Concrete Calculator
Pouring a patio slab, sidewalk, driveway, footing, or sonotube column? Our free concrete calculator computes exact cubic yards or cubic meters required, adds a 10% safety buffer, and tells you whether to order a ready-mix truck or how many 80lb, 60lb, or 40lb bags to buy.
Project Inputs
Enter your measurements to calculate exact materials
Enter measurements above to view calculation results.
Step-by-Step Calculation Example
Scenario: Pouring a 10 ft × 10 ft concrete patio slab that is 4 inches thick with a 10% safety buffer.
Convert 4" thickness to feet: 4 ÷ 12 = 0.333 ft.
Calculate Net Volume: 10 ft × 10 ft × 0.333 ft = 33.33 cu ft.
Add 10% Safety Buffer: 33.33 × 1.10 = 36.67 cu ft.
Convert to Cubic Yards: 36.67 ÷ 27 = 1.36 cubic yards.
Calculate 80 lb Bags: 36.67 cu ft ÷ 0.60 cu ft/bag = 61.1 -> 62 bags.
Concrete Slab Volume Formula
Cubic Yards = (Length [ft] × Width [ft] × Thickness [in ÷ 12]) ÷ 27 × (1 + Waste %) Multiply Length by Width by Thickness (converted to feet) to find volume in cubic feet. Divide cubic feet by 27 to obtain Cubic Yards. Add a 10% buffer to compensate for uneven ground and formwork deflection.
Formula Variables
Calculation Assumptions
- One cubic yard equals 27 cubic feet (0.7645 m³).
- An 80 lb bag yields 0.60 cu ft; a 60 lb bag yields 0.45 cu ft; a 40 lb bag yields 0.30 cu ft.
- Ground subgrade is rarely perfectly level; a 10% overage buffer prevents running out during a pour.
Frequently Asked Questions
Common homeowner and DIY questionsConcrete Calculator
When should I order a concrete truck instead of mixing bags?
As a general rule, if your project requires more than 1 cubic yard (approx 45–60 eighty-pound bags), ordering ready-mix delivery by truck is faster, produces higher structural strength, and saves strenuous manual labor.
How thick should a concrete slab be?
Sidewalks, walkways, and shed slabs should be 4 inches thick. Driveways supporting passenger vehicles should be 4 to 5 inches thick, and heavy RV pads or commercial driveways should be 6 inches thick with rebar reinforcement.
How many bags of concrete make 1 cubic yard?
It takes forty-five (45) 80 lb bags, sixty (60) 60 lb bags, or ninety (90) 40 lb bags to make exactly 1 cubic yard of solid concrete.
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