How to calculate concrete

The basic formula

For any flat, rectangular pour, which covers slabs, driveways, sidewalks, pads, walls and strip footings:

Cubic feet = length (ft) × width (ft) × thickness (ft)

Cubic yards = cubic feet ÷ 27

A cubic yard is a cube 3 ft on each side, and 3 × 3 × 3 = 27, which is where the 27 comes from. The trap is thickness. You measure it in inches, but it has to be in feet before you multiply, so divide it by 12 first. Four inches is 4 ÷ 12 = 0.333 ft, six inches is 0.5 ft.

The 324 shortcut

Because you divide by 12 for inches and then by 27 for yards, and 12 × 27 = 324, you can keep thickness in inches and divide once:

Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 324

For a 12 × 12 ft slab at 4 in: 12 × 12 × 4 = 576, and 576 ÷ 324 = 1.78 cubic yards. Same answer, one step fewer.

Worked example: a patio

A patio 24 ft long and 12 ft wide, poured 4 in thick.

1. Convert thickness: 4 ÷ 12 = 0.3333 ft
2. Volume: 24 × 12 × 0.3333 = 96 cu ft
3. Cubic yards: 96 ÷ 27 = 3.56 cu yd
4. Add 10% for waste: 3.56 × 1.1 = 3.91 cu yd
5. Round up to the quarter yard plants sell in: order 4 cu yd

Check with the shortcut: 24 × 12 × 4 ÷ 324 = 1,152 ÷ 324 = 3.56. It matches.

Round shapes: holes, piers, columns and tubes

A round hole is a cylinder. Its volume is the area of the circle times the depth, and the area of a circle is π (about 3.1416) times the radius squared. Radius is half the diameter.

Cubic feet = 3.1416 × (diameter (ft) ÷ 2)² × depth (ft)

Diameters are usually in inches, so divide by 12 first. For a 12 in Sonotube 4 ft deep: diameter is 1 ft, radius 0.5 ft, area 3.1416 × 0.25 = 0.785 sq ft, volume 0.785 × 4 = 3.14 cu ft. For four of them, 12.57 cu ft, or 0.47 cu yd.

A common mistake is using the diameter instead of the radius, which gives four times too much concrete.

Footings

A strip footing is a long, thin slab. Measure the total run of the trench in feet, and the width and depth in inches, then use the basic formula with both small dimensions divided by 12. An 80 ft trench 16 in wide and 8 in deep: 80 × (16 ÷ 12) × (8 ÷ 12) = 80 × 1.333 × 0.667 = 71.1 cu ft = 2.63 cu yd. Trenches are rough, so order more than the math says. See the waste guide.

Steps

Solid poured steps are a stack of blocks. Counting from the top, the first step is one riser tall, the second is two risers tall, and so on, each one run deep. Adding them gives the series 1 + 2 + 3 + … + N, which equals N(N+1) ÷ 2.

Cubic feet = width × run × rise × N(N+1) ÷ 2, all in feet

Three steps 7 in high and 11 in deep, 4 ft wide: one block is 4 × 0.917 × 0.583 = 2.14 cu ft, times (3 × 4 ÷ 2) = 6, gives 12.8 cu ft. A landing on top adds width × landing depth × total rise as one more block. The stairs calculator does both.

Odd shapes

Split them. An L-shaped patio is two rectangles. A slab with a round end is a rectangle plus half a circle. A slab with thickened edges is a slab plus a footing. Calculate each piece, add the cubic feet, then convert to yards once. The homepage calculator has an “Add to project” button that does the adding for you.

From cubic yards to bags

Bagged mix is sold by weight but yields a known volume. Divide the cubic feet you need by the yield of one bag and round up. An 80 lb bag yields 0.60 cu ft, so the 96 cu ft patio with 10% waste, 105.6 cu ft, needs 105.6 ÷ 0.60 = 176 bags. Yields for every size are in the bag yield guide.

Mistakes that catch people out

• Mixing units. Feet times feet times inches is not cubic feet. Convert thickness first or use 324.
• Forgetting to divide by 27. A 96 cubic foot slab is 3.56 yards, not 96. Ordering in cubic feet will get a surprised look from the plant.
• Using the diameter as the radius in round holes.
• Measuring the slab, not the forms. Measure the inside of the forms where the concrete will actually go.
• No waste allowance. Ground is never flat and forms always bow a little. Add 10%.
• Ordering the exact number. Plants deliver in quarter yards. Round up.

Quick reference

Conversions used in every concrete calculation

From	To	Do this
Inches	Feet	÷ 12
Cubic feet	Cubic yards	÷ 27
ft × ft × in	Cubic yards	÷ 324
Cubic yards	Cubic metres	× 0.7646
Cubic feet	80 lb bags	÷ 0.60, round up
Cubic feet	60 lb bags	÷ 0.45, round up