Concrete Volume Calculator

Concrete volume basics

Volume = length × width × thickness ÷ 27 (since there are 27 cubic feet in a cubic yard). One 80-lb bag of pre-mixed concrete yields 0.6 cubic feet. A 60-lb bag yields 0.45 cubic feet. So 45 bags of 80-lb = roughly 1 cubic yard.

For pours over 1 cubic yard, ready-mix concrete (delivered truck) is almost always cheaper and faster than bagging. Below that, bags from the home center work fine for a DIYer.

Slab thickness by use

• 3 inches: Decorative slab, garden path, sub-base for pavers. Crack risk high without reinforcement.
• 4 inches: Residential patio, sidewalk. Standard for foot traffic.
• 5 inches: Driveway for cars. Some codes still allow 4" for single-car residential.
• 6 inches: Driveway for trucks/RVs, garage floor.
• 8+ inches: Heavy commercial, foundation slab. Code-specified.

Footing depths by climate

Footings must extend below the frost line — varies by region:

• Southern US (FL, TX, southern CA): 12-18". Minimal frost depth.
• Mid-Atlantic, central US: 24-36".
• Northern US, Midwest, New England: 36-48". Check local code.
• Alaska, mountain states: 48-60"+.

A 12-inch diameter post hole, 36 inches deep, needs roughly 1.5 cubic feet of concrete (about 3 80-lb bags).

Reinforcement: when and what

• Wire mesh (6×6 W1.4): Standard for residential slabs. Place at mid-depth — not the bottom.
• Rebar (#3 or #4, 12-18" grid): Driveways, garage floors, anything that bears load. Tied at intersections, suspended on chairs at mid-depth.
• Fiber reinforcement: Synthetic or steel fibers mixed into the batch. Reduces shrinkage cracking but not load capacity — supplement, not replacement for rebar.
• None: Acceptable only for thin decorative pours under 3 ft × 3 ft.

PSI ratings explained

• 2,500 PSI: Most home center pre-mix bags. Adequate for patios, paths, post holes.
• 3,000 PSI: Driveways, garage floors, basements.
• 4,000 PSI: Heavy commercial, exposed-to-weather structural.
• 5,000+ PSI: High-stress applications. Need to specify when ordering ready-mix.

Mixing and curing

• Water: Use the minimum the bag specifies. Excess water = weaker concrete. Aim for a "wet earth" consistency that holds a shape but isn't liquid.
• Mixing time: 3-4 minutes after all water is added.
• Place within 60-90 minutes of mixing in hot weather; 2 hours in cool.
• Curing: Keep moist for 7 days minimum. Mist daily, cover with plastic or burlap. Cured concrete is 50% stronger than dry-cured.
• Don't pour below 40°F without heated blankets — concrete won't hydrate and will be permanently weak.

How much over-order?

• Slabs: Add 5% to your calculated volume for spillage, uneven sub-base.
• Footings/holes: Add 10-15% — soil walls aren't perfect.
• Ready-mix trucks: Many charge a "short load" fee under 4 cubic yards. Round up to avoid running short mid-pour.