How to Calculate How Much Concrete You Need
Whether you are laying a garden patio, pouring a garage floor, or setting fence posts, knowing how much concrete to order is essential. Too little and you will have gaps and weak spots; too much and you are wasting money on a material that cannot be returned once mixed.
The basic formula for a rectangular slab is straightforward: Volume (m³) = Length × Width × Depth. For example, a 5m × 3m patio at 100mm (0.1m) depth requires 5 × 3 × 0.1 = 1.5 m³ of concrete. Always add a waste allowance of at least 10% to account for uneven ground, spillage, and formwork irregularities.
Bags vs Ready-Mix
For small jobs under 1 m³ — such as fence posts or a small shed base — pre-mixed bags from a builders' merchant are the most practical option. A standard 25kg bag yields approximately 0.012 m³, while a 40kg bag yields around 0.02 m³. For anything above 1 m³, ordering ready-mix concrete from a local supplier is almost always cheaper and less labour-intensive. Ready-mix is delivered in a lorry and poured directly into your formwork.
Choosing the Right Concrete Mix
In the UK, concrete mixes are specified by strength class (e.g., C20, C25) or standardised prescribed mixes (ST2, ST3, RC25). For most domestic applications — patios, driveways, and shed bases — a C25 / ST3 mix is appropriate. Structural foundations and reinforced concrete require a stronger mix; always consult a structural engineer for load-bearing applications.