Concrete Calculator

About this tool

Concrete is ordered by volume and sold by the bag. This calculator turns a shape and a few dimensions into the two numbers that actually matter when you are standing at the store: the volume you need, and how many bags that buys.

Three shapes cover most jobs. A rectangular slab is also how you work out a footing or a wall; a round column or pier is a solid cylinder; a hollow tube is a cylinder with the core taken out. They use genuinely different geometry — one multiplies three dimensions, one uses the area of a circle, one subtracts the hollow centre — so they are kept separate rather than folded into a single form.

Units follow the way the work is actually described out loud. In feet-and-inches, the plan dimensions are in feet and the thickness is in inches, which is exactly what "ten by ten, four inches thick" means. Switch to meters and centimeters and the numbers are converted for you, so you can move between a set of plans and a bag label without doing arithmetic in your head. A waste allowance of 10% is added on top by default: running short mid-pour costs far more than one unused bag.

Why the bag count rounds up, and never one bag too many

Bags are sold whole, so the count is always rounded up: 55.6 bags means buying 56. The rounding carries a tiny tolerance, so an exact result such as 27 divided by 0.6 — which the arithmetic gives as 45.00000000000001 — still reports 45.

Ordering the exact figure is not an option; a store sells 80 lb bags, not 55.6 of them. Rounding up is also the safe direction. One leftover bag is far cheaper than a pour that stops halfway, and the cold joint where it stopped is a permanent seam in the slab.

The subtlety is in the last decimal place. Twenty-seven cubic feet divided by the 0.60 ft³ that an 80 lb bag yields does not come out as exactly 45 in binary floating point; it lands just above it. A plain round-up would then answer 46, one bag over the truth. A tolerance of a billionth is subtracted before rounding — far smaller than any real bag, far larger than the noise.

The count is worked out in the unit the bag is quoted in: cubic feet for the imperial bags, cubic metres for the metric ones. The volumes on screen have been converted for display, but the bag arithmetic stays where the yield figure lives, so nothing is lost to a round trip.

Why bag yields come from a table, not from the density

The yield of a bag of mix — 0.60 ft³ for an 80 lb bag, 0.0104 m³ for a 25 kg one — is a figure the manufacturer states, so the tool stores it as data rather than deriving it from the density of wet concrete.

The weight and the mixed volume of concrete are two different things. The 2400 kg/m³ used here is the density of the wet material, and its job is to turn a volume into the load the formwork and the ground have to carry. It says nothing about how much volume a bag of dry mix becomes once water is added, because that depends on the mix design.

That second figure is a declared yield, printed on the bag, and it is the one to trust when counting bags. The imperial yields happen to be tidy — about 0.0075 ft³ per pound, so 40, 60 and 80 lb bags land on 0.30, 0.45 and 0.60 ft³ — but the metric bags follow the same logic and not a formula of their own.

Because the yields come from the bag rather than from the model, they can change. A product reformulated to a different blend can revise its stated yield, and the note at the top of the page asks you to do the one thing that settles it: check the label you are actually buying.

What happens to your numbers when you switch units

Every dimension is converted at its own rate — the plan dimensions from feet to metres, the thickness from inches to centimetres — so a 10 by 10 slab that was four inches thick becomes 3.048 by 3.048 metres at 10.16 cm.

There are two units in play here, not one, because that is how the work gets described out loud: “ten by ten, four inches thick” mixes feet for the plan with inches for the depth. A single unit switch would have to choose one of those rates and would be wrong about the other by a factor of twelve.

The conversion is applied to the numbers in the boxes, not to the labels, and it runs in the right direction: a foot is 0.3048 of a metre, so 10 feet becomes 3.048 metres. Dividing instead of multiplying would turn the same slab into 32.8 metres — and the result would still look like a plausible number, which is exactly why it is worth checking once.

Boxes you left empty stay empty. An empty field is not converted to zero, because zero would then be rejected as a dimension that has to be greater than zero, and the complaint would point at the wrong problem.

A cubic yard is always 3,600 lb of dry mix, split into 45, 60 or 90 bags

An 80 lb bag yields 0.6 cubic feet, so a cubic yard of 27 cubic feet takes 45 bags; 60 lb bags take 60; 40 lb bags take 90. All three come to 3,600 lb, because yield is strictly proportional to bag weight.

Bagged mix is sold by dry weight and placed by volume, and the bridge between the two is the yield. The three figures in the table are the manufacturers stated values: 40 lb yields 0.3 cubic feet, 60 lb yields 0.45, 80 lb yields 0.6.

Divide 27 by 0.6 and you get 45 bags, by 0.45 and you get 60, by 0.3 and you get 90. Every product is 3,600 lb: choosing the small bag only splits the same material into more packages, and what you pay extra for is packaging and handling, not concrete.

The metric side follows the same logic: a 40 kg bag stated at 0.0167 cubic metres needs 59.9 bags per cubic metre, rounded to 60; 25 kg needs 96.2, rounded to 97; 20 kg needs 120.5, rounded to 121. In total weight that is roughly 2,400 kg, matching the density of wet concrete.

Add the default 10% waste and 45 bags become 50. That step is not padding: spillage, levelling and form absorption all consume material, and a mid-pour trip for more costs far more than five extra bags.