Concrete Calculator

How This Is Calculated

For a slab or footing, the calculator multiplies length × width × depth to get a raw volume in cubic metres, then adds your chosen waste allowance. Ten percent is the standard trade allowance for a reasonably level, prepared base — increase it to 15% or more on rough, hand-dug or sloped ground where over-excavation is likely.

For fence post holes, the calculator works out the volume of a cylindrical hole (using the diameter and depth you enter), then subtracts the volume of the post itself for the depth it sits in the ground. This net volume is what actually needs filling with concrete or postcrete.

Bag counts are based on a typical yield of approximately 0.0125m³ per 25kg bag and 0.01m³ per 20kg bag of general-purpose concrete. Actual yield varies by brand and by how wet the mix is, so treat the bag count as a solid starting point and always round up rather than down — running short of concrete mid-pour is far worse than having half a bag left over.

Common Questions

How much extra concrete should I order for waste?

Add 10% to your calculated volume as standard to cover spillage, uneven ground and over-excavation. For a hand-dug footing on rough ground, 15% is safer.

How many 25kg bags of concrete make a cubic metre?

Roughly 80 bags of 25kg general-purpose concrete make one cubic metre, based on a typical yield of about 0.0125m³ per bag. This varies by brand, so check the bag for its exact stated yield on larger jobs.

Is it cheaper to use bagged concrete or ready-mix for a big job?

For most UK regions, ready-mix delivered by lorry becomes cheaper than bagged concrete somewhere between 0.5m³ and 1m³, once you account for the labour of mixing dozens of bags by hand.

How deep should a fence post hole be?

A common rule of thumb is one third of the post's above-ground height, with a practical minimum of around 450-600mm for a standard garden fence post, deeper in exposed or windy locations.