Concrete Calculator
What this calculator does
This calculator estimates the volume of concrete needed for a project—such as a slab, footing, column, or wall—based on the dimensions you provide. It outputs the result in cubic yards and cubic feet (the standard ordering units in the US) as well as the approximate number of pre-mixed bags needed for smaller jobs.
Ordering the right amount of concrete is critical. Too little means a costly second delivery and a cold joint in your pour. Too much means paying for material you cannot use (ready-mix concrete cannot be returned). This calculator helps you get it right the first time by converting your measurements into precise volume estimates with a recommended waste factor built in.
Whether you are pouring a patio, setting fence posts, building a foundation, or filling a sidewalk, accurate concrete estimation saves money and prevents project delays.
How to use it
- Select the shape of your pour: slab/rectangle, cylinder/column, or wall/footing.
- Enter the dimensions (length, width, and depth for slabs; diameter and height for cylinders).
- Enter a waste factor percentage (typically 5–10%) to account for spillage, uneven ground, and formwork irregularities.
- Review the total volume in cubic yards and cubic feet, plus the estimated number of bags (60 lb, 80 lb, or both).
Formulas used
Rectangular slab: Volume (ft³) = Length (ft) × Width (ft) × Depth (ft) Volume (yd³) = Volume (ft³) / 27
Cylinder / column / post hole: Volume (ft³) = π × (Diameter/2)² × Height (all in feet) Volume (yd³) = Volume (ft³) / 27
With waste factor: Adjusted volume = Volume × (1 + Waste% / 100)
Bag estimation: An 80 lb bag yields ≈ 0.6 ft³. A 60 lb bag yields ≈ 0.45 ft³. Bags needed = Volume (ft³) / yield per bag, rounded up.
Examples
Example 1 — Patio slab: 10 ft × 12 ft × 4 inches (0.333 ft) = 40 ft³ = 1.48 yd³. With 10% waste: 1.63 yd³. Order 2 cubic yards from a ready-mix supplier.
Example 2 — Fence post holes (10 holes): Each hole: 10 inches diameter × 36 inches deep = 0.727 ft diameter × 3 ft. Per hole: π × (0.364)² × 3 = 1.25 ft³. Ten holes = 12.5 ft³ ≈ 0.46 yd³. About 21 bags of 60 lb mix.
Example 3 — Garage floor: 20 ft × 24 ft × 5 inches (0.417 ft) = 200 ft³ = 7.41 yd³. With 5% waste: 7.78 yd³. Order 8 cubic yards.
Tips and common mistakes
- Always add a waste factor. Ground is rarely perfectly flat, forms shift, and some concrete sticks to the mixer and chute. 5–10% extra is standard; use 10% for irregular shapes or sloped ground.
- Convert inches to feet. The most common calculation error is forgetting to convert depth from inches to feet. 4 inches = 0.333 feet, not 4 feet.
- Ready-mix vs. bags: For anything over 1 cubic yard, ready-mix delivery is more cost-effective and practical than hand-mixing dozens of bags. Below 0.5 yd³, bags are usually fine.
- Check the minimum order. Many ready-mix companies have a 1-yard minimum and charge a short-load fee for orders under 3–5 yards.
- Prepare before the truck arrives. Concrete begins setting within 60–90 minutes. Have your forms built, rebar placed, and crew ready before delivery.
- Consider reinforcement. Slabs over 4 inches thick or subject to heavy loads should include wire mesh or rebar. This does not change the volume calculation but is critical for structural integrity.