Topsoil, mulch and gravel calculator
Enter the size and depth of a bed, border or path to get cubic metres, bags to buy and, for gravel and sand, the weight in tonnes.
The RHS recommends at least 5 cm, and ideally 7.5 cm, of biodegradable mulch such as bark.
Volume needed
- Volume needed
- 0.788 m³788 L
- Bags to buy
- 1460 L bags, 13.13 before rounding up
- Area covered
- 10 m²
Bag size and other assumptions
Bark, topsoil and compost sizes are from a UK producer. Gravel and sand sizes are worked out from a maker’s coverage at 50 mm. Choose Other size to type the size on your bag.
Bark and topsoil settle after spreading. A UK producer advises 5 to 10% for bark and 5% for topsoil. Other materials start at 0%.
Estimate only. Check the product label and the building regulations where you live before you buy or build.
Quick answer
Multiply the area in square metres by the depth in metres to get cubic metres, then add an allowance for settlement. A 4 × 2.5 m border mulched 7.5 cm deep needs 10 × 0.075 = 0.75 m³ of bark, or 0.788 m³ (788 litres) with 5% extra, which is 14 bags of 60 litres or 2 bulk bags of 0.6 m³.
How to use the topsoil, mulch and gravel calculator
- Choose the material: bark mulch, topsoil, compost, gravel or sand.
- Pick the shape: length and width for a bed, border or path, the diameter for a circle around a tree, or a known area if you have already added up several beds.
- Enter the depth of the finished layer in centimetres (50 mm is 5 cm). The cubic metres, litres, bags and area update as you type.
- Open ‘Bag size and other assumptions’ to choose a litre bag or a 0.6 m³ bulk bag, change the 5% allowance for settlement of bark and topsoil, or change the bulk density used to weigh gravel and sand.
How do you work out how much topsoil or mulch you need?
Multiply the area of the bed in square metres by the depth in metres. Depth is measured in centimetres, so divide it by 100 first: 7.5 cm is 0.075 m. One cubic metre is 1,000 litres, the unit bagged bark, compost and topsoil are labelled in.
Bark and topsoil settle after they are spread, so a UK bark and soil producer recommends allowing 5 to 10% extra for bark and 5% for topsoil. Divide the total by the bag size and round up to whole bags. Gravel and sand are also sold by weight, so their volume is multiplied by a bulk density in tonnes per cubic metre.
area (m²) = length × width, or 3.1416 × (diameter ÷ 2)² for a circlevolume (m³) = area × depth (cm) ÷ 100volume to buy (m³) = volume × (1 + extra % ÷ 100)litres = m³ × 1,000bags = litres ÷ bag size (L), rounded up; bulk bags = m³ ÷ 0.6, rounded upweight (tonnes) = m³ × bulk density (t/m³)How much does a bag or bulk bag cover?
Divide the bag volume by the depth: a 60 litre bag is 0.06 m³, so at 7.5 cm it covers 0.06 ÷ 0.075 = 0.8 m². The figures below are before any allowance for settlement.
| Depth | 60 L bag | 0.6 m³ bulk bag | 1 m³ |
|---|---|---|---|
| 2.5 cm | 2.4 m² | 24 m² | 40 m² |
| 5 cm | 1.2 m² | 12 m² | 20 m² |
| 7.5 cm | 0.8 m² | 8 m² | 13.3 m² |
| 10 cm | 0.6 m² | 6 m² | 10 m² |
| 15 cm | 0.4 m² | 4 m² | 6.7 m² |
A 20 litre bag of topsoil covers 0.4 m² at 5 cm, and a 50 litre bag of compost covers 1 m² at 5 cm. An aggregate maker gives the same 12 m² at 50 mm for a bulk bag of gravel or play sand.
Worked examples
Bark mulch for a 4 × 2.5 m border
The border covers 4 × 2.5 = 10 m². At 7.5 cm deep the bark fills 10 × 0.075 = 0.75 m³. With 5% for settlement that is 0.788 m³, or 788 litres.
In 60 litre bags that is 13.13 before rounding up, so buy 14 bags. In 0.6 m³ bulk bags it is 1.31, so 2 bulk bags.
Gravel for an 8 m garden path
A path 8 m long and 1 m wide covers 8 m². At 5 cm deep it needs 8 × 0.05 = 0.4 m³ of gravel. Gravel and sand start with no allowance, because no source gives one.
At 1.6 tonnes per cubic metre that weighs 0.64 tonnes (640 kg). One 0.6 m³ bulk bag covers it (0.67 of a bag), or 20 bags of 20 litres. If you add 5% for an uneven base, it becomes 0.42 m³, 0.67 tonnes and 21 bags of 20 litres.
How deep should bark mulch be?
