How to use the concrete calculator for cubic yards
- 1 Choose a slab or a round column.
- 2 Enter the length, width and thickness (or column height and diameter) and how many.
- 3 Set the waste and the bag size, and a price per cubic yard if you like.
- 4 Read the cubic yards and the number of bags.
Worked example
One 10 × 10 foot slab, 4 inches thick, with 10% waste. The volume is 10 × 10 × 0.333 = 33.33 cubic feet; with waste, 36.67 cubic feet or 1.36 cubic yards. That is 62 bags of 80 lb premix (82 of 60 lb), weighing about 5,300 lb.
Pouring concrete
- Measure thickness carefully: an extra inch adds 25% to a 4-inch slab.
- Add 10% waste.
- Use ready-mix above about a cubic yard.
- Keep it damp while it cures.
Find the area with the Square Footage Calculator, order a base with the Gravel Calculator, and size post holes with the Cylinder Volume Calculator. Measure round slabs with the Circumference Calculator.
How it works: A slab's volume is length times width times thickness in feet; a column's is pi times the radius squared times the height. The waste allowance is added, then cubic feet are divided by 27 for cubic yards and by each bag's yield for the number of bags.
Features
Slabs and columns
Rectangular pours and round posts or piers.
Bags or ready-mix
80, 60 and 40 lb premix bags, or cubic yards for a truck.
Waste allowance
10% extra by default for spills and uneven ground.
When to use it
Patios and sidewalks
Plan a 4-inch slab. Find the area first with the Square Footage Calculator.
Fence posts
Concrete for post holes. Check a hole's volume with the Cylinder Volume Calculator.
Base layer
Order the gravel base with the Gravel Calculator.
Steps
Work out risers with the Stair Calculator.
Related Construction tasks
Frequently asked questions
How do you calculate concrete?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27 for cubic yards. A 10 × 10 foot slab 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards.
How many bags of concrete do I need?
Divide cubic feet by the bag yield: 0.60 for an 80 lb bag, 0.45 for 60 lb and 0.30 for 40 lb. A 10 × 10 × 4 inch slab with 10% waste needs about 62 bags of 80 lb.
How thick should a concrete slab be?
4 inches for patios, walkways and shed floors; 5 to 6 inches for driveways and garage floors with cars; thicker for heavy vehicles.
How much waste should I allow?
5% to 10% is common for spills, uneven subgrade and form movement.
When should I order ready-mix instead of bags?
For about 1 cubic yard or more, ready-mix is usually cheaper and faster than mixing 45 or more 80 lb bags by hand.
How much does a cubic yard of concrete weigh?
About 4,000 pounds (2 tons), or roughly 145 to 150 pounds per cubic foot.
How do I calculate concrete for a round column or post hole?
Use π × radius² × height. A 12-inch diameter, 4-foot deep hole is about 3.14 cubic feet.
How long does concrete take to cure?
It can be walked on after about 24 to 48 hours and driven on after about a week, and it reaches most of its strength in about 28 days.
Does this include rebar or mesh?
No. Reinforcement takes up very little volume, so it does not change the order.
What is the difference between cement and concrete?
Cement is the powder that binds; concrete is cement mixed with sand, gravel and water.
How many 80 lb bags are in a cubic yard?
About 45: 27 cubic feet ÷ 0.6 cubic feet per bag.
Built and reviewed by Hamza Farooq, founder and lead developer
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