How to Calculate Turf, Topsoil and Fertiliser

Measure the Lawn as Simple Shapes

Turf is normally ordered by area, but gardens are rarely perfect rectangles. Sketch the finished lawn and split it into shapes you can measure:

• Rectangle: length × width
• Triangle: base × height ÷ 2
• Circle: π × radius²
• L-shape: two non-overlapping rectangles added together

Measure the prepared finished edge, not an approximate garden boundary. Beds, paths, patios and large tree pits should be excluded.

The Turf Calculator handles rectangles, circles and L-shapes, including waste, topsoil and starter fertiliser.

Worked Example: L-Shaped Lawn

Split an L-shaped area into two rectangles:

• Section A: 8m × 4m = 32m²
• Section B: 3m × 2m = 6m²

Total measured lawn:

32 + 6 = 38m²

If the turf product is sold as 1m² of coverage and you allow 5% for cuts:

38 × 1.05 = 39.9m²

Round up to 40m² of turf. Suppliers package turf differently, so order from the stated coverage rather than assuming every roll or strip covers the same area.

How Much Extra Turf Should You Order?

Five per cent is a reasonable starting allowance for a simple lawn with straight edges. Consider more where the plan includes:

• tight curves or many small beds;
• diagonal laying or awkward tapers;
• short sections where offcuts cannot be reused;
• damaged edges or variable roll dimensions; and
• no practical way to obtain an exact matching top-up quickly.

Do not add waste twice. If the supplier has already converted your measured area into an order quantity with its own allowance, record that clearly.

Calculate Topsoil by Volume

Topsoil is calculated from area and compacted depth:

Topsoil volume (m³) = area (m²) × depth (m)

Convert millimetres to metres before multiplying. For 40m² at a 25mm dressing depth:

25mm = 0.025m 40 × 0.025 = 1.00m³

At 50mm depth:

40 × 0.05 = 2.00m³

That difference is why “a bulk bag should do it” is not a reliable estimate. Bag weights, loose volume, moisture content and soil density vary. Builder Tools gives an indicative conversion using about 1.3 tonnes per cubic metre, but the supplier's declared bag volume or weight should control the order.

Topsoil cannot fix poor levels or deep construction damage on its own. Confirm whether the job needs a light dressing, replacement growing layer, drainage improvement or full cultivation before pricing.

Fertiliser Quantity

Use the product label wherever one has been selected. As a general preparation reference, the Royal Horticultural Society's turf-laying guidance suggests that light, poor soil may receive general-purpose fertiliser at the manufacturer's rate, with about 35g per m² given as an approximate rate.

At 35g per m² for a 40m² lawn:

40 × 35 = 1,400g = 1.4kg

Do not increase the rate to use up a bag. Too much fertiliser can scorch young turf, and some prepared soils will not need the same treatment. Match the product, season and soil condition.

Ground Preparation Affects the Quantity

Good preparation is more important than squeezing the order to the last roll. The RHS recommends removing perennial weeds, relieving compaction, levelling and firming the soil before turf arrives. Its guidance describes cultivating to roughly 20–25cm where needed and settling disturbed ground before final levelling.

A practical sequence is:

1. Remove existing turf, weeds, roots, rubble and debris.
2. Check drainage and relieve compacted ground.
3. Cultivate and add suitable organic matter where specified.
4. Set finished levels, allowing for the turf thickness.
5. Firm and rake in more than one direction.
6. Measure the final prepared area again.
7. Arrange delivery only when the ground and labour are ready.

Fresh turf is perishable. Lay it promptly in line with the supplier's instructions, working from boards to avoid rutting the prepared soil. Water thoroughly and keep traffic off while roots establish. The RHS's new-lawn aftercare guidance explains how watering and mowing change while the turf is still rooting.

Circular Lawn Example

For a circular lawn 6m in diameter:

Radius = 6 ÷ 2 = 3m Area = π × 3² = 28.27m²

With 7% extra for the curved perimeter:

28.27 × 1.07 = 30.25m²

Order according to the supplier's pack increment, likely rounding to at least 31m² if sold in full square metres. Curves create short triangular offcuts, so agree the cutting detail and avoid relying on every offcut being reusable.

What to Include in the Quote

The material line is only one part of a turf job. Include:

• removal and disposal of the old lawn;
• cultivation, levelling and compaction;
• imported topsoil or soil improver;
• turf and delivery;
• fertiliser if required;
• watering and hose access;
• edging repairs; and
• aftercare instructions and any return visit.

Run the final dimensions through the Turf Calculator, then use the Quote Generator so preparation, materials and aftercare are all visible to the customer.