Concrete Calculator: How to Calculate Cubic Yards of Concrete

The Cubic Yards Formula

Concrete is ordered and priced by the cubic yard. One cubic yard equals 27 cubic feet. The formula works for any rectangular shape — slabs, walls, footings, and piers.

Volume (cu yd) = Length (ft) × Width (ft) × Depth (ft) ÷ 27

The most common mistake is entering depth in inches instead of feet. Always convert first: 4 inches = 0.333 ft, 5 inches = 0.417 ft, 6 inches = 0.5 ft.

Worked Example: 10 × 20 Foot Patio at 4 Inches Thick

Length = 10 ft, Width = 20 ft, Depth = 4 in = 0.333 ft
10 × 20 × 0.333 ÷ 27 = 200 × 0.333 ÷ 27 = 66.6 ÷ 27 = 2.47 cubic yards
Add 10% waste: 2.47 × 1.10 = 2.7 cubic yards

That 10% waste factor is not optional. Subgrade irregularities, spillage, and over-pour to avoid cold joints consistently consume 5–15% of the total order on typical residential pours. Running short mid-pour creates a cold joint — a structural weak point where fresh and hardened concrete meet that can fail under load.

How Many Bags of Concrete Per Cubic Yard

Bag yields come directly from Quikrete and Sakrete published specifications. The math is straightforward: 1 cubic yard = 27 cubic feet, so divide 27 by the yield per bag.

For the 2.7 cubic yard patio example (with waste), you need about 122 bags of 80-lb concrete— roughly 9,760 lbs of material to mix and place by hand. That is why ready-mix wins for anything over about 0.75 cubic yards.

Ready-Mix vs. Bagged Concrete

The choice comes down to project size. Ready-mix trucks have minimum delivery quantities (typically 1 cubic yard) and charge short-load fees for small orders. But hand-mixing bags for large projects is physically punishing and risks inconsistent water ratios.

Ready-mix truck minimum orders are usually 1 cubic yard (~$150–$200), with short-load surcharges of $50–$200 for orders under 3–5 yards. Bagged concrete runs $5–$7 per 80-lb bag at most hardware stores. For small footings and repairs, bags win on both cost and convenience.

Concrete for Different Applications

Thickness and mix strength both matter. Using too thin a slab or too weak a mix is a false economy — a cracked driveway costs $3,000–$7,000 to replace.

Concrete PSI Grades and Uses

PSI (pounds per square inch) measures compressive strength after 28 days of curing. Higher PSI costs more per yard but resists cracking and freeze-thaw damage far better.

The Portland Cement Association (PCA) recommends a minimum of 4,000 PSI for driveways in freeze-thaw climates, where water expands inside micro-cracks and accelerates spalling. In mild climates, 3,000 PSI is adequate for most residential flatwork.

Standard Concrete Mixing Ratio

The classic mix for a 3,000 PSI concrete is 1 part Portland cement : 2 parts sand : 3 parts coarse aggregate (gravel) by volume, plus water. This is the standard mix used for residential flatwork.

The water-to-cement ratio is critical. Too much water weakens the final slab and increases shrinkage cracking. The American Concrete Institute (ACI) recommends a water-to-cement ratio of 0.45–0.50 for residential work. In practical terms: use only enough water to make the mix workable, not soupy.

For bagged concrete mixes, follow the manufacturer’s water instructions exactly. Quikrete 80-lb bags typically specify 3 quarts of water per bag for standard concrete.

Curing Time

Concrete curing is a chemical process (hydration), not just drying. The strength builds over time:

Keep concrete moist and above 50°F during the curing period. Cold concrete cures slowly; below 40°F hydration nearly stops, permanently reducing strength. In hot weather, mist the surface or cover with plastic to prevent rapid moisture loss and surface cracking.

The 28-day cure is the industry standard for testing concrete strength. The ACI specifies all design strengths at the 28-day mark.

Concrete Cost Estimates (2024–2026)

Regional prices vary, but these ranges from Angi and HomeAdvisor 2025 data give a solid baseline:

According to the National Ready Mixed Concrete Association (NRMCA), the U.S. produces approximately 300 million cubic yards of ready-mix concrete per year, making it the most consumed construction material in the country. Concrete outlasts most alternatives — properly installed residential driveways last 30–50 years according to the National Association of Home Builders.

Frequently Asked Questions

How do you calculate cubic yards of concrete?

Multiply the length (ft) by the width (ft) by the depth (ft), then divide by 27 to convert cubic feet to cubic yards. Always add 10% for waste and spillage. For a 10 ft × 20 ft patio at 4 inches (0.333 ft) thick: 10 × 20 × 0.333 ÷ 27 = 2.47 cubic yards. With 10% waste, order 2.7 cubic yards.

How many bags of concrete do I need?

It depends on bag size. An 80-lb bag yields about 0.60 cubic feet — so one cubic yard (27 cubic feet) requires roughly 45 bags. A 60-lb bag yields 0.45 cubic feet, requiring about 60 bags per cubic yard. For projects over 0.75 cubic yards, ready-mix is usually more cost-effective than bags.

How thick should a concrete slab be?

Thickness depends on the application. Sidewalks and patios are typically 4 inches. Driveways for passenger vehicles are 4–5 inches; heavier loads require 5–6 inches. Footings depth varies by frost line depth, but are typically 8–12 inches or deeper. The Portland Cement Association recommends 4 inches as the minimum for residential flatwork.

What is the difference between concrete and cement?

Cement is an ingredient in concrete — not the same thing. Concrete is a mixture of Portland cement, sand, coarse aggregate (gravel or crushed stone), and water. The standard mix ratio is 1 part cement : 2 parts sand : 3 parts aggregate by volume. Cement acts as the binder that holds everything together when it reacts with water through hydration.

How long does concrete take to cure?

Concrete reaches about 70% of its design strength within 7 daysand full strength at 28 days. During curing, keep the concrete moist and above 50°F — cold temperatures slow hydration and can permanently weaken the slab. Light foot traffic is safe after 24–48 hours, but vehicle traffic should wait at least 7 days, and full loads until the 28-day cure is complete.