Concrete Calculator
Calculate exactly how much concrete you need for slabs, footings, and columns in cubic yards and bags.
On this page
Concrete Slab Measurement
Measure the Length and Width of your form boards, and the Depth of the pour. Most standard patios and sidewalks require a 4-inch depth.
How to Use the Concrete Calculator
Calculating how much concrete you need for your project doesn't have to be complicated. Our free calculator takes the guesswork out of ordering ready-mix or buying bags at the hardware store.
- Select your shape: Choose whether you are pouring a rectangular slab or a round column/tube.
- Enter your dimensions: Input the length and width (or diameter) in feet, and the depth/thickness in inches.
- Include waste: We highly recommend leaving the "10% waste factor" box checked to account for spillage, uneven subgrades, and settling.
- Get your results: The calculator will instantly display your required cubic yards, cubic feet, and the exact number of 60 lb or 80 lb bags you'd need.
Concrete Calculator Formula
If you want to do the math yourself, calculating concrete volume is a simple matter of finding the cubic footage and converting it to cubic yards.
Step 1: Convert all measurements to feet.
If your slab is 4 inches thick, divide 4 by 12 to get 0.33 feet.
Step 2: Calculate Cubic Feet
Volume (ft³) = Length × Width × Depth
Step 3: Convert to Cubic Yards
Volume (yd³) = Volume (ft³) ÷ 27
Step 4: Calculate Bags (Optional)
For 80 lb bags: Divide total cubic feet by 0.60.
For 60 lb bags: Divide total cubic feet by 0.45.
Concrete Example Calculation
Let's say you want to pour a concrete patio that is 12 feet long, 12 feet wide, and 4 inches thick.
- Length: 12 ft
- Width: 12 ft
- Depth: 4 inches (0.33 ft)
- Math: 12 × 12 × 0.33 = 48 cubic feet.
- Convert to yards: 48 ÷ 27 = 1.78 cubic yards.
- Add 10% waste: 1.78 × 1.10 = 1.96 cubic yards.
Conclusion: You should order exactly 2 cubic yards of ready-mix concrete for this patio.
Tips for Pouring Concrete
Pouring concrete is a time-sensitive project. A few careful steps during the planning phase will ensure your slab lasts for decades.
- Always add 10% waste: The ground is rarely perfectly level. Subgrades settle, forms bow outward, and spills happen. A 10% buffer prevents you from running out of concrete mid-pour.
- Know when to order ready-mix: If your project requires more than 1 cubic yard (about forty-five 80lb bags), do not try to mix it by hand. Order a ready-mix truck. Hand-mixing that volume will take hours and cause the slab to dry unevenly.
- Prepare a solid base: Concrete is only as strong as what's underneath it. Tamp your soil and add 4 inches of compacted gravel base before pouring.
- Control joints are crucial: Concrete shrinks as it cures. Cut or tool control joints every 8-12 feet to ensure that when it cracks, it cracks in a straight, invisible line.
Frequently Asked Questions
How many cubic yards of concrete do I need for a 10x10 slab?
For a 10x10 ft slab at 4 inches thick, you need approximately 1.23 cubic yards of concrete. Use the formula: 10 × 10 × (4/12) = 33.33 cubic feet ÷ 27 = 1.23 cubic yards. Always add 10% extra for waste.
How many 80 lb bags of concrete make a cubic yard?
It takes exactly 45 bags of 80 lb concrete mix to make one cubic yard. Since one 80 lb bag yields about 0.60 cubic feet, and there are 27 cubic feet in a yard, 27 ÷ 0.60 = 45 bags.
How thick should a concrete slab be for a patio?
A standard concrete patio slab should be 4 inches thick. If you plan to park heavy vehicles or install a hot tub on the slab, you should increase the thickness to 5 or 6 inches and use wire mesh reinforcement.
What is the difference between cubic yards and cubic feet?
A cubic foot is a block measuring 1 foot on all three sides. A cubic yard is a block measuring 1 yard (3 feet) on all sides. Since 3x3x3 = 27, there are 27 cubic feet in one cubic yard.
How much does a cubic yard of concrete cost?
On average, ready-mix concrete costs between $115 to $135 per cubic yard, excluding delivery fees. Bagged concrete is generally more expensive per yard, costing around $150 to $200 per yard.
How do I calculate concrete for a circular slab?
To calculate a circular slab, square the radius (half the diameter) in feet, multiply by Pi (3.14159), and multiply by the depth in feet to get cubic feet. Then divide by 27 to get cubic yards.
Should I use ready-mix or bags for a large pour?
For any project requiring more than 1 to 1.5 cubic yards, you should order ready-mix concrete delivered by a truck. Mixing more than 45-60 bags by hand is highly labor-intensive and runs the risk of the concrete curing unevenly.
What happens if I order too little concrete?
Ordering too little concrete is a major problem known as a 'short load'. You will have a cold joint where the old concrete sets before the new concrete arrives, weakening the slab. This is why a 10% waste factor is highly recommended.
How do I calculate concrete for columns?
Measure the diameter and height. Find the radius (diameter ÷ 2), convert to feet, square it, multiply by Pi (3.14), and multiply by the height in feet to get cubic feet. Divide by 27 for cubic yards.
What is the standard concrete mix ratio for a slab?
The standard mix ratio for a concrete slab is 1 part portland cement, 2 parts sand, and 3 parts gravel or crushed stone. This 1-2-3 ratio yields a strong, durable concrete mix suitable for driveways and patios.