How Much Topsoil for a Raised Bed? UK & US Guide
Filling a raised bed is one of those jobs where a small maths error costs real money. Order too little and you are back at the garden centre paying twice for delivery. Order too much and you have half a tonne of soil sitting on your drive in the rain. This guide shows you exactly how to work out the volume, convert it into the units suppliers actually sell in, and how much to add for settling.
Step 1: Measure the bed and work out the volume
Raised bed volume is simply length × width × depth. The trick is keeping every measurement in the same unit before you multiply.
- Measure the internal dimensions, not the outside of the timber. A bed built from 45 mm (1.75 in) boards loses nearly 90 mm off each internal measurement.
- Choose your fill depth. This is usually less than the full height of the bed. Leave 25 to 50 mm (1 to 2 in) of freeboard at the top so soil does not wash over the edge when you water.
- Convert everything to metres (or to feet, if you are working in imperial) and multiply the three numbers together.
That gives you cubic metres (m³) or cubic feet (ft³). Two conversions get you the rest of the way:
- 1 m³ = 1,000 litres = 35.3 ft³
- 1 cubic yard = 27 ft³ = 0.765 m³
Step 2: Convert volume into what suppliers sell
In the UK, topsoil is sold loose by the tonne, in bulk bags, or in small 20 to 25 litre sacks. In the US and Canada it is almost always sold by the cubic yard, sometimes by the “scoop”. Getting between them needs one more number: bulk density.
Screened topsoil typically weighs 1.3 to 1.5 tonnes per cubic metre (roughly 2,200 to 2,500 lb per cubic yard) when moderately moist. Very dry, loose soil can be as light as 1.2 t/m³; heavy clay or soaking wet soil can exceed 1.6 t/m³. If a supplier quotes you a tonnage, ask what density they are assuming.
A UK bulk bag (often called a tonne bag) holds roughly 700 to 900 litres, which is 0.7 to 0.9 m³. It is sold as “one tonne” but the volume varies between suppliers, so check the litre figure on the product page rather than assuming.
Step 3: Add an allowance for settling
Freshly delivered topsoil is full of air. Once it is watered in and has been through a few weeks of weather, it typically drops by 10 to 20 percent. Beds with a high compost content settle at the higher end of that range.
Add 10 percent to your figure as a minimum, or 15 percent if your mix is compost-heavy. It is much cheaper to over-order slightly than to arrange a second delivery.
Worked example: a 2.4 m × 1.2 m bed
Take a standard bed measuring 2.4 m by 1.2 m internally (8 ft by 4 ft), filled to a depth of 300 mm (12 in).
Volume: 2.4 × 1.2 × 0.3 = 0.864 m³
That is 864 litres, 30.5 cubic feet, or 1.13 cubic yards.
With a 10 percent settling allowance: 0.864 × 1.1 = 0.95 m³ (950 litres, or 1.24 cubic yards).
What to order in the UK: at 800 litres per bulk bag, that is one bulk bag plus about 150 litres, so one bulk bag and six 25-litre sacks. Weight comes to roughly 0.95 × 1.4 = 1.33 tonnes if you are buying loose.
What to order in the US: 1.24 cubic yards, so most suppliers would round you up to 1.5 cubic yards. That weighs roughly 1.6 to 1.8 US tons.
If you are mixing rather than using pure topsoil, a common raised bed recipe is 60 percent topsoil, 30 percent compost and 10 percent horticultural grit. Applied to 0.95 m³, that is 0.57 m³ topsoil, 0.29 m³ compost and 0.10 m³ grit. Avoid filling a bed with pure compost, which slumps badly and drains poorly once it is wet.
Rough costs
Prices move constantly and vary hugely by region, so treat these as starting points and always check local suppliers. In the UK a bulk bag of screened topsoil commonly lands somewhere around £40 to £90 delivered, with loose bulk loads working out cheaper per cubic metre once you are ordering several tonnes. In the US, screened topsoil is often in the region of $30 to $60 per cubic yard before delivery, and delivery is frequently the larger part of a small order. In the eurozone, expect broadly comparable rates to the UK per cubic metre, typically €50 to €100 per bulk bag delivered.
The practical lesson: delivery charges reward bigger single orders. If you are building three beds, work out the total first and order once.
How deep does a raised bed actually need to be?
For salads, herbs and most leafy crops, 150 to 200 mm (6 to 8 in) of good soil is enough. For carrots, parsnips, potatoes and most perennials, aim for 300 to 450 mm (12 to 18 in). If your bed sits on open ground rather than a hard surface, roots can travel below the frame, so you can build shallower than you might think.
Can I fill the bottom of the bed with something cheaper?
Yes, within reason. Filling the lower third with logs, woodchip or turf turned upside down (sometimes called hugelkultur) reduces the soil you need to buy. Be aware it settles a lot in the first two seasons, so expect to top the bed up. Do not use builders’ rubble or anything that might leach contaminants into food crops.
Is topsoil the same as garden soil or compost?
No. Topsoil is mineral soil, the upper layer stripped from a site and usually screened. Compost is decomposed organic matter, much richer in nutrients but structurally weak on its own. Bagged “garden soil” or “raised bed mix” is usually a pre-blended combination of the two. Check the ingredients rather than the marketing name.
Ready to work out your own numbers? Enter your bed dimensions into the free Garden Soil & Topsoil Calculator and it will give you the volume in cubic metres, litres and cubic yards, plus an estimated weight in tonnes. If you are also laying a path or patio alongside the beds, the Gravel Calculator covers aggregate by area and depth, and the Garden Calculators hub has tools for plant spacing and fertiliser rates.