How to work out how much concrete you need
Every concrete quantity comes from one multiplication: volume = length × width × depth. The only thing that trips people up is units — bag yields are quoted in cubic feet, ready-mix is ordered in cubic yards (or cubic metres), and depth is usually given in inches. This calculator takes feet, inches, yards, metres and centimetres in any combination and hands back all three volume units at once, so you can compare bag mixing against a ready-mix delivery on the same numbers.
The numbers behind the answer
- 1 cubic yard = 27 cubic feet = 0.765 m³. A 10 ft × 10 ft slab at 4 in thick is 33.3 ft³, which is 1.23 yd³.
- Bag yields: an 80 lb bag of concrete mix makes about 0.60 ft³, a 60 lb bag about 0.45 ft³, a 40 lb bag about 0.30 ft³, and a 25 kg bag roughly 0.0117 m³ (0.41 ft³). Bag counts are always rounded up, because you cannot buy two thirds of a bag.
- Weight: mixed concrete runs about 145 lb per cubic foot (≈ 2,400 kg/m³), so a cubic yard weighs roughly 2 US tons — that is why the bag route gets heavy fast.
- Water: an 80 lb bag wants about 3 quarts (2.8 L). Add most of it, mix, then trickle in the rest until the mix is workable but still holds its shape.
Practical thicknesses
Four inches is the normal minimum for a garden path or shed base; five to six inches for a driveway, patio or anything taking a vehicle; eight to twelve inches for a footing below the frost line. Post holes for a fence or deck are usually 8–12 in across and 24–42 in deep — deeper in cold climates so the footing sits below frost.
Bags or a truck?
As a rule of thumb, under about one cubic yard bags are cheaper and give you time to work; above that, ready-mix delivery wins on price, speed and your back. Add 5–10% to the calculated volume for spillage, uneven subgrade and the concrete left in the mixer chute. For slabs, also budget for a compacted gravel base, a vapour barrier and reinforcement — none of those are part of the concrete volume itself.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
Forty-five. A cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.60 ft³, so 27 ÷ 0.60 = 45 bags. In practice most people round up to 46 because the last bag is never quite full, and this calculator already rounds every bag count up for you.
How much concrete do I need for a 10×10 slab?
At 4 inches thick, 10 ft × 10 ft × 0.333 ft is 33.3 cubic feet, or 1.23 cubic yards — about 56 eighty-pound bags, 75 sixty-pound bags or 112 forty-pound bags. At 6 inches the same slab needs 1.85 cubic yards, so thickness is what really moves the number.
How thick should a concrete slab be?
Four inches for footpaths, shed floors and garden pads, five to six inches for patios and driveways that take cars, and six to eight inches where heavy vehicles or a hot tub are involved. Slabs should also sit on at least 4 inches of compacted gravel, which is separate from the concrete volume.
Is it cheaper to mix bags or order ready-mix?
Under roughly one cubic yard, bags are usually cheaper and more flexible. Above one yard, delivered ready-mix generally costs less per yard, arrives mixed to a consistent strength and saves hours of labour — expect a minimum charge and a short delivery window.
How deep and wide should post holes be?
A common rule is one third of the post height above ground in depth, with a hole about three times the post width. For a 6 ft fence that means roughly 2 ft deep and 8–12 inches across; in cold climates go below the local frost line, which may be 3 ft or deeper.
Why add waste on top of the calculated volume?
The formula assumes a perfectly flat subgrade and no spillage. Real pours lose concrete to uneven ground, formwork gaps and the mixer chute, so the volume selector lets you add 5% for a carefully prepared slab, 10% for typical ground or 15% where the base is rough. Leave it at zero when you want the pure geometric answer.