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- Volume = length × width × thickness. Divide cubic feet by 27 to get cubic yards.
- A 10 ft × 10 ft slab 4 inches thick is 1.23 cubic yards before waste.
- Add about 10% for waste, spills and uneven ground.
- An 80 lb bag of pre-mix makes about 0.6 cubic feet, so one yard is about 45 bags.
How to use the concrete calculator
Pick the shape
Choose a slab, patio or footing, or a round column or post hole.
Enter the size
For a slab, enter length and width in feet and thickness in inches. For a round shape, enter the diameter in inches and the height in feet.
Set the extra
Leave 10% for waste, or change it. Use the advanced options to choose your bag size and enter a ready-mix price per cubic yard.
Read the result
You get cubic yards to order, cubic feet, cubic meters, the number of bags of pre-mix and the ready-mix cost if you entered a price.
How much concrete do I need?
Concrete is sold by volume. Multiply the three dimensions in the same unit to get cubic feet, divide by 27 for cubic yards, and add a waste allowance. For bagged concrete, divide the cubic feet by the volume one bag makes.
Concrete volume
Slab in cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
Round column in cubic feet = π × (diameter in ÷ 24)² × height (ft)
Cubic yards = cubic feet ÷ 27
Bags = cubic feet with waste ÷ yield per bag
Typical bag yields: 40 lb about 0.30, 50 lb about 0.375, 60 lb about 0.45 and 80 lb about 0.60 cubic feet. Check the bag label.
Worked examples
A 10 ft × 10 ft patio, 4 inches thick
100 sq ft × 4 ÷ 12 = 33.33 cubic feet.
For this size, bags are a practical option. For larger pours, ready-mix is usually easier.
A 20 ft × 20 ft slab, 6 inches thick
400 sq ft × 6 ÷ 12 = 200 cubic feet.
Order from a ready-mix supplier and ask about minimum loads.
A round post hole
12 inch diameter and 3 ft deep.
Multiply by the number of posts for a fence or deck.
Concrete per 100 square feet
Cubic yards for 100 sq ft of slab
Before waste.
Add 10% for waste.
Bags in one cubic yard
At the typical yields above.
Divide 27 cubic feet by the bag yield.
Where a concrete calculator helps
Patios and walkways
Order the right amount for a pad or path. Find the area first with the square footage calculator.
Footings and post holes
Estimate fence, deck and sign post concrete.
Driveways and garage slabs
Plan larger pours. Use the gravel calculator for the base layer.
Comparing bags and ready-mix
Check which option costs less for your volume.
Running short in the middle of a pour can cause cold joints and weak spots, so order slightly more than you calculate. Slab thickness, reinforcement and base preparation depend on the use and your local building rules. For load-bearing or structural work, ask a qualified builder or engineer.
Concrete calculator FAQ
How do I calculate how much concrete I need?
Multiply length by width by thickness to get the volume, convert to cubic yards by dividing cubic feet by 27, then add 10% for waste.
How many cubic yards of concrete are in a 10 × 10 slab?
A 10 ft by 10 ft slab 4 inches thick needs 1.23 cubic yards. With 10% waste, order about 1.36 cubic yards.
How many bags of concrete make a cubic yard?
About 45 bags of 80 lb mix, or 60 bags of 60 lb mix, based on typical bag yields.
How thick should a concrete slab be?
4 inches is common for patios and walkways. Garages and driveways are often thicker. Local rules and use decide the right figure.
How much extra should I order?
About 10% is common. Uneven ground and forms that are not perfectly square can use more.
Should I use bags or ready-mix?
Bags suit small jobs. For jobs over about a cubic yard or two, ready-mix is usually faster and often cheaper per yard.