Spread bark, wood chippings or compost at least 5 cm (2 in) deep, and ideally 7.5 cm (3 in), as the RHS advises for biodegradable mulches. One UK bark producer gives 50 to 100 mm for its ornamental bark, and a peat-free compost maker gives 5 to 8 cm when its compost is used as a mulch.
Do not pile mulch against the stems of trees and shrubs: the RHS says mulch in direct contact with them can soften the stem and make it vulnerable to disease. It gives mid to late spring, when annual weeds have not yet germinated, and autumn as good times to mulch.
How much does a bulk bag of gravel weigh?
A bulk bag of gravel weighs about 0.9 to 1 tonne. Heidelberg Materials UK says a bulk bag covers about 12 m² at 50 mm, which is 0.6 m³, and puts aggregate at approximately 1.5 to 1.7 tonnes per cubic metre. So 0.6 m³ comes to 0.9 to 1.02 tonnes. The calculator starts at 1.6 t/m³, which gives 0.96 tonnes.
Topsoil is lighter: one producer lists its 0.6 m³ bulk bag at 510 to 630 kg and its 20 litre bag at 17 to 21 kg. Weight per cubic metre differs between products, so for a bulk load ask the supplier for its figure and type it under the assumptions.
Questions
How many 60 litre bags of bark do I need per square metre?
At 7.5 cm deep you need 1.25 bags of 60 litres per square metre, because each square metre takes 75 litres. At 5 cm it is 50 litres, or 0.83 of a bag. Add 5 to 10% for settlement, as the producer advises: a 10 m² border at 7.5 cm with 5% extra takes 788 litres, which is 14 bags.
How much does a bulk bag of topsoil cover?
A 0.6 m³ bulk bag of topsoil covers 6 m² at 10 cm deep, 4 m² at 15 cm and 12 m² at 5 cm. Divide 0.6 by the depth in metres for any other depth. The producer recommends allowing 5% for settlement, so plan on about 5.7 m² per bag at 10 cm. One producer lists the bag at 510 to 630 kg.
How much area does a tonne of gravel cover?
A tonne of gravel covers about 12.5 m² at 5 cm deep if it weighs 1.6 tonnes per cubic metre. One tonne is then 0.625 m³, and 0.625 ÷ 0.05 = 12.5. Across the 1.5 to 1.7 t/m³ range an aggregate maker gives, the same tonne covers between 11.8 and 13.3 m². At 2.5 cm deep, double those figures.
How much extra should I order for settling?
Order 5 to 10% more bark and 5% more topsoil than the measured volume, as a UK bark and soil producer recommends for natural settlement. The calculator starts at 5% for those two and at 0% for compost, gravel and sand. For a 10 m² bed at 7.5 cm, 0.75 m³ becomes 0.788 m³ at 5% and 0.825 m³ at 10%. Set it to 0% to see the bare measured volume.
How much sand do I need for a sandpit?
A 1.2 × 1.2 m sandpit filled 20 cm deep needs 0.288 m³ of sand, which is one 0.6 m³ bulk bag. At 1.6 t/m³ that is about 0.46 tonnes. In smaller bags, one maker’s large bag of playpit sand covers 0.35 m² at 50 mm, about 17.5 litres, so 288 litres takes 17 bags. If your sand is labelled by weight, choose Other size and type the weight on the bag.
Sources
- Royal Horticultural Society, Mulches and Mulching (checked 19 October 2026)Biodegradable mulch at least 5 cm and ideally 7.5 cm thick; do not pile mulch against stems; best applied mid to late spring and in autumn.
- Melcourt Industries, Ornamental Bark Mulch (checked 19 October 2026)60 litre bags and bulk bags; lay at 50 to 100 mm; allow 5 to 10% for natural settlement.
- Melcourt Industries, Topsoil Blended Loam (checked 19 October 2026)20 litre bags (17 to 21 kg) and 0.6 m³ bulk bags (510 to 630 kg); allow 5% for natural settlement.
- Melcourt Industries, SylvaGrow Farmyard (checked 19 October 2026)50 litre bags and 0.6 m³ bulk bags; as a mulch, spread 5 to 8 cm and avoid stems and trunks.
- Heidelberg Materials UK, Aggregates (FAQ: How much aggregate do I need?) (checked 19 October 2026)Aggregate density approximately 1.5 to 1.7 tonnes per cubic metre; ask your supplier.
- Heidelberg Materials UK, 20mm Gravel technical data sheet (checked 19 October 2026)As top dressing, a Maxipack covers about 0.4 m² and a bulk bag about 12 m² at 50 mm.
- Heidelberg Materials UK, Playpit Sand technical data sheet (checked 19 October 2026)At 50 mm, a Maxipack covers 0.35 m², a Midipack 0.21 m² and a bulk bag 12 m².
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