Measure your slab, circle or column, enter the depth and a waste allowance, and see the volume in cubic metres and yards, the approximate weight in tonnes, and how many bags to buy.
Results are estimates for planning. Check the figures before you make a financial or health decision.
How the Concrete, gravel and mulch calculator works
The calculator first works out the volume in cubic metres.
- Slab: length × width × depth.
- Circle: π × (diameter ÷ 2)² × depth.
- Column or post hole: π × (diameter ÷ 2)² × height.
Depth can be entered in centimetres, millimetres or inches and is converted. The volume is multiplied by the number of identical areas, then increased by your waste percentage. Cubic yards use 1 yd³ = 0.7646 m³.
Weight is volume × density. The densities offered are approximate: loose gravel is typically around 1.6 tonnes per m³, and you can type your own figure. Bag counts for gravel, sand and other loose material divide the weight by 25 kg, 40 kg or your bulk bag size, rounding up. Concrete bags use the yield per 20 kg bag that you enter (the default is a guide, so check the bag). Mulch bags use the litre size you enter, with 1 m³ = 1,000 litres.
The tool does not allow for compaction, moisture or ground that is not level.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Shape | Slab or rectangle |
| Lengths in | Metres |
| Length | 4 |
| Width | 3 |
| Depth | 10 |
| Depth measured in | Centimetres |
| How many identical areas or columns | 1 |
| Extra for waste and uneven ground | 10 % |
| Material | Gravel or aggregate (loose) |
| Density (approximate) | 1.6 t/m³ |
| Bulk bag size | 850 kg |
Volume needed: 1.32 m³
| Volume before waste | 1.2 m³ |
|---|---|
| Volume including 10% extra | 1.32 m³ (1,320 litres) |
| Cubic yards | 1.73 yd³ |
| Approximate weight | 2.11 tonnes (2,112 kg) |
| 25 kg bags | 85 |
| 40 kg bags | 53 |
Tips and common mistakes
Measure the depth in several places, because ground is rarely flat, and use a higher waste figure for uneven or soft ground. Gravel and sand are compacted when laid, so you may need more than the loose volume suggests. For concrete, use the yield printed on the bag rather than a general figure, and for a large pour ask a ready-mix supplier, who will want cubic metres. Round up and keep a few spare bags. Check the units before you rely on a result: a slab 10 cm deep is 0.1 m, not 10 m.
Frequently asked questions
How do I convert cubic metres to tonnes?
Multiply the volume by the density of the material. For loose gravel an approximate figure is 1.6 tonnes per cubic metre, so 1 m³ is about 1.6 tonnes. Compacted or wet material is heavier.
How many bags of concrete mix do I need?
Divide the volume in litres by the yield of one bag. The yield is printed on the bag, and the calculator lets you enter it. Always round up.
Why add 10% waste?
Spillage, uneven ground, settling and over-excavation all use extra material. Ten per cent is a common allowance, and you can change it.