Calculate Concrete Needed
Select your project type and enter dimensions to calculate concrete volume, bags needed, and estimated cost.
Slab / Patio Dimensions
Bags Needed
Cost Estimator
Volume Visualization
Column / Cylinder Dimensions
Bags Needed
Cost Estimator
Footing Dimensions
Bags Needed
Cost Estimator
Wall Dimensions
Bags Needed
Cost Estimator
How to Calculate Concrete
Calculating concrete accurately saves money and prevents shortages on job day. Follow these steps to determine how much concrete you need for any project.
Volume Formula
For slabs: Length (ft) × Width (ft) × Thickness (in) ÷ 12 = Cubic feet. For columns: π × (Diameter ÷ 2)² × Height. Divide cubic feet by 27 to get cubic yards.
Bag Coverage
40 lb bag = 0.011 cu yd, 60 lb bag = 0.017 cu yd, 80 lb bag = 0.022 cu yd. Divide total cubic yards by coverage per bag to get bags needed.
Waste Factor
Always add 10-15% extra for spillage, uneven excavation, and mixing losses. Complex shapes or inexperienced crews should use 15%. Simple slabs can use 10%.
Mixing Tips
Mix bags one at a time in a wheelbarrow or mixer. Add water gradually - too much weakens concrete. Cure for 7 days by keeping moist with a tarp or periodic spraying.
Concrete Coverage Reference
Standard bag coverage for different sizes of pre-mixed concrete.
| Bag Size | Coverage (cu yd) | Coverage (cu ft) | Bags per cu yd |
|---|---|---|---|
| 40 lb | 0.011 | 0.30 | 90 |
| 60 lb | 0.017 | 0.45 | 59 |
| 80 lb | 0.022 | 0.60 | 45 |
Worked Examples
Example 1: Patio Slab
Dimensions: 10 ft × 12 ft × 4 inches thick
Calculation: (10 × 12 × 4 ÷ 12) ÷ 27 = 1.48 cu yd
With 10% waste: 1.48 × 1.10 = 1.63 cu yd
60 lb bags needed: 1.63 ÷ 0.017 = 96 bags
Example 2: Round Column
Dimensions: 12-inch diameter × 8 ft tall
Calculation: π × (6 in ÷ 12)² × 8 ÷ 27 = 0.23 cu yd
With 10% waste: 0.23 × 1.10 = 0.25 cu yd
80 lb bags needed: 0.25 ÷ 0.022 = 12 bags
Example 3: Foundation Footing
Dimensions: 30 ft × 2 ft × 10 inches deep
Calculation: (30 × 2 × 10 ÷ 12) ÷ 27 = 1.85 cu yd
With 15% waste: 1.85 × 1.15 = 2.13 cu yd
60 lb bags needed: 2.13 ÷ 0.017 = 125 bags
Frequently Asked Questions
How much concrete do I need for a slab?
How many bags of concrete do I need?
What is a waste factor and why do I need it?
How do I calculate concrete for a round column?
What size concrete bag is best for DIY projects?
How much does concrete cost per cubic yard?
Privacy & Limitations
All calculations are performed locally in your browser. No data is sent to our servers. This calculator provides estimates based on standard concrete densities and bag coverage rates. Actual amounts may vary based on concrete mix, moisture content, and job conditions. Always consult with a professional for critical structural projects and check local building codes.
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Concrete Calculator FAQ
How much concrete do I need for a slab?
Calculate volume in cubic feet by multiplying length (ft) × width (ft) × thickness (in) ÷ 12. Then divide by 27 to get cubic yards. For example, a 10ft × 10ft × 4in slab needs (10 × 10 × 4 ÷ 12) ÷ 27 = 1.23 cubic yards. Add 10% waste factor for safety.
How many bags of concrete do I need?
A 40-lb bag covers 0.011 cubic yards, a 60-lb bag covers 0.017 cubic yards, and an 80-lb bag covers 0.022 cubic yards. Divide your total cubic yards by the bag coverage. For 1 cubic yard, you need about 90 bags of 40-lb, 59 bags of 60-lb, or 45 bags of 80-lb concrete.
What is a waste factor and why do I need it?
A waste factor accounts for spillage, over-excavation, uneven surfaces, and mixing losses. The standard is 10%, but use 15% for complex shapes or inexperienced crews. It ensures you have enough concrete to complete the job without running short.
How do I calculate concrete for a round column?
For a cylinder, volume = π × radius² × height. Convert diameter to radius (diameter ÷ 2), ensure height is in feet, and the formula gives cubic feet. Divide by 27 for cubic yards. For example, a 12-inch diameter × 8-ft tall column needs π × 0.5² × 8 ÷ 27 = 0.23 cubic yards.
What size concrete bag is best for DIY projects?
60-lb bags offer the best balance of coverage and portability for most DIY projects. 80-lb bags cover more area but are harder to lift and mix. 40-lb bags are easier to handle but require more bags, increasing total cost and mixing time.
How much does concrete cost per cubic yard?
Bagged concrete costs $4-8 per bag depending on size and brand. For 1 cubic yard, expect to pay $240-360 using 60-lb bags. Ready-mix concrete delivered by truck costs $100-150 per cubic yard, making it more economical for projects over 2 cubic yards.