How Much Topsoil or Aggregate Do I Need? A Simple Guide

Working out how much topsoil or aggregate you need comes down to one sum: length times width times depth, in metres, which gives you cubic metres. Convert that to tonnes using the material’s density and you have your order. Most mistakes happen not in the maths but in the depth people choose, and in forgetting that loose material settles once it is laid.

This guide covers the sum, the depths each job typically calls for, and the margin worth adding.

The sum

Measure the area in metres, decide the depth in metres too, then multiply all three.

Length (m) x width (m) x depth (m) = volume in cubic metres.

A border 5m long and 4m wide, filled to 0.3m deep, needs 5 x 4 x 0.3, which is 6 cubic metres. Keeping everything in metres keeps the sum simple: 50mm is 0.05m, and 150mm is 0.15m.

Turning cubic metres into tonnes

Aggregates are usually sold by weight, so you need the material’s bulk density to convert. As a rough rule, screened topsoil is around 1.5 tonnes per cubic metre, most gravels and shingles fall between 1.5 and 1.8, and a compacted sub-base such as MOT Type 1 is heavier again at roughly 2 tonnes or more.

On that basis the 6 cubic metre border above works out at approximately 9 tonnes of topsoil. Densities do vary between products, so check the figure for the exact material rather than assuming.

Typical depths

Depth is where most orders go wrong. These are the depths jobs usually call for:

• Topsoil for a new lawn: 100 to 150mm.
• Topsoil for planting borders: 300 to 450mm, deeper for shrubs and trees.
• Decorative gravel over a membrane: around 50mm. Much deeper and it becomes hard to walk on.
• Gravel path: about 50mm of gravel over a compacted sub-base.
• Driveway: typically 150 to 200mm of compacted sub-base, with the surface layer on top.

If the job is structural, such as a driveway taking vehicle weight, the build-up matters more than the headline volume, so confirm the specification before you calculate.

A worked example

Say you are laying a gravel driveway 8m long and 3m wide. The sub-base at 150mm is 8 x 3 x 0.15, which is 3.6 cubic metres, and at roughly 2 tonnes per cubic metre that comes to about 7.2 tonnes of MOT Type 1. The gravel surface at 50mm is 8 x 3 x 0.05, or 1.2 cubic metres, which at around 1.6 tonnes per cubic metre is a little under 2 tonnes. Add the margin below to each figure and you have your order.

Awkward shapes, and the margin to add

For an irregular area, split it into rectangles, work each one out and add them together. For a circle, multiply the radius by itself, then by 3.14, then by the depth.

Then add a margin. Loose material settles and compacts once it is laid and watered in, and some is always lost to spillage and levelling. Adding 5 to 10% on top of the calculated figure is sensible, and a little more for topsoil, which settles noticeably over the first season. Ordering slightly over is cheaper than arranging a second delivery for the last square metre.

Bulk bags or loose tipped

Small jobs suit bulk bags and larger ones suit a loose tipped load. Bag weights vary between suppliers, so check what a bag actually contains rather than assuming, and compare the price per tonne both ways. Above a few tonnes, loose delivery is normally better value, provided you have somewhere for it to go.

Think about that tipping point before the lorry arrives: a firm, clear area as close as possible to where the material is needed, with room for the vehicle to get in and out. Moving several tonnes of stone twice by wheelbarrow is a job nobody enjoys.

Combining a delivery with a collection

If the job also produces spoil, such as digging out a border or a driveway, plan both movements together. Budget Waste runs grab hire for waste out and aggregates in, so the same lorry can bring the new material and take the excavated material away, saving a separate trip.

Choosing the right product

Budget Waste stocks topsoil, sands, sub-base, gravels and shingles, and decorative stone through the aggregates range. The soil products cover topsoil for lawns and borders, while the stone products cover sub-base, gravels and decorative options. Specification matters as much as quantity, so read the product details before ordering, particularly for anything load-bearing.

The bottom line

Measure in metres, multiply length by width by depth, convert with the right density, then add 5 to 10% for settling. Get the depth right for the job and the rest of the calculation looks after itself.