Concrete Calculator
Determine required cubic yards, cubic meters, and premix bags for slabs, footings, and circular columns.
Parameters & Inputs
Summary & Breakdown
Pre-Mix Bag Alternatives
About the Concrete Calculator
Pouring concrete requires ordering the correct batch volume from the ready-mix plant, as underestimating causes disastrous "cold joints" where concrete hardens midway through a pour.
In the United States, commercial concrete is ordered by the cubic yard (27 cubic feet), while home improvement stores sell pre-mixed bagged dry concrete in 60-lb and 80-lb bags.
This calculator computes exact volumes for rectangular slabs, footings, and cylindrical sonotube columns, factoring in the recommended 10% safety margin for ground deflection and form bowing.
How the Calculations Work
Calculates slab volume as Length * Width * (Thickness / 12), converts cubic feet to cubic yards by dividing by 27, and adds the specified waste percentage buffer.
Cubic Yards = [Length(ft) * Width(ft) * (Thickness(in) / 12)] / 27 * (1 + Waste)
Variables & Definitions
- Length & Width: Plan dimensions of the slab in feet
- Thickness: Depth in inches (converted to feet by dividing by 12)
- 27: Conversion factor from cubic feet to cubic yards (3 x 3 x 3 ft)
For a 20 ft x 15 ft patio slab that is 4 inches thick: Volume = 20 * 15 * (4/12) = 100 cu ft. In cubic yards: 100 / 27 = 3.70 yd³. With a 10% buffer: 3.70 * 1.10 = 4.07 cubic yards (or 184 bags of 80-lb premix).
Key Terms Explained
Cubic Yard
Standard commercial volume unit equal to 27 cubic feet (0.764555 cubic meters).
Cold Joint
A structural plane of weakness created when fresh concrete is poured against already hardened concrete.
Slump
A measure of concrete consistency and workability before setting, tested via ASTM slump cone.
Subgrade Compaction
Tamping gravel base beneath concrete to prevent uneven settling and future surface cracking.
Practical Tips & Pitfalls to Avoid
Always round up to the next quarter yard
Ready-mix concrete plants deliver in 0.25 cubic yard increments; order 4.25 yards rather than exactly 4.07.
4-inch depth is standard for patios and sidewalks
Sidewalks and patios use 4 inches (10 cm); residential driveways carrying heavy vehicles require 5 to 6 inches.
Use bagged concrete for small jobs under 1 yard
If a project needs under 1 cubic yard (45 bags of 80 lb), mixing bags yourself avoids minimum-truck dispatch fees.
Account for form deflection
Wooden 2x4 forms bow slightly outward when holding heavy wet concrete (~150 lbs per cubic foot).
Frequently Asked Questions
It takes 45 bags of 80-lb pre-mixed concrete to equal 1 cubic yard (each bag yields 0.60 cubic feet).
Excavated soil is never perfectly flat, forms deflect slightly under hydrostatic pressure, and spillage during wheelbarrow transport is inevitable.
Normal-weight concrete weighs approximately 4,050 pounds (~2 short tons) per cubic yard.
Multiply cubic yards by 0.764555 to convert to cubic meters (1 m³ = 1.308 yd³).
Standard passenger car driveways should be at least 4 inches thick over a compacted subbase, with 5 inches recommended for heavy trucks.