How much concrete do I need?
The arithmetic is area times depth, and almost nobody gets that part wrong. What goes wrong is everything around it: ordering in the wrong unit, forgetting that four inches is a third of a foot, buying bags for a job that wanted a truck, and adding no waste to a pour that cannot be topped up an hour later.
This is what each of those decisions actually costs, and how to arrive at a number you can read down a phone to a supplier.
Everything starts as a volume, then gets converted
Concrete is a volume problem wearing several different hats. Multiply the area by the depth and you have cubic feet. Everything after that is a conversion into whatever unit the thing you are buying is sold in, and that unit is set by the supplier rather than by the maths.
Ready-mix is sold by the cubic yard in the United States and by the cubic metre almost everywhere else. Bagged concrete is sold by the bag, and each bag prints a yield in cubic feet. Aggregate under the slab — the gravel or crushed stone base — is quoted by the ton, so that one has to go through a density as well.
The trap is the depth. A slab is measured in feet across and inches deep, and those two numbers cannot be multiplied together. Four inches is a third of a foot, not four, and getting that wrong is a factor of twelve rather than a rounding error.
A worked example, and why the number is not round
Take a driveway pad twenty feet by twelve, poured four inches thick. That is 240 square feet of area and a third of a foot of depth, so eighty cubic feet of concrete. A cubic yard is twenty-seven cubic feet, which makes it 2.96 cubic yards before any allowance.
With ten per cent added for waste it comes to 3.26 cubic yards, or about eighty-eight cubic feet. In metric that is 2.49 cubic metres, and it weighs roughly six and a half short tons — which is worth knowing if anything about the access is marginal.
20 ft x 12 ft = 240 sq ft
240 sq ft x 4 in (0.333 ft) = 80 cu ft
80 / 27 = 2.96 cubic yards
+ 10% waste = 3.26 cubic yards (2.49 cu m, 6.59 short tons)
Work it out: Concrete Slab Calculator →
Why ten per cent, and when it should be more
A waste allowance is not padding for clumsiness. It covers the ground being less level than you measured, forms bowing outward under the weight of wet concrete, the last barrow that goes stiff before it is placed, and the fact that a slab poured slightly deep is invisible while one poured slightly shallow is a crack in two winters.
Ten per cent is the usual figure for a formed slab on prepared ground. Push it to fifteen if the sub-base is rough, if the ground slopes, or if you are pouring against soil rather than into forms — an unformed edge takes whatever the earth gives it.
The reason to be generous is the asymmetry. Ordering half a yard too much wastes the price of half a yard. Ordering half a yard too little on a slab means a cold joint through the middle of it, because the second batch does not arrive for an hour and the first is no longer wet. One of those is a cost and the other is a defect.
Bags or a truck
The crossover is around half a cubic yard. Below it, bags win on convenience and there is no minimum-load charge to swallow. Above it, bags stop being sensible fast, and the reason is not the price per yard — it is that mixing is the slowest part of the job and a slab has to be placed while the first of it is still workable.
The driveway pad above is 147 eighty-pound bags. That is over five and a half tons of material to buy, move, split, mix and place, and it has to happen quickly enough that the first barrow has not gone off before the last is down. A mixer and two people can do it. One person with a wheelbarrow cannot.
Bag yields are printed on the bag and worth using rather than estimating: an eighty-pound bag makes about 0.6 cubic feet, a sixty-pound 0.45, a fifty-pound 0.375 and a forty-pound 0.30. They are not proportional to weight in the way people assume, because the mixes differ.
Work it out: Concrete Bag Calculator →
The base underneath counts too
Most slabs sit on four inches of compacted gravel or crushed stone, and that is a second material with a second unit. Aggregate is sold by the ton rather than the cubic yard, so the volume has to be multiplied by a density to become an order.
Densities move with moisture, which is why any tonnage is an estimate to order against rather than a promise. Wet gravel weighs meaningfully more than the same gravel dry, and neither figure is wrong — they are the same stone with different amounts of water in it.
Compaction is the other half. A base laid loose and then compacted loses depth, so ordering the finished depth leaves you short. Adding a little on the base is much cheaper than adding it on the concrete.
Work it out: Gravel Calculator →
Questions
How many cubic feet in a cubic yard?
Twenty-seven — three feet by three by three. It is the conversion every concrete order goes through, and the one people most often do from memory and get wrong by taking it as nine.
How thick should a slab be?
Four inches is standard for a path, patio or garage floor. Six is usual where vehicles heavier than a car will stand on it. Thickness is a structural question rather than a quantity one, so take it from whatever governs the job locally rather than from a calculator.
How much waste should I add to a concrete order?
Ten per cent for a formed slab on prepared ground, fifteen if the base is rough or you are pouring against soil. Running short mid-pour is a cold joint through the slab; running over is the price of a few extra cubic feet.
Is it cheaper to use bags or ready-mix?
Below about half a cubic yard, bags. Above it, ready-mix — not mainly on price, but because a slab has to be placed before the first of it stiffens, and mixing by hand cannot keep up past a certain size.
How many 80lb bags in a cubic yard?
Forty-five. An eighty-pound bag yields about 0.6 cubic feet and a cubic yard is twenty-seven, so twenty-seven divided by 0.6 is forty-five bags — around 3,600lb of material for one yard.
Does concrete weigh the same as the aggregate under it?
No. Cured concrete runs around 2,400kg per cubic metre, and gravel and crushed stone are lighter and vary with moisture. They are also sold differently — concrete by volume, aggregate by weight — so the two orders never look alike even for the same depth.