How to measure for concrete
Every pour is a simple solid, or several. Pick the piece, enter its sizes as they are on your plans, and add the next one: the calculator adds the volumes before it rounds anything.
- Slab. Length × width × thickness. A round slab takes its diameter; an L-shaped one its overall size and the corner that is missing. For a thin pour over a floor, “Slab by its area” takes square feet.
- Footing. The length of the run, its width and its depth: 20 ft of footing 16 in wide and 8 in deep is 17.78 cu ft, 0.66 cubic yards.
- Round columns and tube forms. Diameter, height, and how many. Six forms 12 in across and 4 ft high hold 3.14 cu ft each, 0.70 cubic yards in all. A cardboard tube form is often called by one maker’s name, Sonotube: it is the same shape.
- Steps. Solid steps are a stack: the first is one rise high, the second two. Three steps 36 in wide, with a 7 in rise and an 11 in run, are 9.63 cu ft, 0.36 cubic yards.
- Post holes. The hole, less the post that stands in it. A hole 10 in across and 30 in deep round a 4 × 4 post takes 1.15 cu ft; fourteen of them, 16.11 cu ft.
Worked example. A slab 10 × 10 ft, 4 in thick.
- Volume: 10 ft × 10 ft × 4 in = 33.33 cu ft
- In cubic yards: 33.33 ÷ 27 = 1.23 cubic yards
- With 10% extra: 1.36 cubic yards. A truck delivers by the quarter yard, so order 1.5.
- Weight: 33.33 cu ft × 150 lb = 5,000 lb
Cubic yards, bags or a truck?
A cubic yard is 45 bags of 80 lb, each to be carried, opened and mixed: the slab above is 56 of them before any extra. Bags suit post holes, a few steps, a small pad. Past a yard or so, ask a ready-mix supplier two things: the smallest load they deliver, and what a short load costs. Many charge for a minimum whatever you order.
The link under the result opens the concrete bag calculator on the same pieces, with the bag sizes and that minimum side by side.
Why order extra
The calculator adds 10% unless you say otherwise. Ground that is not quite flat makes a slab thicker than its plan, forms bow outward under the weight, and some concrete stays in the chute, the wheelbarrow and the mixer. Running short in the middle of a pour costs more than a little left over: the first batch sets while you wait for the second.
Choose “No extra” to see the bare figure: for the slab, 1.25 cubic yards to order instead of 1.5.
What it weighs
Hardened concrete weighs about 150 lb a cubic foot, 4,050 lb a cubic yard (Inch Calculator); the Federal Highway Administration’s estimating table has 155. The weight line is there for the day a slab is broken up and hauled away, or to check what a trailer will carry. It changes nothing in the order.
What this calculator doesn’t do
It does not choose a size. How thick a slab is, how wide and deep a footing, whether there is rebar or mesh in it, how deep a post goes: those come from your plans, your building department or an engineer, and they depend on your soil, your frost line and what the concrete carries. Enter the sizes you were given, and the calculator counts the concrete.
Questions about concrete
How many yards of concrete for a 10×10 slab?
At 4 in thick, 1.23 cubic yards, or 1.5 to order with 10% extra. Thickness is the figure that moves it: enter the one on your plans.
How many bags of concrete make a yard?
45 bags of 80 lb, 60 of 60 lb or 90 of 40 lb, at the yields on Quikrete’s data sheet: 0.60, 0.45 and 0.30 cu ft a bag.
How much concrete for a post hole?
The volume of the hole less the post. A hole 10 in across and 30 in deep with a 4 × 4 post takes 1.15 cu ft. How deep and how wide the hole should be is for your local frost depth and your plans to say.
How much does a yard of concrete weigh?
About 4,050 lb, at 150 lb a cubic foot. Mixes run from about 145 to 155 lb a cubic foot.
Sources
- Inch Calculator, Concrete Weight Calculator. A typical concrete mix weighs 150 lb per cubic foot, 4,050 lb per cubic yard, or 2,400 kg per cubic metre; the figure changes with the aggregate, the water and the air in the mix. Read on .
- Federal Highway Administration, Central Federal Lands, Application Rates and Weights for Estimating Purposes. Aggregate base: 139.0 lb per cubic foot. Hot asphalt mix: 145.2 lb per cubic foot. Concrete: 155.0 lb per cubic foot. Read on .
- Quikrete, Concrete Mix (No. 1101), data sheet. A 40 lb bag yields approximately 0.30 cu ft, a 50 lb bag 0.375, a 60 lb bag 0.45, an 80 lb bag 0.60 and a 90 lb bag 0.675. An 80 lb bag takes about 6 pt of water to start with, and 6 to 9 pt in all. Read on .
- Archtoolbox, Lumber Dimensions: Nominal vs Actual. Surfaced softwood lumber: a 2 x 10 is 1 1/2 x 9 1/4 in and a 2 x 12 is 1 1/2 x 11 1/4 in; a 4 x 4 is 3 1/2 x 3 1/2 in and a 6 x 6 is 5 1/2 x 5 1/2 in; a 1 x 4 is 3/4 x 3 1/2 in and a 1 x 6 is 3/4 x 5 1/2 in. Standard lengths run from 6 to 24 ft in steps of 2 ft. Read on .
Updated . How we calculate. Found a mistake? Tell us.