How much topsoil, compost or bark do I need?

Almost every garden material is sold by volume and almost every garden is measured in a mixture of metres and millimetres, which is where the arithmetic goes wrong. The sum itself takes ten seconds. Getting the depth into the same units as the length and width is the whole job, and it is where the errors of a factor of a thousand come from.

What you'll need

• A tape measure
• A calculator
• Something to write the answer on (optional)

Step by step

1. Measure length and width in metres.For an irregular bed, split it into rectangles, work each one out and add them up. For a circle, radius × radius × 3.14. For a curved border, pace the length and take an average width — it is close enough at this scale.
2. Convert your depth to metres before it goes anywhere near the sum.50mm = 0.05m. 75mm = 0.075m. 100mm = 0.1m. 300mm = 0.3m. This is the only step where the mistake is expensive.
3. Multiply the three.Length × width × depth gives cubic metres. A 4m by 1.2m bed being mulched 75mm deep: 4 × 1.2 × 0.075 = 0.36 cubic metres.
4. Choose your depth honestly.Mulch to suppress weeds needs 50 to 75mm — thinner and weed seed still germinates through it. Compost as an annual soil improver, 25 to 50mm. Filling a new raised bed, the full internal depth. Topsoil to level a lawn area, the depth of the low spot.
5. Convert to what the supplier sells.One cubic metre is 1,000 litres. A 50-litre bag is 0.05 cubic metres, so twenty bags to the cubic metre. A bulk bag holds roughly 0.7 to 0.8 cubic metres of loose material, though this varies by supplier and by how settled it is — ask, rather than assuming a full cubic metre.
6. Add for settlement.Bark and wood chip settle by roughly a tenth once they have been walked on and rained on, so order about 10 per cent over. Compost used as a bed fill settles far more than that — a bed filled with compost alone drops a quarter to a third in year one, which is why the fill should be topsoil-led.
7. Sanity-check with a bulk bag as a mental unit.One bulk bag spread 50mm deep covers roughly 14 to 16 square metres. If your answer says one bag will mulch a whole front garden, check the depth conversion again.
8. Compare prices per cubic metre, not per bag.Bagged compost is typically several times the price of the same volume bought loose or in bulk bags. Below about half a cubic metre bags win on convenience; above that the difference is large enough to be worth arranging access for a delivery.

Tips

• Weight is a separate question from volume and it matters for delivery and handling. A cubic metre of topsoil is well over a tonne; a cubic metre of bark is a few hundred kilos. Check what your access and your back can take.
• As-dug topsoil is cheaper than screened, and comes with stones, roots and weed seed. Screened is worth the difference for a raised bed you are going to plant immediately.
• Bulk bags cannot usually be craned more than a few metres from where the lorry can park. Establish that before ordering, or you will be barrowing a tonne.
• Buying slightly over is nearly always right. The second small delivery of the same material costs almost as much as the first, and surplus compost is never wasted.

Common mistakes

• Putting the depth into the sum in millimetres — Multiplying metres by metres by millimetres gives an answer a thousand times too big. The classic version is ordering 27 cubic metres of bark for a border.
• Assuming a bulk bag is a cubic metre — Most hold 0.7 to 0.8 cubic metres of loose material and the industry is inconsistent about it. Ask the supplier for the actual volume before you count bags.
• Filling a raised bed entirely with compost because the sum said so — The volume is right and the material is wrong. It will drop by a third and dry out. Order two-thirds topsoil and a third compost for the same total volume.

Questions people ask