Concrete Calculator

How it's calculated

Concrete is sold by the cubic yard, which is 27 cubic feet. The calculator converts every dimension to feet, multiplies to get cubic feet, adds your waste percentage, then divides by 27.

Ready-mix orders are rounded up to the next quarter yard, which is how most batch plants take orders. Bag counts are rounded up to whole bags using typical yields: 0.60 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb and 0.30 ft³ for 40 lb.

For post holes, the volume of the post is subtracted from each hole, since the post takes up space the concrete would otherwise fill.

Worked example

A 12 ft × 16 ft patio poured 4 inches thick: 12 × 16 × (4 ÷ 12) = 64 ft³. Add 10% waste and you need 70.4 ft³, or 2.61 yd³. Order 2¾ yards of ready-mix, or about 118 bags of 80 lb pre-mix. At that size, a truck is almost always the better choice.

Field notes

• Measure the formed depth in several spots. A slab that is ½" deeper than planned across 200 sq ft needs roughly 0.3 more yards.
• Under about one yard, bags usually beat ready-mix once you count short-load fees.
• On frost-prone sites, bell the bottom of post holes slightly wider than the top and add that extra volume to your waste.
• Have a plan for leftover concrete on a truck pour: a few extra forms for pads or stepping stones put it to use.

Common questions

How many 80 lb bags make a yard?

About 45. One 80 lb bag yields roughly 0.60 cubic feet, and a yard is 27 cubic feet.

How thick should a patio or driveway slab be?

Four inches is standard for patios and walkways. Driveways that carry vehicles are typically 5 to 6 inches. Check your local code and the plans.

Should I include rebar or mesh in the volume?

No. Reinforcing steel displaces so little concrete that it falls well inside the waste allowance.

How many bags of concrete per fence post?

For a 10" hole, 3 feet deep, around a 4×4 post, plan on about 2½ to 3 bags of 80 lb mix per hole with waste. Use the Post holes tab for your exact size.