Soil / Compost Volume Calculator, South Africa
Calculate exactly how much topsoil, compost or mulch you need for any SA garden project. Volume in m³, bulk bags (assumed 750L each) and retail bags with cost estimate.
Quick answer: A topsoil depth of 150mm (15cm) is a typical depth for a new garden bed or lawn, equal to 0.15m³ (150 litres) of topsoil per m². A 20m² garden bed at that depth needs 3m³ of topsoil net, or 3.45m³ including our 15% settling buffer, which is approximately 5 bulk bags at 750 litres each.
Soil / Compost Volume Calculator
Topsoil: 150mm deep, plus a 15% settling allowance
For planning purposes only. Material volumes vary with supplier and moisture content. Always inspect material before accepting delivery.
How to Use This Calculator
- Enter garden/bed areaTotal area to cover in m². Add multiple beds or zones together.
- Select material typeTopsoil, compost, mulch, top-dressing or fill. Each has a typical depth.
- Adjust application depthThe calculator suggests a typical depth for your chosen material, which you can adjust.
- Select compaction factorMaterials settle after delivery, so order slightly extra. 15% is our default.
- Click Calculate VolumeResults show m³ and litres, the number of 750L bulk bags versus 30L retail bags needed, and an estimated 2026 SA cost range.
Garden Volume Calculations for South African Projects
The simple formula for any garden volume calculation is: Volume (m³) = Area (m²) × Depth (m). The tricky part is knowing the right depth for your material and use case, and then converting cubic metres into the bags or loads your supplier actually sells.
| Material | Typical Depth | Use Case | Bulk Bag Price (2026) | Notes |
|---|---|---|---|---|
| Topsoil (garden grade) | 100–150mm | New beds, raising levels, lawn preparation | R600–R860/750L | Quality varies widely, so inspect before accepting |
| Compost / humus | 50–100mm | Dig into new beds; annual top-dressing | R600–R860/750L | Best added at planting time or in autumn |
| Bark chip mulch | 75mm | Weed suppression and moisture retention | about R1,125/750L | Replenish annually as it breaks down |
| Lawn top-dressing | 25mm | Level uneven lawns, improve soil structure | R675–R860/750L | Apply in spring before growth season |
| Fill / construction soil | As required | Levelling sites, raising paved areas | R200–R350/750L | Not suitable for planting, as it has no organic matter |
Bulk prices are 2026 delivered prices from South African suppliers per cubic metre: Johannesburg topsoil R800 to R950; Cape Town topsoil, compost and lawn dressing R1,149; bark mulch R1,499 (read 2026-10-07). The table shows them per 750L bag. Fill soil is our estimate. Retail 30L bags cost R35 to R48 each at Builders and Leroy Merlin.
Bulk bags vs retail bags: which to buy
In South Africa, a bulk bag (builder's bag) usually holds 0.75 to 1 cubic metre (this calculator uses 750 litres, so check your supplier's bag size) and typically costs R600 to R1,125 per 750 litres depending on material; 750 litres equals 25 retail 30L bags. For volumes above 300 litres, ordering bulk bags from a landscaping or building supplier is almost always cheaper than buying retail bags from a garden centre. Below 300 litres (about 4–5 retail bags), the convenience of retail bags outweighs the cost saving of arranging a bulk delivery. A bulk bag also eliminates the labour of carrying and emptying 25 individual bags.
Topsoil vs compost: what South African soils actually need
Most South African garden soils are deficient in organic matter. Sandy soils (coastal, Highveld) drain too fast and hold little moisture or nutrients. Clay soils (certain inland areas) compact and drain poorly. In both cases, the solution is the same: regular addition of compost. Topsoil provides physical volume, so use it to fill raised beds or level sites. Compost improves fertility and structure, so dig it in whenever planting or as an annual mulch. The best SA garden soil is a mix of 70% garden topsoil and 30% quality compost by volume.
Soil pH and Amendment in South African Gardens
South African garden soils vary widely in pH. Coastal KwaZulu-Natal and Western Cape soils tend toward acidity (pH 5.5–6.5). Highveld and inland soils are often more alkaline (pH 6.5–7.5). Most vegetables and ornamentals prefer pH 6.0–7.0. A basic pH test kit (R50–R150 from most nurseries) before ordering soil amendment tells you whether you need lime to raise pH on acidic soils or sulphur-based amendment to lower pH on alkaline soils. Adding organic matter without testing pH first can fail to solve the underlying fertility problem even when the volume calculation is correct.
Compost quality varies significantly across South African suppliers. Municipal compost from green waste is affordable but often coarser and lower in nutrients than matured garden compost. For a productive vegetable garden, blend about 30% compost with 70% topsoil (our suggestion) and apply a balanced organic fertiliser at planting. For established lawn renovation, a 10–15mm topdressing of fine compost worked into the existing turf is more effective than replacing the entire surface layer.
