Volume is the starting quantity
Soil, compost, bark, and mulch fill a three-dimensional bed. For a rectangular bed, cubic metres equal length × width × settled depth in metres. The calculator converts centimetres to metres before multiplying, then can convert the volume into bags using the bag volume. If a material is sold by mass, the optional bulk density produces an approximate tonne or kilogram figure. Volume and mass are not interchangeable: damp soil, fluffy mulch, and compacted compost occupy different volumes for the same weight.
Raised-bed example
A bed 4.8 m long and 1.25 m wide is 6.0 m². Filling it to 18 cm means 6.0 × 0.18 = 1.08 m³ before settlement. With a 10% settling allowance, the order volume is 1.188 m³. For 50 L bags, divide by 0.05 m³: 23.76 becomes 24 bags. If a topsoil supplier quotes 1,300 kg/m³, that same order is about 1,544 kg. A delivery sold as 1.2 m³ is therefore plausible; a single 50 L bag plainly is not.
Choose depth for the job
A decorative mulch layer may be 5–8 cm after settling, while a new vegetable bed may need a much deeper layer of soil or compost depending on what already lies beneath. Do not use the same depth simply because the calculator accepts it. Existing topsoil, drainage, roots, edging height, and the intended plants determine the target. Soil added to a container or raised bed often settles after watering, so a modest allowance is sensible. Mulch also decomposes and compresses, which is why a spring depth may not match the autumn measurement.
Irregular beds need simple pieces
For a triangular bed, area is base × height ÷ 2; for a circular bed, area is πr². Split kidney-shaped borders into rectangles, triangles, and semicircles, then total them before entering an equivalent area or use their average measured width with caution. Avoid measuring the outer edge of a curved border and multiplying it by a guessed width; that can overstate the fill. A slope changes depth as well as area, so calculate the actual profile if the material is intended to level it.
Bag labels and delivered loads
Bagged product is commonly labelled in litres, while bulk yards use cubic metres or cubic yards and may quote a minimum delivery. Convert litres by dividing by 1,000: 50 L is 0.05 m³. Do not type ‘50’ into a cubic-metre bag field. Bulk density helps estimate trailer and wheelbarrow loads, but supplier material can be wet or dry and contains variable particle sizes. Ask whether the supplier’s stated volume is loose, screened, or compacted; a calculation cannot recover volume lost when a load settles in transit.
Mulch and soil are different materials
Mulch is a surface layer used to moderate weeds, evaporation, and temperature; it is not automatically a planting medium. Topsoil supports roots but can be poor in structure or nutrients; compost is an amendment, not a universal replacement for native soil. Use this calculator for the quantity of the selected material, then choose the material based on plants and site conditions. Putting a deep layer of fine compost directly against stems, burying root flares, or piling mulch against a building can create horticultural or moisture problems that arithmetic cannot identify.
Common errors
The biggest error is entering depth in centimetres as metres: 15 cm is 0.15 m, not 15 m. Another is calculating the outside dimensions of a bed but forgetting a pond, path, or established shrubs that displace fill. Ordering by bags without accounting for settlement produces a short delivery. Conversely, adding a large reserve to a poorly measured bed merely hides uncertainty. Measure length and width at several points, write the intended settled depth, and keep the allowance visible rather than burying it inside a guessed dimension.
How it differs from the existing mulch page
The existing Mulch Calculator returns a simple rectangular volume from length, width, and depth. This soil-and-mulch estimator adds settlement, bag conversion, and optional mass from bulk density, which makes it useful when comparing bagged product with a bulk delivery. It is not a gravel calculator: mineral aggregate has a different density, compaction behaviour, and often requires a specified base depth. Nor does it prescribe soil chemistry, fertiliser, drainage, or planting depth.
A sensible ordering check
Calculate the base volume, then inspect whether the resulting bags or delivery load matches the physical bed. Keep a small amount of material for topping up after settlement, but arrange delivery access before ordering a tonne of wet soil. Verify the bag’s litres or the supplier’s cubic-metre measure and use density only as a transport estimate. Inputs stay in the browser; the result is private, yet the final decision should still account for local soil, drainage, plant requirements, and the supplier’s own product specification.
One last practical check
A volume estimate should be checked against the site before delivery day. A cubic metre is a pile roughly one metre wide, one metre long, and one metre high when loose; visualising that scale catches many decimal errors. Make sure a bulk load can be placed without blocking drains, pavements, or access, and plan where bags will be stored dry. Spread material in shallow passes and measure the settled depth rather than emptying every bag at one end. This avoids burying plants or discovering too late that an edged bed has less usable depth than its outside dimensions suggested.