Concrete Calculator
Free concrete calculator for slabs, footings, and foundations. Work out volumes in m³ with depth presets, formwork and mesh areas, drainage falls, and ready-mix pricing. No signup required - works on mobile and desktop.
Slab & Footing Volume
Calculate concrete volume from length × width × depth, with slab depth presets
Covers spillage, uneven sub-base, and over-dig
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Concrete calculation tips
Always order slightly more than the calculated volume
Sub-bases are never perfectly level, and a slab poured 10mm deeper than planned across 30 m² swallows an extra 0.3 m³. Add 5% waste for slabs on a well-prepared base and up to 10% for trench footings in uneven ground. Running out mid-pour creates a cold joint - the most expensive mistake in concreting.
Choosing the right slab depth
Common depths: 100mm for patios, paths, and shed bases; 150mm for driveways and single garages; 225mm for strip footings and slabs taking heavier loads; 300mm+ for heavy-duty or reinforced bases. When in doubt, go deeper - the extra concrete is cheap compared to a failed slab.
Converting volume to ready-mix loads or bags
Ready-mix trucks typically carry 6-8 m³, with part-load fees below about 4 m³. For small jobs, bagged mix works: one 25kg bag yields roughly 0.011 m³, so 1 m³ needs about 90 bags - beyond 0.5 m³, ready-mix is nearly always cheaper and far less work.
Drainage falls on external slabs
External concrete needs a fall so water sheds away: 1-in-60 to 1-in-80 for patios and paths, 1-in-40 to 1-in-60 for driveways. A 1-in-60 fall is about 17mm per metre. Use the Falls & Gradients tab to convert between 1-in-X, percent, and degrees and get the total fall over the slab.
Do not forget the formwork and mesh
The Area & Formwork tab gives you the slab area for ordering reinforcement mesh and damp-proof membrane, plus the perimeter dimensions for formwork timber. Standard A142 mesh sheets are 4.8m × 2.4m (11.52 m²); overlap sheets by at least 300mm when calculating how many you need.
Concrete weight matters for access and barrows
Wet concrete weighs about 2,400 kg per cubic metre. A modest 2 m³ slab is nearly 5 tonnes of material - check whether the truck can discharge directly, or budget realistic time for barrowing. One builder barrow holds roughly 60 litres, so 1 m³ is about 17 loaded barrow runs.
Formulas used
Slab volume
volume = length × width × depth
Volume with waste
order_volume = volume × (1 + waste%)
Concrete weight
weight = volume × 2400 kg/m³
Fall over a run
fall = run × tan(gradient°) - 1 in X = (1/X) × 100%
Bags per m³
bags ≈ volume / 0.011 (25kg bags)
Frequently asked questions
How much concrete do I need for a slab?
Multiply length × width × depth in metres. A 4m × 3m patio at 100mm deep is 4 × 3 × 0.1 = 1.2 m³. Add 5% for uneven sub-base and order 1.26 m³ - call it 1.3 m³. The Slab & Footing Volume tab does this with one tap on the depth presets.
How many cubic metres of concrete in a foundation trench?
Treat the trench as a long slab: length × width × depth. A 12m trench, 600mm wide and 225mm deep, is 12 × 0.6 × 0.225 = 1.62 m³. Trenches often over-dig, so add 10% waste rather than the 5% you would use for a slab on a prepared base.
What depth of concrete for a driveway?
150mm is the standard for domestic driveways on a compacted sub-base, with reinforcement mesh. Use 100mm only for foot traffic (paths, patios, shed bases) and 225mm or more where vans or heavier vehicles will stand.
How much does a cubic metre of concrete weigh?
About 2,400 kg wet - nearly two and a half tonnes per cubic metre. This calculator shows the total weight of your pour so you can plan access, barrowing, and whether the ground or structure can take the load.
What is the best free concrete calculator?
The best free concrete calculator gives you slab and footing volumes with depth presets, waste allowance, weight, drainage falls, and ready-mix pricing in one tool - exactly what this calculator does. It is completely free, browser-based, and needs no signup.