construction ToolUpdated 2026-09-19
Concrete Calculator
Estimate the exact amount of concrete required for slabs, patios, footings, and driveways, with waste margin allowances.
Construction & Material Estimate Notice: Material quantity calculations are theoretical estimates. Actual requirements depend on sub-base conditions, irregular surface cuts, and structural engineering specifications. Always verify with your supplier or licensed contractor prior to ordering.
Formula & Mathematical Methodology
Cubic Yards = (Length(ft) × Width(ft) × (Thickness(in) ÷ 12)) ÷ 27
Converts thickness to feet, calculates cubic feet, and divides by 27 to find cubic yards.
How to Use This Concrete Calculator
- Enter the length and width of your slab in feet, and thickness in inches (4 inches is standard for patios and walkways).
Step-by-Step Worked Example
Scenario: 20 ft × 10 ft slab at 4 inches thick
1. Thickness in feet: 4 in ÷ 12 = 0.333 ft
2. Cubic feet: 20 × 10 × 0.333 = 66.67 cu ft
3. Cubic yards: 66.67 ÷ 27 = 2.47 cu yd
4. With 10% waste buffer: 2.72 cu yd (or approx 112 bags of 80 lb concrete)
Final Calculated Result: 2.47 cu yd (2.72 cu yd with 10% buffer)
What Does Your Result Mean?
When ordering ready-mix concrete from a truck, round up to the nearest quarter or half yard to avoid running short.
Common Practical Applications:
- Patio and sidewalk slabs
- Garage foundation footings
- Fence post anchoring
Important Limitations & Assumptions:
- Sub-base variations and irregular ground grading require ordering at least 10% additional concrete buffer.
Frequently Asked Questions
How many 80 lb bags of concrete make a cubic yard?
It takes 45 bags of 80 lb pre-mixed concrete to equal one cubic yard (27 cubic feet).
What is standard concrete slab thickness?
4 inches is standard for residential walkways, patios, and garage floors. Driveways supporting heavy vehicles typically require 5 to 6 inches.
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