Which Soil Do You Need?

Most people reach for a calculator before they have decided what they are buying. Topsoil, compost and a raised-bed mix are three different products at three different prices, and the wrong one costs money twice — once when you buy it and again when the plants sulk. This page settles that choice, then sends you to the right tool.

Topsoil, compost, raised-bed mix — what each one is for

Topsoil is the mineral soil layer plants root into. It is what you use to build up a level, fill a low spot, or lay a lawn on. It is sold against a British Standard, BS 3882, which is the thing to ask for by name.¹

Compost is decomposed organic matter. It feeds and conditions; it is not a structural soil. Filling a bed with compost alone gives you something that slumps as it rots down and dries out hard.

A raised-bed mix is a blend — topsoil for body, organic matter for fertility, sometimes grit for drainage. It costs more per litre than topsoil because you are paying for the blending.

What BS 3882 actually promises

BS 3882:2015 is the current edition, published 30 April 2015.¹ It covers natural and manufactured topsoils that are moved or traded, and it specifies two things: multipurpose topsoil, fit for most situations, and specific-purpose topsoil, which covers acidic, calcareous and low-fertility material for particular jobs.¹

Worth knowing, because merchants sell on names the standard does not use: "premium", "economy" and "grade 1" are trade descriptions, not BS 3882 classifications. Asking a supplier to confirm the material is multipurpose topsoil to BS 3882:2015, and asking for the test certificate, is a sharper question than asking whether it is premium.

Improve what you have, or buy it in?

Buying soil is the expensive answer and often the wrong one. Existing ground that drains badly or grows weakly is usually a structure and organic-matter problem, not a reason to replace the soil. Digging in organic matter and leaving it to the worms is cheaper than importing tonnes.

Buy it in when you are raising a level, filling a new raised bed, replacing soil lost to a build, or laying turf over subsoil a builder left behind. Improve instead when the ground is there and simply tired.

If you are weighing it up on price, the per-square-metre figures on what topsoil costs per m² are from real supplier listings rather than a rule of thumb.

How deep, before you work out how much

Depth decides the bill far more than area does, so it is worth settling first. The RHS gives two figures for bought-in topsoil: at least 10cm for laying a lawn, and about 20cm or more for new beds.² Double the depth and you double the tonnage, which is why a 5cm estimate and a 10cm estimate for the same patch look like two different jobs.

The RHS page does not give a raised-bed depth, so neither will we — that one depends on what you are growing, and the raised bed calculator takes the height you have actually built.

The three tools on this site, and when each one helps

• Topsoil calculator — you know the area and the depth and you want the volume, in litres, bags and bulk bags. Start here for levelling, lawns and borders.
• Raised bed soil calculator — you have a bed of a given length, width and height and want the fill volume, including the mix ratio rather than a single material.
• Topsoil cost per m² — you want to know what it will actually cost before you order, from current supplier prices rather than an average.

Sources and checking notes

Checked 19 September 2026.

1. BSI — BS 3882:2015, Specification for topsoil. knowledge.bsigroup.com. Checked 19 September 2026: published “30 Apr 2015”, status “Current”. The standard covers natural and manufactured topsoils that are moved or traded, specifying requirements for multipurpose topsoil and for specific-purpose topsoils that are acidic, calcareous or of low fertility.
2. Royal Horticultural Society — Topsoil: buying. rhs.org.uk/soil-composts-mulches/topsoil-buying. Checked 19 September 2026: “For laying lawns you will need at least 10cm (4in) layer of top soil”; “New beds will need about 20cm (8in) or more”. The page states no raised-bed depth, which is why none is quoted here.

Left uncited on purpose: the improve-or-replace guidance is GardenCalc’s own editorial judgement from the jobs behind these calculators, not a claim attributed to anyone else.