Concrete volume, mass, and bag count for a slab, footing, or column.
Density 2400 kg/m³ (C25/30 standard). Bag yield rough: 1 m³ ≈ 40 × 25 kg = 25 × 40 kg pre-mix bags. Bulk delivery is cheaper above 1 m³.
Concrete is sold by the cubic metre at the ready-mix plant and by the 25 kg or 40 kg bag at the hardware store, but every project is specified in metres of length, centimetres of thickness, and millimetres of rebar. Translating between dimensions and order quantities is the first task of any DIYer pouring a slab, building a strip footing, or casting a round column — and the most common error is under-ordering. A 20 m² shed slab with 12 cm thickness needs 2.4 m³ of concrete; a 20 cm slab needs 4 m³. Mixing up the thickness unit (cm versus m) shifts the result by 100×. The calculator handles three standard shapes (slab, strip footing, round column), the unit conversions, the standard 5 % waste margin (formwork loss, spillage, roundness imperfection), and the conversion from cubic metres to typical bag counts at both 25 kg and 40 kg sizes — and the cost from your concrete price per cubic metre.
For a slab or strip footing: volume (m³) = length (m) × width (m) × thickness (m). The calculator takes thickness in cm and divides by 100. For a round column: volume = π × (diameter/2)² × height, with diameter in cm and height in m, and the calculator handles the conversion. The result is multiplied by 1.05 to apply the 5 % waste factor (the standard recommendation across construction guides; for complex pours with multiple obstacles, increase to 10 %). Mass is volume × 2 400 kg/m³, the density of standard C25/30 concrete used for residential slabs and footings. Bag-count conversions: 1 m³ ≈ 40 bags of 25 kg pre-mix, or 25 bags of 40 kg pre-mix — these are practical rules of thumb based on typical pre-mix yields (the bag itself contains cement + aggregate, and 1 kg of dry mix produces about 0.55 L of cured concrete). For larger projects (above 1 m³), bulk delivery from a ready-mix truck is cheaper per cubic metre and avoids hauling 40+ bags.
Pick the shape from the dropdown (slab, strip footing, round column). The form rearranges to ask for the relevant dimensions: slab and footing want length, width, and thickness; column wants diameter and height. Defaults are a 5 m × 4 m × 12 cm slab — a typical garden shed footprint. Enter your concrete price per cubic metre (defaults to €120/m³, the 2024 retail ready-mix price in France for standard C25/30 with delivery within 15 km). The result panel shows the volume to order (with waste already added) as the headline, the exact volume alongside, the bag counts at both 25 kg and 40 kg, the total mass, and the estimated cost. The cost figure is for the concrete only — formwork, rebar, vapour barrier, gravel sub-base, and labour are separate line items.
A 5 m × 4 m garage slab at 12 cm thickness. Volume = 5 × 4 × 0.12 = 2.4 m³. With 5 % waste = 2.52 m³. Mass = 2.52 × 2 400 = 6 048 kg. Bags 25 kg = ceil(2.52 × 40) = 101. Bags 40 kg = ceil(2.52 × 25) = 63. Cost at €120/m³ = €302. A round column, 30 cm diameter × 2.5 m tall: volume = π × (0.15)² × 2.5 = 0.177 m³. With waste = 0.186 m³. Mass = 446 kg. Bags 25 kg = 8. Cost at €120/m³ = €22. A strip footing for a small wall, 12 m long × 50 cm wide × 30 cm deep: volume = 12 × 0.5 × 0.3 = 1.8 m³. With waste = 1.89 m³. Mass = 4 536 kg. Bags 25 kg = 76. Cost at €120/m³ = €227. The slab is the dominant order; the column is bag-territory; the footing sits in the awkward 1–2 m³ band where bulk delivery starts to make economic sense.
First, mixing units. Thickness in cm and length in m is the standard for residential pours, and the calculator does the conversion — but many online calculators expect everything in m or everything in cm, and a 12 instead of 0.12 produces a 100× over-order. Always sanity-check: a 10 m² slab at 12 cm thick should be just over 1 m³; if your number is 100 m³, you have a unit error. Second, omitting the waste factor. A 0 % waste calculation is mathematically correct but practically wrong: formwork seepage, pump-line flushing, levelling adjustments, and last-minute oversights consume 3 to 8 % of any pour. The 5 % default is a sensible minimum for simple geometries. Third, picking the wrong concrete grade. C25/30 is standard for residential slabs and footings; a driveway or load-bearing column may need C30/37 or higher. The density (2 400 kg/m³) is similar across grades, but cost and bag conversions differ slightly. Fourth, ignoring rebar. Reinforcement steel adds significant project cost and weight (10 mm bars at 200 mm spacing weigh about 5 kg per m² of slab) but does not displace concrete volume meaningfully — the calculator returns the concrete number, not the total project mass. Fifth, ordering in the wrong unit at the ready-mix supplier. French and continental suppliers quote and deliver in cubic metres; the US uses cubic yards (1 yd³ = 0.7646 m³). Confirm the order in the supplier's native unit.
Concrete pricing is volatile. The 2024 French retail price for ready-mix C25/30 with delivery within 15 km of a plant is €110–€150/m³ depending on supplier and quantity. Bag pre-mix retails at €4–€8 per 25 kg bag, which works out to €160–€320/m³ — significantly more expensive per cubic metre than ready-mix, but the only viable option for sub-1 m³ pours where the truck minimum doesn't make sense. Fibre-reinforced concrete adds polypropylene or steel fibres for crack resistance, eliminating some rebar in light-duty slabs; cost premium is 10–25 %. Self-compacting concrete (SCC) flows into formwork without vibration, useful for densely-reinforced columns; cost premium is 20–40 %. Eco-concrete with cement substitutes (slag, fly ash) reduces CO₂ by 30–60 % and is increasingly available from major suppliers; cost is similar or slightly higher than standard. The mass calculation in the tool uses the standard 2 400 kg/m³ density, valid for most general-purpose mixes; lightweight concrete (1 500–1 800 kg/m³) and heavyweight concrete (3 000–3 500 kg/m³, used for radiation shielding) require different density inputs not exposed by the tool.