How Much Concrete for Patio?
A concrete patio provides a versatile outdoor living space that can be left as a plain slab, brushed for a textured finish, or used as a sub-base for paving slabs or porcelain tiles. For a 6m x 4m patio at 100mm depth, you may need approximately 2.4m³ of concrete, or around 294 x 20kg bags of ready-mix. This guide covers site preparation, drainage planning with a gentle slope away from the house, and techniques for achieving a smooth, level finish.
Planning and Preparation
Mark out the patio area with string lines and measure the diagonals to ensure it is square. The patio should slope gently away from the house at a gradient of 1 in 60 (approximately 16mm per metre) to prevent rainwater from pooling against your walls. Excavate to a depth of 200mm or more, depending on ground conditions: 100mm of compacted Type 1 sub-base plus 100mm of concrete. Remove any topsoil completely, as organic matter will decompose and cause the slab to sink. Build formwork from straight timber boards, securing them firmly so they do not bow under the weight of wet concrete. A patio of 6m x 4m is a substantial pour, so consider whether you can manage it in sections.
Mixing Concrete
For this project our estimate suggests around 2.4m³ of concrete. You have two options: use approximately 294 x 20kg bags of ready-mix concrete (such as Blue Circle, Postcrete, or Hanson Multicem), or mix your own using a C20 mix ratio of 1:2:3 by volume. To mix your own, you'll likely need around 660kg of Portland cement (27 x 25kg bags), 1408kg of sharp sand, and 2376kg of 20mm gravel aggregate — a total of roughly 4444kg of materials. For small batches, mix by hand on a clean board or in a wheelbarrow: combine the dry materials first until the colour is uniform, then gradually add water until the mix holds its shape when squeezed but is still workable. For larger volumes, hire a cement mixer — a half-bag (electric) mixer handles most DIY jobs and costs around £25-35 per day from tool hire shops. Avoid adding too much water, as this weakens the finished concrete significantly.
Pouring Technique
A patio this size is best poured in sections (bays) of no more than 3m x 3m at a time, using temporary formwork to divide the area. This keeps the work manageable and creates natural expansion joints between bays. Pour the concrete and screed it level, maintaining the drainage slope away from the house. A brushed finish is the most practical for a patio as it provides grip when wet. Alternatively, a smooth power-floated finish looks sleek but can be slippery in rain. If you plan to lay paving slabs or porcelain tiles on top of the concrete later, a rough tamped finish is fine — the adhesive will bond better to a textured surface. Make sure the finished level allows for the thickness of the final surface material plus adhesive.
Weather Considerations
For a patio this size, weather planning is critical. Check the forecast for a window of at least 48 hours without heavy rain or frost. Concrete takes 24-48 hours to achieve its initial set and remains vulnerable to weather damage during this period. In summer, pour in the early morning or late afternoon to avoid the midday heat. Keep the slab damp by misting with water and covering with polythene or damp hessian for at least three days. Proper curing in the first week accounts for most of the final strength. If you are pouring the patio in bays over several days, ensure each bay is protected from rain and frost individually. Expansion joints between bays allow for thermal movement and prevent random cracking across the slab surface.
Curing Times
A concrete patio should be left undisturbed for at least 48 hours after pouring. Light foot traffic is acceptable after 48 hours, but avoid placing heavy garden furniture or planters on the slab for at least 7 days. Full design strength is reached at 28 days. Proper curing is essential for a patio, as the large surface area is prone to drying out and developing shrinkage cracks. Cover the slab with polythene sheeting for a minimum of 3 days, and ideally 7 days, keeping the surface damp underneath. If you are laying paving slabs or tiles on top, wait at least 14 days before starting — this allows the concrete to cure sufficiently and reduces the risk of adhesive failure due to residual moisture in the slab.
Estimated Cost
Prices for concrete materials in the UK vary by region and supplier. Using ready-mix bags, approximately 294 x 20kg bags may cost around £1176-£1764 at typical retail prices of £4-6 per bag. If you mix your own, the individual materials may cost roughly £325-£506 (cement £135-£216 for 27 x 25kg bags, sharp sand £70-£110 for 2 bulk bags, aggregate £120-£180 for 3 bulk bags). For a job this size, mixing your own concrete from bulk materials may be significantly cheaper than buying individual ready-mix bags. Consider ordering a bulk bag (dumpy bag) of sand and aggregate from a local builders’ merchant for the best price. If you need to hire a cement mixer, expect to pay £25-35 per day from most tool hire companies. A wheelbarrow, shovel, and spirit level are also essential tools you may need to buy or borrow.