Concrete Volume Calculator
Work out how much concrete a slab, footing, column, ring or set of steps needs — cubic yards or cubic metres, bags, weight and cost.
Fill in the dimensions — the volume, bag count and cost update as you type.
- Concrete volume (net)
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- Order this much (10% waste)
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- Same volume, other units
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- Pre-mixed bags needed
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- Approx. wet weight
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- Total cost
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Mixing it yourself? Cement, sand and gravel breakdown
- Cement
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- 50 kg cement bags
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- Sharp sand
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- Gravel / aggregate
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Dry ingredients shrink when mixed, so the dry volume is taken as 1.54 × the wet concrete volume — the standard bulking factor. One 50 kg bag of cement is treated as 0.0347 m³ (34.7 litres) of loose powder.
Estimates only. Sub-base dips, over-dig, formwork bulge and pump losses all eat concrete, so most people order 5–10% more than the bare figure. Check the yield printed on your bags and confirm the order with your supplier before the truck rolls.
How it works
Concrete is sold by volume, so every estimate starts with length × width × depth. A
rectangular slab or footing is L × W × D; a round column is
π × (d ÷ 2)² × h; a circular slab or tube is
π ÷ 4 × (outer² − inner²) × depth; and solid steps use the standard stair
formula width × run × rise × n(n+1) ÷ 2, which adds one more tread-sized block
for every step down the flight, plus width × platform depth × total rise if
there is a landing at the top. Every dimension is converted to metres first, so the cubic
metre figure is exact and the cubic feet (÷ 0.0283168466) and cubic yards
(÷ 0.764554857984) values are conversions of the same number, never separate rounding.
The waste percentage is applied once at the end, and the order volume drives everything else. Bag counts use the printed yield of each pre-mixed bag — 0.60 ft³ for an 80 lb bag, about 11.7 litres for a 25 kg bag — and always round up, because half a bag never finishes a pour. Wet weight assumes 2,400 kg/m³ (about 150 lb/ft³), the usual density of normal-weight concrete. Cost multiplies the order volume by your price per cubic metre or cubic yard. Everything is computed in your browser with plain JavaScript: no uploads, no sign-up, no ads, and the page keeps working offline once it has loaded.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length × width × thickness in the same unit, then convert. A 4 m × 3 m slab poured 10 cm deep is 4 × 3 × 0.1 = 1.2 m³. In imperial, a 12 ft × 10 ft slab at 4 in deep is 12 × 10 × (4 ÷ 12) = 40 cubic feet, and 40 ÷ 27 = 1.48 cubic yards. Add a waste allowance on top — 5 to 10% is normal, because the sub-base is never perfectly level and some concrete is always lost to spillage, over-dig and the pump line. This calculator does all of that in one step and also handles round columns (π × r² × h), circular slabs and rings (π ÷ 4 × (outer² − inner²) × depth) and solid steps (width × run × rise × n(n+1) ÷ 2).
How many bags of concrete are in a cubic yard?
It depends on the bag's printed yield, not its weight alone. An 80 lb bag of standard pre-mixed concrete yields about 0.60 cubic feet, so a cubic yard (27 cubic feet) takes 45 bags. A 60 lb bag yields 0.45 cubic feet — 60 bags per yard — and a 40 lb bag yields 0.30 cubic feet, so 90 bags. In metric, a 25 kg bag yields roughly 11.7 litres, which is about 86 bags per cubic metre. Bag counts are always rounded up, since half a bag never finishes a pour. Past about half a cubic yard, ready-mix delivered by truck is usually cheaper and far less work than mixing bags by hand.
How much does concrete weigh, and can I mix it myself?
Normal-weight concrete is about 2,400 kg per cubic metre, or roughly 150 lb per cubic foot — so a modest 1.3 m³ pour weighs over 3 tonnes, which matters for formwork, wheelbarrow trips and floor loading. For hand mixing, a common general-purpose ratio is 1 part cement to 2 parts sharp sand to 3 parts gravel by volume; 1:1.5:3 is used for structural work and 1:3:6 for mass fill. Dry ingredients shrink when combined, so the dry volume is taken as about 1.54 × the finished wet volume, and one 50 kg bag of cement is roughly 0.0347 m³ of loose powder. The calculator shows that breakdown alongside the bag count.