Concrete Calculator - Cubic Yards and 60/80 lb Bags

How to measure for concrete

Concrete is sold by volume, so you need three dimensions. Length and width in feet, and thickness or depth in inches because that is how everyone talks about slabs. A patio or sidewalk is usually 4 inches thick. A driveway for cars is 4 to 5 inches, and 6 inches if it carries trucks or an RV. Wall footings are commonly 16 to 24 inches wide and 8 to 12 inches deep, but your local code and frost line decide the real numbers.

Measure the inside of the forms, not the area you roughly marked with paint. And measure depth in several spots. A base that dips half an inch across a patio adds up fast.

The formula

Volume in cubic feet = length × width × (thickness ÷ 12). Divide by 27 to get cubic yards, because a cubic yard is 3 × 3 × 3 feet. For a round column or post hole: π × (diameter in inches ÷ 24)² × height in feet. For solid steps, each step is a block as deep as the tread and as tall as its own height, so three steps with a 7-inch rise use 1 + 2 + 3 = 6 rise-heights of concrete.

For bags, use the manufacturer's yield. Quikrete's Concrete Mix data sheet lists an 80 lb bag at about 0.60 cubic feet, a 60 lb bag at 0.45, and a 40 lb bag at 0.30. That means one cubic yard takes 45 bags of 80 lb, or 60 bags of 60 lb. Yes, that is 3,600 pounds of dry mix per yard. Plan your back accordingly.

Worked example: a 10 × 10 patio

A 10 × 10 ft slab at 4 inches is 10 × 10 × 0.333 = 33.3 cubic feet. Add 10% for spillage, uneven base, and form bulge and you need 36.7 cubic feet, or 1.36 cubic yards.

In bags, 36.7 ÷ 0.60 = 61.1, so you buy 62 bags of 80 lb mix. That is almost 2.5 tons of material, which is more than a half-ton pickup should carry in one trip. At this size, a ready-mix truck is usually the better call: order 1.5 yards, and ask about the short-load fee, since many plants charge extra below 3 or 4 yards.

Now four fence posts, each in a 12-inch hole 4 feet deep: π × 0.5² × 4 × 4 = 12.6 cubic feet, or 13.8 with waste. That is 24 bags of 80 lb, a very bag-friendly job.

Bags or a truck?

The rough rule: under about 1 cubic yard, use bags. Above that, get a quote for ready-mix. Mixing 45 bags by hand or with a rented mixer takes hours, and the first batch starts to set before the last one is placed, which risks cold joints. Ready-mix also comes at a consistent strength, typically 3,000 to 4,000 psi, while hand-mixed bags vary with how much water goes in. For pours between 1 and 3 yards, a rent-a-trailer mixer from a local yard is a good middle ground.

Common mistakes

Forgetting that thickness is in inches. 10 × 10 × 4 is 400 cubic feet, not 33. Always divide inches by 12.

Skipping waste. Ordering exactly the calculated volume is how pours end with a truck gone and a corner still empty. 5 to 10% is standard; use more for irregular excavation.

Adding too much water. Soupy concrete is easy to place and weak once cured. Follow the bag's water amount, about 3 quarts per 80 lb bag for Quikrete, and add only what you need to make it workable.

Pouring on soft ground. A 4-inch layer of compacted gravel under a slab is not optional in most soils. Budget for it; the gravel calculator will size it.

When to hire a pro

A small pad, steps, or post footings are reasonable DIY projects. Call a concrete contractor for driveways, garage floors, anything structural like a foundation or a retaining wall footing, and any pour over about 2 yards, because finishing a big slab before it sets needs a crew. Many cities also require a permit and inspection for driveways and footings, and some want the rebar inspected before you pour.