How to calculate concrete, mulch, gravel or soil
- Choose metric or US units and the material.
- Pick a rectangle (slabs, beds, paths) or a circle (round pads, tree rings, post holes) and enter the size and depth. Use Quantity for several identical pieces such as footings.
- Add an extra margin: 10% is typical for concrete, 5% for loose materials.
- Read the volume, weight and number of bags, or order by the cubic metre or cubic yard.
How the volume and bags are worked out
Volume is length × width × depth (or π × radius² × depth for a circle), with depth converted from centimetres or inches. A 3 × 3 m patio slab 10 cm thick is 0.9 m³, or 0.99 m³ with 10% extra: about 1.3 cubic yards, 2.4 tonnes of concrete and roughly 110 bags of 20 kg premix. That many bags is hard work, which is why ready-mix trucks are used for larger jobs.
Weights use typical densities: concrete about 2,400 kg/m³, gravel and sand about 1,600 kg/m³, topsoil about 1,300 kg/m³ and bark mulch about 400 kg/m³; moisture can change these by 10–20%. Bag yields come from typical product labels, such as 0.6 cu ft for an 80 lb concrete bag, so check the bag you buy. Loose materials also settle after spreading and compacting, so it helps to round up. All calculations run in your browser.
How many bags of concrete for a post hole?
Choose Circle, enter the hole diameter and depth, subtract the post if you like, and set the quantity to the number of posts.
How thick should a concrete slab be?
About 10 cm (4 in) for patios and paths; 15 cm (6 in) or more for driveways and garage floors.
How much does a cubic yard cover?
At 3 in deep, one cubic yard covers about 108 sq ft; at 2 in, about 162 sq ft.