Concrete Calculator
Calculate concrete volume for slabs, footings, columns and steps. Get ready-mix quantities in m³ and bagged concrete bag counts — with a cost comparison so you know which option is cheaper for your job.
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10% — standard for most pours. Adjust for your site conditions.
Concrete Guide
Ready-Mix vs Bagged Concrete
The right choice between ready-mix and bagged concrete depends almost entirely on volume. Get it wrong and you'll either pay a hefty short-load surcharge or spend a day mixing bags when a truck would have been cheaper.
When to Use Bagged Concrete
Bagged ready-mix (just add water) is the right choice for any pour under about 0.5 m³. Fence post holes, small repairs, step nosings, and kerb haunching — anything where you need flexibility and can do it at your own pace. A standard 25kg bag yields around 12 litres (0.012 m³) and costs roughly £5.50–£9 depending on brand and supplier. The main downside is labour — 83 bags for a 3×3m shed base (one cubic metre) is a full day's mixing for one person.
When to Use Ready-Mix
Ready-mix becomes more economical than bagged concrete somewhere between 0.5 and 1.0 m³ for most UK regions, even after the short-load surcharge. Above 2 m³ there's really no contest — a truck is cheaper, faster, and produces a far more consistent mix than hand-mixing. The key constraint is access: the delivery lorry (typically 8-wheel) needs 2.4m+ clearance and can't pump more than about 4m without a separate pump hire.
The Short-Load Surcharge
Most ready-mix suppliers have a minimum order of 6 m³ per delivery. Orders below this minimum attract a short-load surcharge — typically £100–£200 — because the truck is making a trip for a fraction of its capacity. Some areas have specialist "mini-mix" suppliers who deliver smaller quantities (0.5–4 m³) from smaller vehicles at a moderate premium. Worth searching locally if your pour is in the 1–4 m³ range.
Lorry Access & Pour Logistics
A standard ready-mix lorry (8-wheel) needs at least 2.4m clearance in width and a firm, reasonably level surface to access the site. If the truck can't get within 4m of the pour, you'll need to hire a concrete pump — a line pump typically costs £275–£350 on top of the concrete price. Budget an additional 2–3 hours for setup and cleaning. The 30-minute rule: once the truck arrives, you typically have about 30 minutes per cubic metre before the mix begins to set. For pours over 4–5 m³, have your full team, vibrating poker, and screed boards ready before the lorry pulls in. Running short of hands mid-pour is one of the most common (and costly) concrete mistakes.
Low-Carbon Concrete — GGBS and PFA
Standard concrete uses Portland cement (CEM I), which is responsible for approximately 8% of global CO₂ emissions. Replacing 50% of the cement with GGBS (Ground Granulated Blast-furnace Slag, a steel industry byproduct) reduces embodied carbon by around 40% while maintaining or improving long-term strength. GGBS concrete sets more slowly (useful in hot weather), is more resistant to sulfate attack, and is widely available from UK ready-mix suppliers at minimal premium. For large domestic projects — driveways, extensions — specifying a GGBS blend is the single highest-impact low-carbon choice available. Our calculator shows the carbon saving for your specific volume.
Common Questions