Hong Life CalculatorsUnderstanding the gravel, sand, soil and mulch calculator — concept and principle
This calculator converts an area and a depth into the volume of gravel, sand, topsoil, bark mulch or crushed stone you need, then turns that volume into a weight in tonnes and a number of 25 kg bags. Use it before ordering landscaping materials, since bulk suppliers usually sell by the cubic metre or by the tonne while garden centres sell by the bag.
How it is calculated
- Volume (m³) = area (m²) × depth (cm) ÷ 100.
- Weight (t) = volume × density (t/m³).
- 25 kg bags = ⌈weight × 1,000 ÷ 25⌉.
Typical loose densities used here: gravel and dry sand about 1.6 t/m³, topsoil about 1.3, bark mulch about 0.4 and compacted crushed stone, such as a Type 1 sub-base, about 2.1 (typically 2.0–2.2).
The principle
A layer of uniform depth is a flat prism, so its volume is simply area times thickness; depth is divided by 100 to convert centimetres to metres. Weight depends on bulk density, the mass of a cubic metre including the air spaces between particles. That is why loose bark is light while compacted stone is heavy: more voids mean less mass per cubic metre. Moisture also matters, because water fills those voids and adds weight.
Worked example
Default inputs: area 20 m², depth 5 cm, material dry sand (1.6 t/m³).
- Volume = 20 × 5 ÷ 100 = 1 m³.
- Weight = 1 × 1.6 = 1.6 t.
- Bags = 1.6 × 1,000 ÷ 25 = 64 bags.
| Area (m²) | Volume (m³) |
|---|---|
| 5 | 0.25 |
| 10 | 0.5 |
| 15 | 0.75 |
| 20 | 1 |
| 25 | 1.25 |
| 30 | 1.5 |
| 40 | 2 |
How to read the result
A tonne bulk bag usually holds less than 1 m³ of heavy material, so compare by weight or volume according to how the supplier sells. Typical depths are 3–5 cm for decorative gravel paths, 5–8 cm for bark mulch on beds, and 10–15 cm or more for topsoil on new lawns or for a driveway sub-base. If the material will be compacted, its final depth will be lower than the loose depth, so order extra. For a sloped or irregular area, split it into rectangles and triangles and add the volumes.
Common mistakes
- Entering depth in metres or millimetres instead of centimetres.
- Mixing up loose and compacted densities.
- Forgetting the extra needed for compaction and uneven ground.
- Ordering by tonnes for light materials such as mulch, which is normally sold by volume.
Frequently asked questions
How much does 1 m³ of gravel weigh?
Roughly 1.5–1.7 tonnes for loose gravel; wet or compacted material can be heavier.
How many 25 kg bags make one cubic metre of sand?
About 64 bags at 1.6 t/m³.
How deep should a gravel path be?
A decorative top layer of about 3–5 cm over a firm base is common; thicker loose layers are hard to walk on.
Sources: ASTM C29/C29M — Standard Test Method for Bulk Density ("Unit Weight") and Voids in Aggregate