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Concrete Calculator

How to Calculate Concrete Volume

Calculating concrete volume depends on the shape of your project. For rectangular slabs and footings, multiply length × width × thickness (depth). For cylindrical columns and post holes, use π × radius² × height. For stairs, each step forms a wedge shape — volume is calculated as width × run × rise × steps, plus a solid base underneath. Convert the result to cubic yards by dividing cubic feet by 27. One cubic yard of concrete weighs approximately 4,000 lbs (about 2 tons). Always order 5-10% extra to account for waste, spillage, and uneven ground. Ready-mix concrete is sold by the cubic yard and typically costs $125-170 per yard depending on your location and mix specifications.

Bags vs. Ready-Mix Concrete

Pre-mixed concrete bags come in 40, 60, and 80 lb sizes. An 80 lb bag yields approximately 0.6 cubic feet (0.022 cubic yards). You need about 45 bags of 80-lb concrete to make one cubic yard. For projects under 1 cubic yard, bags are practical for DIY mixing. For larger projects, ready-mix delivery from a concrete truck is more economical ($125-170/yard vs $200-300/yard from bags) and ensures consistent quality. The minimum truck delivery is typically 1 cubic yard. Mixing concrete properly requires the right water ratio — too much water weakens the concrete, while too little makes it difficult to work with.

Frequently Asked Questions

How much concrete do I need for a 10×10 slab?

A 10×10 foot slab at 4 inches thick requires approximately 1.23 cubic yards, which is about 56 bags of 80 lb concrete (before waste factor). With a 10% waste factor, order about 62 bags or 1.36 cubic yards. For a 6-inch thick slab, you'll need approximately 1.85 cubic yards or 84 bags of 80 lb concrete.

How many bags of concrete are in a cubic yard?

One cubic yard requires approximately 45 bags of 80 lb concrete, 60 bags of 60 lb concrete, or 90 bags of 40 lb concrete. These numbers can vary slightly by brand, so always check the bag yield on the packaging. For large projects over 1 cubic yard, ready-mix delivery is usually more practical and cost-effective.

How much does a cubic yard of concrete weigh?

A cubic yard of standard ready-mix concrete weighs approximately 3,700-4,100 lbs (about 2 tons). The exact weight depends on the mix design and moisture content. Lightweight concrete weighs about 2,800 lbs per cubic yard, while heavy-duty structural concrete can weigh up to 4,400 lbs per cubic yard.

Should I use bags or ready-mix concrete?

Use bags for small projects under 0.5-1 cubic yard (fence posts, small repairs, steps). Use ready-mix truck delivery for anything over 1 cubic yard — it's more economical ($125-170/yd vs $200-300/yd from bags), saves hours of manual mixing, and ensures consistent quality. Most suppliers have a minimum delivery of 1 cubic yard.

How thick should my concrete slab be?

Standard residential thicknesses: 4 inches for patios, walkways, and sidewalks. 5-6 inches for driveways and garage floors that support vehicles. 6-8 inches for heavy-duty applications like RV pads or commercial floors. Footings are typically 12+ inches deep. Thicker slabs should include rebar or wire mesh reinforcement.

How do I calculate concrete for stairs?

Concrete stairs are calculated as stacked wedges. Each step adds a rise (typically 7 in) on top of the previous one, so the bottom step supports the full height. The formula accounts for the cumulative volume: width × run × (sum of rises from 1 to N steps). Standard residential stairs have a 7-inch rise and 11-inch run. Always add 10% waste for stairs since the formwork is more complex.