Concrete
How to Calculate Yards of Concrete
To calculate yards of concrete, multiply length × width × thickness in feet and divide by 27. With the thickness in inches, multiply length × width (ft) × thickness (in) and divide by 324.
By Kumar Baniya, MEng in Structural Engineering (University of Leeds) · Published
What is the formula for yards of concrete?
Cubic yards = length × width × thickness (all in feet) ÷ 27. A cubic yard is 3 × 3 × 3 = 27 cubic feet. When the thickness is in inches, use length (ft) × width (ft) × thickness (in) ÷ 324, because 27 × 12 = 324.
- Measure each dimension of the form, not the finished surface you hope for.
- Convert inches to feet by dividing by 12, or use the ÷ 324 shortcut for the thickness only.
- Calculate each part of the pour separately and add them.
How many yards of concrete for a slab?
A slab 20 ft × 12 ft at 4 in thick needs 20 × 12 × 4 ÷ 324 = 2.96 yd³. The same slab at 6 in needs 4.44 yd³ — thickness changes the volume in direct proportion. Thickness depends on the design and local code; this article only does the arithmetic.
How many yards of concrete for a footing?
Treat a continuous footing as a long, narrow slab. A footing 40 ft long, 16 in wide and 8 in deep is 40 × 1.333 × 0.667 ÷ 27 = 1.32 yd³. For a perimeter footing, add the length of every side (without counting the corners twice).
How do I calculate a round column or post hole?
Use the cylinder formula: π × radius² × height ÷ 27, all in feet. A 12 in round form 4 ft tall is 3.1416 × 0.5² × 4 ÷ 27 = 0.116 yd³, so eight of them need 0.93 yd³. If a post will stand in the hole, subtract the post’s volume.
How should I round when ordering concrete?
Ready-mixed concrete is sold by the cubic yard of fresh concrete as discharged from the truck. The National Ready Mixed Concrete Association’s guidance on ordering (CIP 31) suggests ordering 4% to 10% more than the volume calculated from the plan dimensions, to cover uneven subgrades, form movement and waste. Your supplier also sets its own minimum and order increments — the concrete calculator applies them once you enter them.
How do I calculate concrete for steps?
Split solid steps into stacked blocks, one per step, each as wide as the stair and one tread deep, with a height equal to all the risers up to that step. Three steps 4 ft wide with 11 in treads and 7 in risers are 4 × 11⁄12 × (7 + 14 + 21)⁄12 = 12.83 ft³ = 0.48 yd³.
What mistakes make concrete estimates come up short?
- Using nominal lumber sizes. Forms built from 2 × 4s are 3.5 in deep, not 4 in; 2 × 6s are 5.5 in.
- Assuming a flat base. Low spots in the subgrade add volume that plan dimensions do not show.
- Forgetting parts of the pour. Thickened edges, footings and steps poured with a slab must each be added.
- Rounding each part. Add every part at full precision, then round the total once.
How do I calculate concrete in cubic meters?
Multiply length × width × thickness in meters. One cubic yard is 0.7646 m³, so to convert, multiply cubic yards by 0.7646 — the 2.96 yd³ slab above is 2.27 m³. The concrete calculator accepts feet, inches, meters and centimeters in any mix.
Related calculations
To buy bags instead, see how many bags of concrete are in a yard. To go the other way — area from volume — see how many square feet a yard of concrete covers.