Topsoil Calculator UK - How Much Topsoil Do I Need?

How Much Topsoil Do I Need Per m², By Depth?

Volume is simply area times depth, but the depth you need depends entirely on the job. A thin 25mm top-dressing over an existing lawn needs 0.025 m³ per m² - 25 litres for every square metre. New lawn or turf preparation typically wants 100-150mm to give roots somewhere to establish, which is 0.1-0.15 m³ per m². Borders and planting beds want more again, around 300mm, and raised beds need at least 400mm depending on what's going in them.

These are starting points, not rules - a lawn going over good existing subsoil needs less made-up topsoil than one going over builder's rubble. Use the calculator above with your own depth for an exact figure.

Why "Bulk Bag" Isn't a Standard Size

Unlike a 25kg bag of cement or an 850kg bulk bag of building sand, which are close to industry-standard across UK merchants, a "bulk bag" of topsoil is whatever the supplier decides to call one. Travis Perkins sells a 600 litre bag. Wickes sells a 500 litre bag. B&Q's bulk bag holds 830 litres. Specialist turf and topsoil suppliers like Rolawn offer both 500 litre and 750 litre bulk bags under the same name. That's a genuine 500-1,200 litre range across the market for something sold under one label.

The result: a "you need 4 bulk bags" answer is meaningless without knowing which supplier's bulk bag that is. This calculator asks for your own supplier's bag size in litres or m³ instead of assuming one, and reports volume in m³ and litres as well as bags, so you can compare two suppliers on the same basis before you order from either.

Screened vs Unscreened Topsoil vs Subsoil

• Screened topsoil - passed through a screen to remove stones, roots and clods above a certain size. Lighter and more workable, the standard choice for lawns, borders and anywhere that needs a fine, plantable finish.
• Unscreened topsoil - straight from the ground with stones and clods still in it. Cheaper, heavier (more stone content per m³), and fine for bulk filling or areas that will be cultivated afterwards, but harder to rake to a fine finish or seed directly.
• Compost / growing medium - much lighter, organic-rich material, often blended with topsoil rather than used alone. Settles more than mineral soil as the organic content breaks down and compresses.
• Subsoil - the mineral layer beneath topsoil, low in organic matter and nutrients. Fine as bulk fill or for levelling beneath a topsoil layer, but should never be the final layer wherever anything needs to grow. BS 3882:2015, the UK topsoil specification, explicitly excludes subsoil from its scope for exactly this reason - it isn't classified as a growing medium at all.

Settling: How Much and Why

Loose topsoil is full of small air gaps that close up over time as it's rained on, watered and walked on, so a layer that looks like enough on delivery day ends up shallower a few weeks later. 10% extra is a reasonable default for screened topsoil on a typical garden job. Compost and growing medium are lighter and more organic, and tend to settle more, often 15-20%. A thin top-dressing layer barely settles at all since there's so little depth to compress, so 5% is usually enough there.

This is a real physical effect, not padding for profit margin, and it's worth building it into your order rather than finding out three months later that your new lawn has a dip where the topsoil sat thinnest.

Loose Tipped vs Bulk Bags: When Each Makes Sense

Loose tipped delivery, straight off the back of a tipper lorry, is usually the cheaper option per m³ once you're ordering enough of it - roughly above 5 m³ as a rule of thumb. The catch is that most suppliers set a minimum loose order, commonly 5-8 tonnes, so it simply isn't available at all for a small border or a single raised bed. Below that minimum, bulk bags cost more per litre but are the only practical option, and Travis Perkins and Wickes both offer them in small enough sizes for a single delivery to sit on a driveway. For anything approaching a full garden's worth of topsoil, it's worth asking your local merchant or B&Q for a loose tipped price before defaulting to bags.

Enter your own bulk bag and loose-tonne prices into the calculator above to see which actually works out cheaper for your specific job, rather than relying on the rule of thumb alone.

Topsoil By Area: Quick Reference

Screened topsoil, 10% settling allowance, figures regenerated directly from the calculator above.

Figures include a 10% settling allowance, at 1,500 kg/m³ (screened topsoil, damp, as delivered). Use the calculator above for your own bag size, soil type and settling allowance.

Frequently Asked Questions

How much topsoil do I need per m2?

It depends entirely on depth. At 25mm (lawn top-dressing) you need 0.025 m³ per m²; at 150mm (new lawn preparation) that rises to 0.15 m³ per m²; at 300mm (borders) it's 0.3 m³ per m². Multiply by your area, then add a settling allowance since loose topsoil compresses after it's spread. Use the calculator above for an exact figure at your own depth and area.

Is a bulk bag of topsoil always the same size?

No, and this catches a lot of people out. "Bulk bag" isn't a standard size in the way a 25kg cement bag is. Travis Perkins sells a 600 litre bag, Wickes a 500 litre bag, B&Q an 830 litre bag, and specialist suppliers like Rolawn offer both 500 litre and 750 litre bags. Always compare litres or m³, never bag counts, when comparing suppliers - which is exactly why this calculator asks for your supplier's bag size rather than assuming one.

How much does a cubic metre of topsoil weigh?

Around 1.5 tonnes for screened topsoil at a typical damp, as-delivered moisture level, though this varies more than most materials: dry screened topsoil can be as light as 1.2 tonnes/m³, and topsoil straight off the screen after rain can reach 1.8 tonnes/m³. Unscreened topsoil is heavier again at around 1.7 tonnes/m³ since it still contains stones and clods. Always confirm with your supplier if you're ordering close to a vehicle weight limit.

Why does topsoil settle, and how much allowance should I add?

Loose topsoil contains air pockets that close up as it's walked on, rained on and watered in, so its depth drops after it's spread. 10% is a reasonable default allowance for screened topsoil on most garden jobs; compost and growing medium settle more (nearer 15-20%) since they're lighter and more organic; a thin top-dressing layer needs less. This calculator lets you adjust it.

Should I use subsoil for planting?

No. Subsoil has little organic matter and far less fertility than topsoil, and BS 3882:2015 (the UK topsoil specification) explicitly excludes subsoil from its scope altogether, since it isn't intended to support plant growth. Subsoil is fine as bulk fill beneath a topsoil layer, for levelling, or for structural fill, but always finish with a proper topsoil layer wherever anything needs to grow.

Is loose tipped topsoil cheaper than bulk bags?

Usually, once you're ordering enough of it. Loose tipped delivery is typically the more economical option above roughly 5 m³, but most suppliers have a minimum loose order of 5-8 tonnes, so it isn't available at all for small jobs. Below that, bulk bags are the practical choice even though they cost more per m³. Enter your supplier's prices in the calculator above to compare the actual cost for your job.