Concrete Volume Calculator
Calculate how many cubic yards of concrete you need for any slab, footing, column, or circular pour.
I used this before pouring a backyard patio slab. Concrete is sold by the cubic yard for ready-mix and by the bag for smaller pours, knowing which threshold you're near (about 1 cubic yard) helps you decide whether to rent a mixer and buy bags or call a ready-mix truck. Overordering ready-mix is expensive and results in a truck you have to find something to do with.
Ready-mix concrete has a minimum order of about 1 cubic yard at most suppliers. For pours under 0.5 cubic yards, bagged concrete is usually more practical. Above 1 yard, ready-mix becomes significantly more cost-effective.
How concrete volume is calculated
Volume is simply length × width × depth in consistent units. A 20 × 12 foot slab at 4 inches thick: 20 × 12 × (4/12) = 80 cubic feet. Divide by 27 to convert to cubic yards (80/27 = 2.96 cubic yards). Always add a waste factor, 5–10% for simple pours, more for irregular shapes or if the subgrade has any variation in depth.
Slab thickness standards
Standard residential patio and walkway slabs are 4 inches thick. Driveways are typically 4–6 inches, with 6 inches recommended for vehicles over 4,000 lbs. Garage floors are usually 4–6 inches. Structural footings follow local building codes, typically 12 inches deep and wide for one-story construction, deeper in frost-prone climates. Always check local code for any structural application.
Ready-mix vs bagged concrete
An 80-lb bag of concrete covers approximately 0.6 cubic feet. A cubic yard (27 cubic feet) requires 45 such bags. At $8–12 per bag, that's $360–540 in materials alone, plus significant labor mixing. Ready-mix concrete costs $120–180 per cubic yard delivered, making it competitive at even one yard and substantially cheaper for larger pours. Ready-mix also produces more consistent results than hand-mixed bags for large pours.
Reinforcement
Most structural concrete pours require reinforcement, rebar or wire mesh, to control cracking and add tensile strength (concrete is strong in compression but weak in tension). Patio slabs may get away with fiber-reinforced concrete or control joints alone, but driveways, footings, and anything load-bearing should have rebar. Local codes specify minimum rebar sizes and spacing for structural applications.
Working time
Standard concrete begins setting within 20–30 minutes of water contact and reaches initial set in 2–4 hours. Hot weather, wind, and low humidity accelerate setting. Cold weather (below 50°F) slows it significantly, fresh concrete must be protected from freezing for at least 24 hours. Plan your pour logistics accordingly: the larger the pour, the more critical it is that you have enough hands and equipment ready before the truck arrives.
Frequently asked questions
How much does concrete cost?
Ready-mix concrete ranges from $120–180 per cubic yard for standard 3,000 psi mix, plus delivery charges (often $75–150 for the truck). Specialty mixes (high-strength, fiber-reinforced, colored) cost more. Bagged concrete costs $8–12 per 80-lb bag at home improvement stores. Labor for flatwork finishing runs $2–7 per square foot depending on complexity.
What strength concrete do I need?
Standard residential flatwork uses 3,000–3,500 psi concrete. Driveways and areas exposed to freeze-thaw cycles often use 4,000 psi. Footings and structural applications may require 4,000+ psi per engineering specs. Higher strength mixes cost slightly more. Your ready-mix supplier can recommend the appropriate mix for your application and climate.
How long does concrete take to cure?
Concrete reaches approximately 70% of its design strength in 7 days and 99% in 28 days. Light foot traffic is usually safe after 24–48 hours. Vehicles should stay off for 7 days minimum, and heavy vehicles longer. Curing can be accelerated with heated enclosures in cold weather or extended by misting the surface in hot, dry conditions.