Garden Materials Calculator UK
Not sure how much mulch, compost, topsoil or gravel to order? Our free UK garden materials calculator works out the quantities for one border or an entire project. Enter your measurements, choose the materials and get a shopping list showing litres, cubic metres, estimated weight and how many bags or bulk bags you need.
You can combine borders, paths, raised beds and lawn jobs in one estimate, compare material costs using your own supplier prices, then copy, print or save your results. No account needed.
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How to use the garden materials calculator
- Measure your space. Enter length and width in metres, choose a circular or triangular area, or use the known area in square metres. Split awkward shapes into separate areas.
- Choose the materials. Add bark mulch, compost, soil, gravel, slate, sand, sub-base or lawn topdressing. Select the job and adjust the suggested layer depth.
- Enter buying details. Check the bag capacity or loose-load unit shown by your supplier. Add optional prices to compare material-only costs.
- Check your shopping list. The calculator combines matching materials across areas before rounding up to whole bags. Copy, print or download the results.
How much bark mulch do I need?
For any even layer, the calculation is area (m²) × depth (mm) ÷ 1,000 = volume (m³). One cubic metre is 1,000 litres. The calculator does the maths and adds any extra allowance you choose.
Example: a 6 m × 1.5 m flower border. The border covers 9 m². At 75 mm deep it needs 675 litres of bark. Add a 5% allowance and the buying quantity becomes 708.75 litres, or 11 bags of 70 litres. The shopping list rounds up to whole bags.
The RHS advises a layer of at least 5 cm, ideally 7.5 cm, for organic mulches. Apply mulch over moist, weed-free soil and leave a clear circle of about 10 cm around woody stems rather than piling material against them.
How much area will mulch cover?
The figures below show the coverage of 1 m³ (1,000 litres) before any extra allowance. For example, 1 m³ covers about 20 m² at a 50 mm depth.
| Depth of layer | Area covered by 1 m³ | Area covered by one 70 L bag |
|---|---|---|
| 25 mm | 40 m² | 2.8 m² |
| 50 mm | 20 m² | 1.4 m² |
| 75 mm | 13.3 m² | 0.93 m² |
| 100 mm | 10 m² | 0.7 m² |
How much topsoil or compost should I order?
Example: topping up a 4 m × 2.5 m bed by 100 mm. The area is 10 m², so you need 1 m³ (1,000 litres). With 10% extra for settling or uneven ground, order approximately 1.1 m³, subject to the supplier’s stated bag capacity.
Topsoil and compost serve different purposes. Topsoil creates or replaces a growing layer, while mature garden compost is generally used to add organic matter and improve existing soil. The best depth depends on what is already there, so do not automatically use the same depth for a new bed and a surface mulch.
For more help, see our guide to improving soil structure and our guide to sand, silt and clay soils.
Gravel calculator: paths, gravel gardens and sub-base
A gravel path may need more than one material. Use the project estimator to add decorative gravel and, where appropriate, a compacted MOT Type 1 sub-base to the same area. Keep their depths separate: the finished sub-base depth is not the same as its loose delivery volume.
Example: a 5 m × 1 m path. A 40 mm gravel layer needs 0.20 m³ before allowance, or 0.21 m³ with 5% extra. If the design also calls for a 75 mm compacted sub-base, that is 0.375 m³ of finished stone. With an illustrative 1.3 compaction factor and 5% extra, the loose sub-base ordering volume is about 0.51 m³. Actual construction depths depend on the use and ground conditions.
For heavy stone products, estimated tonnes depend on material density and moisture. Check the product weight, delivery limits and the supplier’s grading before ordering.
Raised bed soil calculator: one bed or several
Enter the inside dimensions of a raised bed, not the outside of its frame. Measure the depth you actually need to fill, allowing for soil already in the bed and a little clearance below the top edge.
Example: three raised beds, each 2.4 m × 1.2 m, with 250 mm of fill. The combined fill volume is 2.16 m³ before settling allowance. Use the calculator’s raised-bed mix option to divide this between topsoil, compost and horticultural washed sharp sand.
The RHS describes a general mix of 7 parts topsoil, 3 parts organic matter and 2 parts sharp sand; its guidance uses 7:3:4 for beds constructed on a hard surface, alongside proper drainage provision. These are starting recipes rather than a requirement for every garden. Use horticultural-grade sand for growing mixes, not builders’ sharp sand.
Small bags, bulk bags or loose deliveries?
- Small bags: sold by litres or kilograms, often convenient for smaller quantities.
- Bulk bags: capacity differs between suppliers. An 850 kg bag is not automatically 1 m³, and a volume-labelled bag is not always one tonne.
- Loose loads: often sold by tonne or cubic metre, with possible minimum orders or access requirements.
Enter your supplier’s actual sizes and prices. The estimator compares material costs only; delivery charges, access restrictions and minimum-order fees can change which option works out cheapest.
Frequently asked questions
How many bags of mulch do I need?
Multiply the area in m² by the layer depth in mm to get litres, then add any extra allowance. Divide the result by the litres stated on the bag and round upwards. A 9 m² border at 75 mm with 5% extra needs 708.75 litres, which is 11 bags of 70 litres.
Is a bulk bag one cubic metre or one tonne?
Not necessarily. Bulk bags may be sold by volume or by weight, and loose density varies by material. Enter the quantity stated by your supplier rather than assuming that every bulk bag is the same size.
How much gravel does one tonne cover?
Coverage depends on the stone’s density and the depth laid. As an illustration, at 1.7 tonnes per m³, one tonne provides about 0.59 m³, which covers roughly 11.8 m² at 50 mm before any allowance. Check the supplied product’s density and grading.
Does the estimator calculate the cheapest way to buy?
It can compare small bags, bulk bags and loose loads when you enter prices. The comparison is for materials only, so add supplier delivery and minimum order charges when making your final buying decision.
Can I save a project and come back later?
Yes. Use “Save on this device” to keep your estimate in the same browser. You can also copy, print or download a shopping list. A saved estimate is not automatically shared between devices.
More practical soil and garden guidance
Planning a wider project? Find out how to make and use leafmould, learn how to improve soil structure, or browse all our Soil & Compost guides.
About these estimates: Figures are approximate and depend on accurate measurements, layer depth, density, compaction and the product sold. For large or load-bearing projects, confirm quantities and construction requirements with a suitable supplier or qualified professional.