Raised Beds: Practical SA Considerations
Raised garden beds allow complete control of the growing medium independent of native soil. A raised bed 400mm deep filled with a 50/50 blend of good topsoil and compost outperforms a well-prepared ground-level bed in most South African conditions. Calculate the volume carefully before ordering: a 3m × 1.5m × 0.4m raised bed requires 1.8m³ of growing medium, equivalent to two to three large bulk bag deliveries. Order about 10 to 15% more than calculated to allow for settling after the first watering cycle (our estimate; this calculator defaults to 15%). When sourcing topsoil in South Africa, always inspect the product before ordering, because quality varies enormously between suppliers. Good topsoil should be dark (indicating organic content), free of large rocks and should not compact into a solid block when squeezed in the hand. Pale, sandy or clay-heavy topsoil with low organic content will need significant amendment before it is productive, which adds to the cost beyond the initial volume calculation. When sourcing topsoil locally in South Africa, ask suppliers for a basic soil analysis report: reputable bulk suppliers provide pH, organic content and texture results, which confirm the soil is suitable for the intended planting without additional amendment cost.
Frequently Asked Questions
A topsoil depth of 150mm (15cm) is typical for a new bed or lawn (our default; suppliers quote 100 to 150mm). At 150mm depth, you need 0.15m³ of topsoil per m², or 150 litres per m². For a 20m² garden bed at 150mm deep, you need 3m³ net (3.45m³ including our 15% settling buffer), or approximately 5 bulk bags (750L each). For a top-dressing application over existing lawn (to level or improve), 25–50mm depth is typical, requiring 0.025–0.05m³ per m².
In South Africa, a bulk bag (also called a builder's bag) usually holds 0.75 to 1m³ of material; this calculator assumes 750 litres, so check your supplier's bag size. This is the most common unit for ordering topsoil, compost, mulch, plaster sand and garden materials from suppliers. To calculate how many bulk bags you need, divide your required volume in litres by 750, and round up to the next whole bag.
For a new garden bed in South African soil (typically sandy or clay-heavy depending on region): dig in 75–100mm of compost to the full depth of the bed (typically 300mm). For an established bed as an annual top-dressing: apply 50mm of compost across the surface and dig in lightly. As a mulch layer to suppress weeds and retain moisture: apply 75mm of bark chips or compost as a top layer without digging in. South African soils generally benefit significantly from regular organic matter addition.
Topsoil is the upper layer of naturally occurring soil. It contains mineral particles, organic matter and micro-organisms. It is used to raise bed levels, fill low spots and create new lawn or garden areas. Compost is fully decomposed organic material (kitchen scraps, garden waste, manure). It does not provide the structural volume of topsoil but massively improves soil fertility, drainage in clay soils and water retention in sandy soils. Most SA garden projects benefit from a mix: topsoil to fill volume, compost to improve fertility.
Apply 75mm (7.5cm) of mulch around shrubs and in garden beds in South Africa. This depth suppresses most weed germination, retains significant soil moisture (reducing irrigation needs by 20–30%) and moderates soil temperature in both summer heat and winter cold. Thinner mulch layers (less than 50mm) do not suppress weeds effectively. Avoid piling mulch directly against plant stems or tree trunks, and leave a 100mm clear gap to prevent stem rot.
For a 10m² garden bed at 150mm deep: volume = 10m² × 0.15m = 1.5m³ net, or 1.725m³ (1,725 litres) once you add our 15% settling buffer. At 30 litres per retail bag, you need 58 bags, which at R35–R48 per bag would cost R2,030–R2,784. In this case, ordering 3 bulk bags (750L each = 2,250L total) from a landscaping supplier would be significantly cheaper and is the recommended option for volumes above 300L. Retail bags are economical only for very small areas or top-dressing.
Yes, freshly delivered topsoil, compost and mulch are loosely tipped and will compact by roughly 10–15% (our estimate) once watered, walked on or rained on, especially over the first few weeks. If you calculate exactly 1.0m³ for a bed and order exactly 1.0m³, you'll likely end up 100–150L short once it settles. Always add a 10–15% buffer (our estimate) to your calculated volume. For example, order 1.15m³ instead of 1.0m³. This is especially important when ordering bulk bags, since arranging a second small delivery for a shortfall often costs more proportionally than ordering slightly extra upfront. Materials with more fine particles (compost, fine topsoil) tend to settle more than coarse mulch.
For volumes above 300L, bulk bags are almost always cheaper and reduce the labour of handling many small bags. A 750L bulk bag at R600–R860 is equivalent to 25 × 30L retail bags at R35–R48 each (R875–R1,200), potentially saving up to about R600 on just one bag's worth of material. For small touch-up jobs under 150L, retail bags from a garden centre are more practical.