Concrete
calculator.

How do you calculate how much concrete you need?

Multiply length × width × thickness with every measurement in feet, then divide by 27 to get cubic yards. A 10 × 10 ft slab 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.33 ft³, or 1.23 yd³. Adding 10% for waste gives 36.67 ft³, or 1.36 yd³. Thickness is the measurement people most often get wrong: 4 inches is 0.333 ft, not 0.4 ft.

cubic feet  = length (ft) × width (ft) × thickness (in) ÷ 12
cubic yards = cubic feet ÷ 27
order       = cubic yards × (1 + waste % ÷ 100)

How many bags of concrete do I need?

Divide the volume by what one bag makes, and round up. QUIKRETE's data sheet for its Concrete Mix says an 80 lb bag yields approximately 0.60 ft³, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.30 ft³. One cubic yard is 27 ft³, so it takes 27 ÷ 0.60 = 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb. The 10 × 10 ft slab above needs 36.67 ÷ 0.60 = 61.1, so 62 bags of 80 lb.

In metric, River Sands' Easy Mix data sheet gives 0.01 m³ (10 litres) for a 20 kg bag and 0.0151 m³ for a 30 kg bag. Yields differ between brands and products. High-yield and fast-setting mixes in particular can be quite different, so choose “Other” and type the yield printed on your bag.

bags = total volume ÷ yield per bag, rounded up
1 yd³ = 27 ft³ = 45 × 80 lb bags at 0.60 ft³ each

How do you calculate concrete for a round column or post hole?

Use the area of a circle, π × diameter² ÷ 4, times the height. A 12-inch tube form 4 ft tall holds 3.1416 ÷ 4 × 1² × 4 = 3.14 ft³, about 0.12 yd³, or six 80 lb bags with no waste. For a post hole, enter the hole's diameter and depth. To subtract a round post, enter its diameter as the inner diameter. A square post takes up less room than people expect: a 3.5 × 3.5 in post is 0.085 ft² of a 10-inch hole's 0.545 ft².

volume = π ÷ 4 × (outer diameter² − inner diameter²) × height

How do you calculate concrete for steps?

Treat each step as a solid block from the ground up. With rise r, run d and width w, the first step is w × d × r, the second w × d × 2r, and so on, so n steps add up to w × d × r × n(n + 1) ÷ 2. Four steps 7 in high, 11 in deep and 4 ft wide come to 4 × (11 ÷ 12) × (7 ÷ 12) × 10 = 21.39 ft³. A 3 ft landing at the top, at the full 28-inch height, adds 4 × 3 × (28 ÷ 12) = 28 ft³.

steps   = width × run × rise × n(n + 1) ÷ 2
landing = width × landing depth × n × rise

How do you calculate concrete for curb and gutter?

Add the two parts of the cross-section and multiply by the length. The curb is curb depth × (curb height above the flag + flag thickness), and the gutter is gutter width × flag thickness. A 6-inch curb standing 6 in above a 12-inch gutter flag 6 in thick is 0.5 × (0.5 + 0.5) + 1 × 0.5 = 1.0 ft² of section, so 50 ft of it is 50 ft³, or 1.85 yd³ before waste.

section = curb depth × (curb height + flag) + gutter width × flag
volume  = section × length

How much does a cubic yard of concrete weigh?

QUIKRETE gives a typical unit weight of about 140 lb/ft³ for its Concrete Mix, measured by ASTM C138. That makes a cubic yard 27 × 140 = 3,780 lb, nearly two US tons, and a cubic metre about 2,240 kg. Mixes with different aggregates weigh more or less, so change the unit weight if your supplier gives one. The calculator weighs the concrete that ends up in the forms (before waste). It does not include reinforcing steel.