Concrete Calculator

How Concrete Volume Is Calculated

Concrete volume depends on the shape being poured:

Slab or footing:

Volume (cu ft) = Length x Width x Thickness $$V=L\times W\times T$$

Column (cylinder):

Volume (cu ft) = pi x (Diameter/2)^2 x Height $$V=\pi \times {\left(\frac{d}{2}\right)}^2\times h$$

Convert cubic feet to cubic yards by dividing by 27. Convert to cubic meters by dividing cubic feet by 35.315.

Variable Definitions

Length: The longest horizontal dimension of the slab or footing in feet. Measure along the longest side of the formwork.

Width: The shorter horizontal dimension of the slab or footing in feet. For footings, this is typically 12-24 inches (1-2 feet).

Thickness: The depth of the slab or footing in feet. Standard slabs are 4 inches (0.333 ft). Enter the value in feet, not inches.

Diameter: The width across a circular column in feet. Standard Sonotube sizes range from 6 inches to 24 inches.

Worked Example

A patio project with one slab and two post footings:

1. Patio slab: 12 ft x 10 ft x 0.333 ft (4 in) = 39.96 cu ft = 1.48 cu yd
2. Footing 1: 2 ft x 2 ft x 1 ft = 4.00 cu ft = 0.148 cu yd
3. Footing 2: 2 ft x 2 ft x 1 ft = 4.00 cu ft = 0.148 cu yd

Total = 47.96 cu ft = 1.776 cu yd = 1.358 cu m

60lb bags: 1.776 / 0.017 = 105 bags. 80lb bags: 1.776 / 0.022 = 81 bags.

With 10% extra: order 1.95 cu yd (about 116 sixty-pound bags or 89 eighty-pound bags).

Edge Cases and Advanced Scenarios

Ordering 10% extra for spillage: Concrete pours rarely match exact calculations. Ground is uneven, formwork is imperfect, and some concrete sticks to the mixing equipment. Always add 10% to your calculated volume. For sloped or rough terrain, add 15%.

Mixing ready-mix with bagged concrete: For projects between 1 and 3 cubic yards, you are in a gray zone. A ready-mix truck delivery (minimum order often 1 yard) may be more efficient than mixing 60+ bags by hand. Above 2 cubic yards, ready-mix is almost always faster and cheaper per yard.

What to Do with Your Result

• For projects under 1 cubic yard, buy bagged concrete from a hardware store. For larger projects, call a ready-mix supplier for a truck delivery quote.
• Rent a concrete mixer for projects between 0.5 and 2 cubic yards to save time and ensure consistent mixing.
• Schedule your pour on a mild day (50-80 degrees F) to allow proper curing. Extreme heat or cold compromises concrete strength.
• Build your formwork before ordering. Once concrete arrives or is mixed, you have about 90 minutes before it becomes unworkable.