Concrete Volume Calculator
Find the concrete volume for a rectangular slab or footing, or a round column or post. Get results in cubic yards, cubic feet, cubic meters, and bag counts, with a built-in waste allowance.
Standard slabs run 4 in; driveways and garage floors often use 5-6 in.
Extra for spillage, uneven subgrade, and over-excavation. Most pros use 5-10%.
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Concrete Volume Calculator
Concrete volume is length x width x thickness for a slab or footing, or pi x radius squared x height for a round column. Convert every dimension to feet first, then divide the cubic-foot result by 27 to get cubic yards, which is how ready-mix concrete is ordered in the US.
The formulas
- Convert every measurement to feet. Thickness and diameter are usually given in inches — divide by 12 first.
- Apply the formula for your shape to get cubic feet.
- Add your waste factor — typically 5-10% — before converting to cubic yards.
- Divide by 27 to get the cubic yards you’ll order from the ready-mix supplier.
Worked examples
Slab: a 10 ft by 10 ft patio at 4 inches (0.33 ft) thick.
10 × 10 × 0.33 = 33 ft³ → 33 ÷ 27 = 1.22 yd³ before waste. Add 10% waste: 1.22 × 1.10 ≈ 1.34 yd³ to order.
Column: a 12-inch diameter post base, 4 ft deep.
Radius = 6 in = 0.5 ft. V = π × 0.5² × 4 = 3.14 ft³ → 3.14 ÷ 27 ≈ 0.116 yd³. At that size, most suppliers would round up to their minimum load or suggest bagged concrete instead.
Bag yields by size
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | ~90 bags |
| 60 lb | 0.45 ft³ | ~60 bags |
| 80 lb | 0.60 ft³ | ~45 bags |
| 90 lb (high-yield) | 0.68 ft³ | ~40 bags |
Bagged concrete makes sense for small pours like posts and footings. Anything over roughly 1-1.5 cubic yards is usually cheaper and faster as a ready-mix truck order.
Assumptions this calculator makes
- The main cubic-yard figure includes your waste factor; the “base volume” cell does not.
- Bag count uses an 80 lb bag yielding 0.60 cubic feet, a common published figure for standard pre-mix bags.
- The column mode assumes a solid cylinder poured full-depth, with no rebar cage or sonotube wall thickness subtracted.
- Results are a material-quantity estimate only, not a structural design — footing depth, width, and reinforcement should follow local code and, for load-bearing work, an engineer’s specification.
Common mistakes
- Forgetting to convert inches to feet. Plugging “4” in for thickness instead of 0.33 ft overstates the volume by roughly 12x.
- Ordering the exact calculated volume with no waste allowance. Subgrade is rarely perfectly flat, and running short mid-pour means a costly second truck and a visible cold joint.
- Using diameter where the formula wants radius, or vice versa. Cylinder volume needs the radius — half the diameter — squared.
- Mixing bag-yield figures. Yield varies by bag size and brand; check the bag label rather than assuming one number covers 40, 60, 80, and 90 lb bags equally.
FAQ
How do I calculate concrete volume for a slab?
Multiply length by width by thickness, all in feet. A 10 ft by 10 ft slab at 4 inches (0.33 ft) thick is 10 x 10 x 0.33 = 33 cubic feet, or about 1.22 cubic yards.
How much concrete do I need for a 10×10 slab?
At a standard 4-inch thickness, a 10×10 slab needs about 1.22 cubic yards of concrete before waste. Most contractors add 5 to 10 percent extra, bringing the order to roughly 1.3 to 1.35 cubic yards.
Should I add extra concrete for waste?
Yes. Uneven subgrade, spillage, and slight over-excavation typically use 5 to 10 percent more concrete than the calculated volume. Running short mid-pour means a second truck and a visible cold joint, so most pros round up rather than order exact.
How many 80 lb bags of concrete equal a cubic yard?
An 80 lb bag yields about 0.60 cubic feet of concrete, so it takes roughly 45 bags to make one cubic yard (27 cubic feet). For 60 lb bags it takes about 60 bags, and for 40 lb bags about 90 bags.
What is the formula for a round concrete column or post?
Volume equals pi times the radius squared times the height: V = pi x r^2 x h. Radius is half the diameter. Convert diameter to feet before squaring, since height is normally measured in feet.
Why do suppliers sell concrete by the cubic yard?
Ready-mix trucks in the US are calibrated and priced in cubic yards, and most have a minimum order (often around 1 yard) plus a short-load fee below that. Knowing your volume in yards lets you compare it directly to the truck’s minimum and pricing.