This concrete calculator tells you exactly how much concrete to order for a slab, footing, or round column — in cubic yards for a ready-mix truck, or as a bag count for 40, 60, and 80-pound bags of ready-mix. Enter your dimensions, keep the recommended 10% waste allowance, and you have your order quantity.
Concrete is sold by the cubic yard (27 cubic feet). Ordering short is the most expensive mistake in a pour: a second truck trip or an emergency bag run mid-pour can cost more than the original load, and a cold joint between old and new concrete weakens the slab. That is why contractors always order 5–10% over the exact geometric volume.
How Concrete Volume Is Calculated
Each shape reduces to a simple volume formula, converted to cubic yards:
- Slab: length (ft) × width (ft) × thickness (in) ÷ 12 = cubic feet, then ÷ 27 for cubic yards
- Footing: length (ft) × width (in)/12 × depth (in)/12, then ÷ 27
- Round column: π × (diameter/24)² × height (ft) gives cubic feet directly (diameter in inches)
Example: a 10 × 10 ft patio slab at 4 inches thick is 10 × 10 × 4/12 = 33.3 cubic feet = 1.23 cubic yards. With 10% waste, order 1.36 cubic yards — round up to 1.5 yards for a ready-mix delivery.
Bags vs. Ready-Mix Truck
Bagged concrete yields per manufacturer specs:
- 40-lb bag → 0.30 cubic feet
- 60-lb bag → 0.45 cubic feet
- 80-lb bag → 0.60 cubic feet
That same 10 × 10 ft slab with waste needs about 62 eighty-pound bags — nearly 5,000 lbs of material to haul and mix by hand. As a rule of thumb: under about half a cubic yard (roughly 25 bags), bags are fine; beyond one cubic yard, call for ready-mix. A truckload is mixed consistently, arrives ready to place, and eliminates the biggest DIY failure mode — inconsistent water ratios between batches.
Common Slab Thicknesses
Typical residential specifications:
- Sidewalks and patios: 4 inches
- Driveways (passenger cars): 4–5 inches
- Driveways (trucks/RVs) and garage slabs: 5–6 inches
- Shed slabs: 4 inches
- Footings: below the local frost line — check your building department; 8-inch depth and 16-inch width are common for one-story structures
Thicker costs more but going too thin causes cracking under load, which cannot be fixed without re-pouring. When in doubt, follow your local code — it overrides any rule of thumb.
Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
At the standard 4-inch thickness, a 10 × 10 ft slab needs 1.23 cubic yards of concrete (33.3 cubic feet). With the recommended 10% waste allowance, order about 1.36 cubic yards — in practice, a 1.5-yard ready-mix delivery or about 62 eighty-pound bags.
How many 80-lb bags of concrete make a yard?
One cubic yard is 27 cubic feet, and an 80-lb bag yields 0.60 cubic feet, so you need 45 eighty-pound bags per cubic yard. That is 3,600 lbs of bags — which is why anything approaching a full yard is usually better ordered as ready-mix.
Should I order extra concrete?
Yes — 5–10% over the calculated volume is standard practice. Subgrades are never perfectly level, forms flex, and some concrete is always lost in the mixer, wheelbarrow, and hose. Running short mid-pour creates a cold joint that permanently weakens the slab, so the small cost of a safety margin is cheap insurance.
How thick should a concrete slab be?
Four inches is standard for patios, sidewalks, and shed slabs. Use 5–6 inches for driveways that carry trucks or for garage floors. Footings must extend below the local frost line, so their depth is set by your building code rather than a universal rule.
How much does a yard of concrete cost?
Delivered ready-mix typically runs $125–$180 per cubic yard in the US, with small-load fees often added below 3–4 yards. Bagged concrete works out to roughly $180–$270 per yard equivalent (45 eighty-pound bags at $4–$6 each) plus your mixing labor, which is why ready-mix wins on both cost and quality for larger pours.