Concrete, Gravel and Sand Calculator

Guide

How the amount is worked out

Slab volume = length × width × thickness, in metres. Add waste, then divide by how much one bag makes.

Example. A 3 × 3 m slab 100 mm thick is 3 × 3 × 0.1 = 0.9 m³. A 25 kg bag makes roughly 0.012 m³ here, so that is 75 bags with no waste allowance, and 83 bags with 10% extra.

Bag yields: an honest note

Bag yields differ between brands and mixes, and the figures here for 20 kg and 25 kg bags are editorial rules of thumb, not figures from a manufacturer’s data sheet. The yield is printed on the bag or on the product data sheet, so enter that in the yield box before you buy.

Ready-mix or bags?

For more than about a cubic metre it is often easier to order ready-mixed concrete than to mix bags by hand. Suppliers have minimum loads and short-load charges, so ring and ask.

Gravel, sand and topsoil

Suppliers sell these by the tonne or by the bulk bag. To go from volume to weight multiply by a density. The densities here are editorial starting points, not standards: aggregates vary with moisture and grading, so ask your supplier.

Safety and rules

Before you dig, check for buried pipes and cables, for example with the free LSBUD service, and check local rules and any permissions that apply to your property. This is not building or structural advice. Foundations and slabs that carry loads may need approval under the Building Regulations.

Common questions