Concrete Volume Calculator & Conversion Guide
In the United States, concrete is ordered and sold by the cubic yard. Most other countries use cubic meters. Whether you're pouring a driveway, patio, or foundation, knowing how to calculate volume accurately — and then convert to the right unit — prevents costly over-orders and frustrating short pours.
How Concrete Volume is Measured
Concrete volume is calculated by multiplying the length, width, and depth of the area to be poured — all in the same unit. In the US, contractors work in feet and inches, then convert to cubic yards for ordering ready-mix.
Converting to cubic yards: Since there are 27 cubic feet in one cubic yard (3 × 3 × 3), divide your cubic-foot result by 27. Always add 5–10% overage to account for spillage, uneven subgrades, and consolidation.
Common slab thicknesses: 3.5 inches for interior floor slabs, 4 inches for residential driveways and patios, and 6 inches for areas that will bear heavy vehicles or equipment.
Using bags: If you're mixing your own concrete, an 80-lb bag yields approximately 0.60 cubic feet (about 0.022 cubic yards), and a 60-lb bag yields approximately 0.45 cubic feet. For pours larger than about 1 cubic yard, ready-mix is usually more practical and economical.
Volume Formulas
Volume (cu ft) = Length (ft) × Width (ft) × Depth (ft)
Volume (cu yd) = Volume (cu ft) ÷ 27
Depth conversion: inches ÷ 12 = feet
Bags needed (80-lb) = Volume (cu ft) ÷ 0.60
Bags needed (60-lb) = Volume (cu ft) ÷ 0.45
Add 5–10% to your final order for waste and overage.
Common Slab Sizes — Cubic Yards & Bags Needed
Bag counts include a 5% overage allowance. Round up when purchasing bags.
| Slab Dimensions | Cu Ft | Cu Yd | 80-lb Bags | 60-lb Bags |
|---|---|---|---|---|
| 10 ft × 10 ft × 4 in | 33.33 | 1.23 | 56 | 74 |
| 10 ft × 20 ft × 4 in | 66.67 | 2.47 | 112 | 148 |
| 20 ft × 20 ft × 4 in | 133.33 | 4.94 | 223 | 297 |
| 4 ft × 8 ft × 4 in | 10.67 | 0.40 | 18 | 24 |
| 12 ft × 12 ft × 6 in | 72.00 | 2.67 | 120 | 160 |
| 20 ft × 30 ft × 4 in | 200.00 | 7.41 | 334 | 445 |
Related Conversion Tools
Frequently Asked Questions
How many cubic yards is a typical concrete truck?
A standard ready-mix concrete truck holds 8–10 cubic yards. Most trucks can deliver a minimum of 1 cubic yard, though short-load fees may apply for orders under 3–4 yards. For small projects, consider renting a concrete mixer or using bags.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27. There are 27 cubic feet in one cubic yard (3 ft × 3 ft × 3 ft = 27 cu ft). For example, 81 cubic feet ÷ 27 = 3 cubic yards.
How many 80-lb bags make a yard of concrete?
An 80-lb bag yields approximately 0.60 cubic feet of concrete. Since there are 27 cubic feet in a cubic yard, you need about 45 bags (27 ÷ 0.60 = 45). Always buy a few extra to account for waste and overfill.