Concrete volume for slabs, footings, columns, stairs and cylinders — with bag counts, mix quantities by grade, rebar weight and the point where ready-mix becomes cheaper than bagging it yourself. That crossover is the decision most people get wrong, and it arrives sooner than expected.
Concrete Calculator
LiveThe formulas
A 6 × 4 m slab at 150 mm: 6 × 4 × 0.15 = 3.60 m³ net, or 3.96 m³ with a 10% allowance. That is 5.18 cubic yards and weighs about 9.5 tonnes. Bagged, it needs 441 bags of 20 kg — which is the number that usually settles the ready-mix question.
Bags or ready-mix? The crossover is around 1 m³
The arithmetic is unforgiving. A 20 kg bag yields about 9 litres of wet concrete, so one cubic metre takes roughly 110 bags — over two tonnes of material to carry, open and mix by hand.
| Volume | Bags of 20 kg | Realistic approach |
|---|---|---|
| Under 0.3 m³ | ~35 | Bags. A wheelbarrow or small mixer handles it comfortably |
| 0.3 – 1 m³ | 35 – 110 | Bags with a hired mixer, or a mix-on-site delivery. Hard work but feasible |
| 1 – 3 m³ | 110 – 330 | Ready-mix, almost always. Most suppliers have a minimum charge here, but the labour saved outweighs it |
| Above 3 m³ | 330+ | Ready-mix without question. Hand-mixing this volume is not realistic in the working time available |
Beyond the labour, there is a quality argument that matters more. Hand-mixed concrete varies batch to batch — water content drifts, ratios slip, and consecutive batches poured against each other create cold joints if the pace falls behind. Ready-mix arrives consistent and continuous. For anything structural, that consistency is worth more than the cost difference.
Mix ratios, and what each is for
| Ratio | Grade | Typical use |
|---|---|---|
| 1:3:6 | C15 / M10 | Mass fill, blinding, non-structural bases under slabs |
| 1:2:4 | C20 / M15 | Paths, patios, shed bases, general domestic slabs |
| 1:1.5:3 | C25 / M20 | Structural slabs, foundations, reinforced work |
| 1:1:2 | C30 / M25 | Columns, beams, anything carrying significant load |
The ratios are cement : sand : coarse aggregate by volume. Water is the part nobody quotes because it is the part people get wrong — a water-cement ratio around 0.45 to 0.55 by weight is the target for most work.
What the calculator's quantities assume
Dry material volumes are computed with a bulking factor of 1.54. This accounts for the fact that dry ingredients occupy substantially more space than the wet concrete they produce — the sand fills voids between aggregate, and cement paste fills voids between sand grains. Ordering by wet volume without applying it leaves you roughly a third short.
Cement is converted at 1,440 kg/m³, the standard bulk density for loose Portland cement. Concrete weight uses 2,400 kg/m³ for normal-weight mixes; lightweight aggregate concrete runs 1,800–2,000 and should be checked against the supplier's figure.
Curing: where the strength actually comes from
Concrete does not dry — it hydrates. Cement reacts chemically with water, and that reaction stops when the water leaves. A slab that dries out at three days stops gaining strength at three days, permanently. No amount of later wetting restarts it.
ACI recommends curing until the concrete reaches 70% of its design strength, which at 20 °C takes about seven days. But that seven-day figure is a temperature-specific answer that gets quoted as though it were universal, and it is not:
| Curing temperature | Days to match 7 days at 20 °C | What is happening |
|---|---|---|
| 5 °C | 14 days | Hydration is barely proceeding. Below 5 °C it nearly stops |
| 10 °C | 10.5 days | Half the rate of a mild day. Plan for it |
| 15 °C | 8.4 days | Slightly slower than reference |
| 20 °C | 7 days | The reference condition every rule of thumb assumes |
| 30 °C | 5.2 days | Faster early gain — but see the warning below |
This is the maturity method, standardised as ASTM C1074. It combines time and temperature into a single index, and the calculator above applies it: enter your average curing temperature and days elapsed, and it estimates the strength reached as a percentage of the 28-day figure.
Hot weather is worse than it looks
| Condition | What ACI advises |
|---|---|
| Placement temperature | Keep between 10 °C and 32 °C. Above that, cool the mix with chilled water or ice, and place at night |
| Curing above 5 °C | Maintain for at least 48 hours minimum, and preferably through the full curing period |
| Cold weather | Below 10 °C, insulate to hold the concrete above 5 °C. Never place on frozen subgrade — it settles when it thaws |
| Mass pours | Keep the core-to-surface temperature difference under about 20 °C, or thermal stress cracks the element from the inside |
| Evaporation | Wind and low humidity dry a surface faster than heat alone. Wet coverings also cool by evaporation, which helps twice over |
Sources: ACI 308 (Guide to External Curing of Concrete) · ACI 301-16 · ACI 209 (Prediction of Creep, Shrinkage and Temperature Effects) · ASTM C1074 (Maturity Method). Cited for the specific figures above.
Reinforcement, roughly
| Element | Typical steel | Note |
|---|---|---|
| Ground slab with mesh | 40–60 kg/m³ | Often A142 or A193 mesh rather than loose bar |
| Suspended slab | 80–120 kg/m³ | Two-way reinforcement, top and bottom in places |
| Footings and pads | 60–90 kg/m³ | Depends heavily on soil and load |
| Columns | 150–250 kg/m³ | Congested — the highest ratio of any common element |
| Beams | 120–200 kg/m³ | Concentrated in the tension zone plus shear links |
Budgetary figures for estimating steel tonnage. Actual reinforcement comes from a structural engineer's drawings — bar size, spacing, cover and lap lengths are design decisions, not estimates.
