applied under the turf as it is laid, not spread on top afterwards.
Rules that touch a turf job
Laying a lawn is a ground-works job before it is a turf job, and three points from the regulations library apply:
- BS 3882 — topsoil. Topsoil is a specification, not a bag volume, and the standard is what a supplier can be held to. Read the requirement.
- Approved Document C — site preparation. What has to happen to the ground, and what must not be built over. Read the requirement.
- Approved Document H — drainage. Where surface water is allowed to go, and the separation rules that rule out a soakaway in a particular position. Read the requirement.
Guidance only. The quantities above are planning estimates from the ProjectCalc calculators, not a quote and not a specification. Check your own measurements and the area your supplier's rolls actually cover before ordering. Topsoil delivery is the point at which a wrong specification becomes expensive, so ask for it in the standard's terms.