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Concrete Calculator — Volume, Cubic Yards & Bags
Calculate cubic yards, cubic feet, pre-mix bags, and material cost for any pour.
Project Specifications
Rectangular Concrete Volume Formula
Transparent mathematical methodology explained in plain English.
Cubic Yards = (Length ft × Width ft × (Thickness in / 12)) / 27 First calculate the 3D volume in cubic feet by multiplying length by width by depth in feet, then divide by 27 to obtain cubic yards.
Variables Explained:
- Length & Width: Surface footprint dimensions measured in feet.
- Thickness: Depth of the concrete pour in inches, divided by 12.
- 27: The number of cubic feet in one cubic yard.
Estimating Concrete for a 20 ft × 12 ft Patio (4 in Depth)
Calculating the concrete order for a standard residential outdoor patio.
- Calculate area: 20 ft × 12 ft = 240 sq ft.
- Convert 4" thickness to feet: 4 ÷ 12 = 0.333 ft.
- Calculate volume in cubic feet: 240 × 0.333 = 80 cu ft.
- Convert to cubic yards: 80 ÷ 27 = 2.96 cubic yards.
- Add 10% waste buffer: 2.96 × 1.10 = 3.26 cubic yards.
- For 80 lb bags (0.60 cu ft yield): 88 cu ft ÷ 0.60 = 147 bags.
Frequently Asked Questions: Concrete Calculator
Click any question below to expand quick answers, formulas, and expert tips.
How do I calculate how much concrete I need?
Multiply length by width by depth in feet to find cubic feet, then divide by 27 to convert into cubic yards. For example, a 10 ft × 10 ft slab at 4 inches (0.333 ft) thick equals 33.3 cubic feet ÷ 27 = 1.23 cubic yards. Always add 10% extra for waste and uneven ground.
How many 80 lb or 60 lb bags of concrete make one cubic yard?
It takes 45 bags of 80 lb concrete mix or 60 bags of 60 lb concrete mix to yield one cubic yard (27 cubic feet). Each 80 lb bag yields approximately 0.60 cubic feet, while a 60 lb bag yields 0.45 cubic feet.
How much extra concrete should I order for waste?
Order an extra 10% for standard rectangular slabs, sidewalks, and footings. For curved walkways, stamped decorative pours, or rough uncompacted excavations, increase the safety buffer to 15%.
What is the standard concrete thickness for patios, driveways, and slabs?
Standard residential walkways and light patio slabs are 4 inches thick. Driveways supporting passenger vehicles should be 5 to 6 inches thick, and commercial or heavy RV parking slabs should be at least 6 to 8 inches thick with rebar reinforcement.
What is the formula to convert square feet to cubic yards of concrete?
Multiply your total square footage by the thickness in inches, then divide by 324 (Formula: [Sq Ft × Depth (in)] ÷ 324 = Cubic Yards). For example, 300 sq ft at 4" depth: (300 × 4) ÷ 324 = 3.70 cubic yards.
When should I order ready-mix concrete vs. mixing bags?
If your project requires more than 1.5 cubic yards (equivalent to roughly 70 80-lb bags), ordering ready-mix concrete delivered by a transit mixer truck saves significant time, physical labor, and ensures consistent structural strength.
How long does concrete take to cure and set?
Concrete sets initial hardness within 24 to 48 hours for light foot traffic. It reaches roughly 70% of its rated compressive strength after 7 days, and achieves full design strength at 28 days.
How much does a cubic yard of concrete cost on average?
Ready-mix concrete costs between $125 and $175 per cubic yard delivered in the US, plus short-load fees if ordering less than full truckloads (usually 7 to 10 yards). Pre-mix 80-lb bags average $5.50 to $7.00 per bag.