Concrete Calculator

How to Calculate Concrete Volume

Concrete volume is length times width times depth — the same box-volume formula as any rectangular slab, footing, or pour. The only trick is unit consistency: depth is usually specified in inches (a 4-inch slab, a 6-inch footing) while length and width are in feet, so depth needs converting to feet before multiplying, and the result — in cubic feet — needs dividing by 27 to convert to cubic yards, since ready-mix concrete is ordered by the cubic yard.

$$\text{Volume (yd}^3\text{)} = \frac{L \times W \times (D \div 12)}{27}$$

Worked Example

A 10 ft × 10 ft patio slab at a 4-inch depth: volume = 10 × 10 × (4 ÷ 12) = 100 × 0.333 = 33.3 cubic feet. Divided by 27, that's 1.23 cubic yards. At 0.45 cubic feet of yield per 60 lb bag, that's 33.3 ÷ 0.45 ≈ 74 bags — or, at 0.60 cubic feet per 80 lb bag, 33.3 ÷ 0.60 ≈ 56 bags. For anything beyond a small patch, a ready-mix truck delivering by the cubic yard is almost always cheaper and less labor than mixing dozens of bags by hand.

Metric Version of the Formula

Outside the US, the same slab is ordered in cubic meters, and the calculation is actually simpler since there's no cubic-yard division step: multiply length and width in meters by depth in meters (depth is usually given in centimeters, so divide by 100 first).

$$\text{Volume (m}^3\text{)} = L \times W \times (D \div 100)$$

A 3 m × 3 m patio slab at a 10 cm depth: volume = 3 × 3 × (10 ÷ 100) = 9 × 0.1 = 0.9 cubic meters (900 liters). Switch the US/Metric dropdown above to see this calculator work in either system — it converts your currently entered dimensions automatically rather than resetting them.

Bagged Mix vs. Ready-Mix Delivery

Bagged concrete (60 lb or 80 lb bags, mixed on-site with a mixer or by hand) makes sense for small pours — a fence post footing, a small step, a patch repair — generally under about 1 cubic yard. Beyond that, the labor of mixing dozens of bags adds up fast, and ready-mix delivered by truck, priced per cubic yard, becomes the more practical and often cheaper option. Most ready-mix suppliers have a minimum order (commonly around 1 cubic yard) and may charge a short-load fee below that minimum.

Why Ordering Extra Matters

Real subgrade is never perfectly flat, forms can bow slightly under wet concrete's weight, and some mix is always lost to spillage and the mixer or truck chute — all reasons contractors routinely order 5-10% more than the calculated volume. Running short mid-pour is a serious problem: concrete sets quickly, and a "cold joint" where an old batch has begun to set before new concrete is added creates a visible seam and a weaker bond than a continuous pour.

Common Concrete Calculation Mistakes

Forgetting to convert depth from inches to feet before multiplying is the single most common error — it inflates or (more often) deflates the volume by a factor of 12. Confusing cubic feet and cubic yards is the second most common — remember to divide cubic feet by 27, not 12 or 3, since a cubic yard is 3×3×3 = 27 cubic feet, not 3. Not accounting for waste, as covered above, is the third — always round up, especially for a ready-mix order that can't easily be topped off once the truck leaves.

Concrete Terms You Should Know

Ready-Mix — concrete mixed at a plant and delivered wet by truck, priced and ordered by the cubic yard.

Cold Joint — a visible seam where one batch of concrete has begun to set before the next batch is placed against it, weaker than a continuous pour.

Subgrade — the compacted soil or base material beneath a concrete slab, which needs to be level and stable before pouring.

Yield — the volume of mixed concrete a given bag produces once water is added — about 0.45 cubic feet for a 60 lb bag, about 0.60 cubic feet for an 80 lb bag.