Cold weather, and why it is different from heat
Heat makes concrete set too fast; cold makes it stop setting altogether, and the two failures need opposite responses:
| Condition | What happens | What it needs |
|---|---|---|
| Above 30 °C | Rapid set, plastic shrinkage cracking, lower ultimate strength | Pour early or late, chilled water, shade, cover immediately. Never add water to restore workability |
| 10 to 25 °C | Normal hydration | Keep damp for at least seven days |
| Below 10 °C | Hydration slows sharply. Strength gain roughly halves for every 10 degrees below 20 | Extend curing, delay striking formwork, consider heated water |
| Below 5 °C | Hydration nearly stops | Insulated blankets. Do not rely on the 28-day figure |
| Freezing before set | Permanent damage. Water expands as ice and tears the paste apart before it has strength to resist | Nothing recovers it. The pour is lost |
Slump, and the water that costs you strength
Slump measures workability — how far a cone of fresh concrete settles when the mould is lifted. Adding water raises it and lowers the strength, and the trade is worse than it looks:
| Slump | Suits | Note |
|---|---|---|
| 25–50 mm | Pavements, slipform, heavily vibrated work | Stiff. Needs mechanical compaction |
| 75–100 mm | Most slabs, footings and walls | The usual specification for general work |
| 125–150 mm | Congested reinforcement, pumped work | Should come from a plasticiser, not from a hose |
| Above 175 mm | Self-compacting mixes only | In an ordinary mix this means too much water |
The water-cement ratio governs strength more than anything else in the mix. Adding roughly 20 litres of water to a cubic metre raises slump by about 25 mm and costs somewhere around 10% of the 28-day strength — a trade nobody would accept if it were stated that plainly on site.
Common mistakes
Frequently asked questions
How many bags of concrete do I need for a cubic metre?
About 110 bags of 20 kg, 88 of 25 kg, or 58 of 40 kg. Each 20 kg bag yields roughly 9 litres of wet concrete. That is over two tonnes of material to carry and mix for a single cubic metre, which is why ready-mix wins above about 1 m³ for most people regardless of the price comparison.
How thick should a concrete slab be?
Common minimums are 100 mm for footpaths and patios, 150 mm for a driveway carrying cars, and 200 mm or more where heavy vehicles are involved. Those are starting points rather than answers — the required thickness depends on the subgrade bearing capacity, the loading and the reinforcement. Anything structural should come from an engineer.
What is the right mix ratio?
1:2:4 cement to sand to aggregate suits paths, patios and general domestic slabs. Use 1:1.5:3 for structural work and foundations, and 1:1:2 for columns and beams. Water is the part that matters most and gets quoted least — aim for a water-cement ratio of 0.45 to 0.55 by weight, and resist adding more to improve workability.
Why does adding water weaken concrete?
Because only a limited amount of water reacts with the cement. The excess remains in the mix, then evaporates and leaves voids — and voids are where strength goes. Raising the water-cement ratio from 0.5 to 0.65 can cost around a third of the compressive strength. The concrete is easier to place and permanently weaker, and no amount of curing recovers it.
How much waste should I allow?
Ten percent for most pours, five if the subgrade is level and properly formed, and fifteen to twenty for footings against earth or on uneven ground. Running short mid-pour creates a cold joint — a permanent plane of weakness — so over-ordering slightly is much cheaper than the alternative. Have a use planned for the surplus before the truck arrives.
How long before I can walk on it or drive on it?
Roughly 24 to 48 hours for foot traffic, seven days for light vehicles, and 28 days for full design strength and heavy loads. Those are typical and depend on temperature — cold weather slows hydration substantially, and below about 5 °C it nearly stops. Keep it damp throughout: curing is where the strength comes from, and it is the cheapest part of the job.
Does the calculator include reinforcement?
It gives a budgetary steel tonnage from typical ratios per cubic metre, which is useful for pricing. It does not size or detail anything. Bar diameter, spacing, cover and lap lengths come from a structural engineer's drawings — those are design decisions, and getting them wrong is not visible until something fails.
Is my input stored?
No. Everything runs in your browser with no server request, and works offline once the page has loaded.
Sources
- American Concrete Institute, ACI 308R. Guide to External Curing of Concrete. The seven-day damp curing recommendation and the strength-gain figures behind the diagram above.
- American Concrete Institute, ACI 305R. Guide to Hot Weather Concreting. Covers rapid set, plastic shrinkage cracking and why adding water on site is the wrong response.
- American Concrete Institute, ACI 306R. Guide to Cold Weather Concreting. The protection requirements below 5 °C and the consequences of an early freeze.
- American Concrete Institute, ACI 211.1. Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete. The mix ratios and water-cement relationships quoted here.
- ASTM C143 / C143M. Standard Test Method for Slump of Hydraulic-Cement Concrete. Defines the slump test and its ranges.
- BS EN 206 and BS 8500. The European and UK equivalents, using strength classes such as C25/30 rather than the American psi grades.
- Neville AM. Properties of Concrete. Pearson. The standard textbook reference for hydration, the water-cement ratio and temperature effects.
- Volume arithmetic. One cubic yard is 27 cubic feet; one cubic metre is 35.3147 cubic feet. Bag yields — 0.30, 0.45 and 0.60 cubic feet for 40, 60 and 80 lb bags — are the manufacturers’ published figures.
Quantities are computed from the dimensions you enter and the published bag yields. Concrete work carries real structural consequences — for anything load-bearing, a slab over about 100 mm, or any element supporting a building, the specification should come from a qualified engineer rather than from a web page.
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See the full list of Construction calculators, or try:
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- Brick Calculator — masonry with mortar and openings
- Paver Calculator — the alternative to a poured slab
- Material Weight Calculator — loads from dimensions and density
- Beam Deflection Calculator — how a concrete beam behaves under load