Concrete Calculator
Cubic yards and 60/80 lb bag counts for slabs, footings, and posts.
Input sheet
DIY at your own risk. Calcora's calculators and guides are general estimates and information only — not professional, engineering, legal, or safety advice. Always verify local building codes and permit requirements, and hire a licensed pro for electrical, gas, plumbing, structural, or any work you're not fully comfortable doing yourself.
Concrete is sold two ways — by the bag at the hardware store or by the cubic yard from a ready-mix truck — and the breakpoint between them decides your whole budget. This calculator gives you the volume, the bag count, and a nudge toward truck delivery once bags stop making sense.
How it works
Length × width × thickness (in feet), divided by 27 for cubic yards, plus a waste allowance for spillage and uneven subgrade.
Volume is the honest part of a concrete job; the rest is logistics. The calculator multiplies length, width, and thickness (converted from inches to feet) to get cubic feet, then divides by 27 to reach cubic yards — the unit ready-mix plants quote.
The waste allowance is not optional padding. Subgrade is never perfectly level, forms bow slightly, and you always lose a little to spillage and the bottom of the wheelbarrow. Five percent is the sane default; bump it to 10% for footings dug by hand where the trench walls aren't clean.
Cubic feet = length × width × (thickness ÷ 12) × (1 + waste); cubic yards = cubic feet ÷ 27. Bags: 80 lb bag ≈ 0.6 cu ft, 60 lb bag ≈ 0.45 cu ft.
Worked examples
A 10 ft × 10 ft patio slab poured 4 inches thick with 5% waste. → About 1.3 cubic yards, or 59 × 80 lb bags.
10 × 10 × (4 ÷ 12) × 1.05 = 35 cu ft. That's 35 ÷ 27 ≈ 1.30 yd, or 35 ÷ 0.6 ≈ 59 bags — right at the point where a truck starts winning.
A 3 ft × 40 ft sidewalk at 4 inches thick. → About 1.5 cubic yards, roughly 70 × 80 lb bags.
3 × 40 × 0.333 × 1.05 ≈ 42 cu ft ≈ 1.56 yd. Mixing 70 bags by hand is a brutal day — order ready-mix.
Tips & gotchas
- Once you cross about one cubic yard, ready-mix delivery is usually cheaper and far less work than mixing bags one at a time.
- Order a touch extra rather than a touch short — a half-finished pour with a cold joint is a permanent flaw, not a fixable one.
- Have everything staged before the truck arrives; most plants give you a free window and then charge by the minute for waiting.
- Thickness drives volume more than people expect — going from 4 to 6 inches is a 50% jump in concrete, not a small tweak.
FAQ
How many bags in a yard?
About 45 × 80 lb bags or 60 × 60 lb bags per cubic yard. Past ~1 yard, ready-mix is usually cheaper.
How thick should a concrete slab be?
Four inches is standard for patios, walkways, and shed floors. Go to 5–6 inches if vehicles or heavy equipment will sit on it, and add rebar or wire mesh for any slab that bears real load.
Do I need rebar in a small slab?
For a patio or walkway, welded wire mesh or fiber-reinforced mix is usually enough. Driveways, footings, and anything carrying a structure want rebar on a grid.
How long before I can walk on it?
Foot traffic after about 24–48 hours, but concrete keeps curing for weeks. Wait a full 7 days before parking on a driveway and ideally 28 days for full strength.
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