Measure the rectangular slab
Enter length and width in feet, then enter slab thickness in inches or feet. Twelve inches equals one foot. The calculator uses your entered thickness; it does not recommend a structural thickness, reinforcement or concrete strength.
Concrete volume and allowance formulas
Calculated cubic yards = length (feet) × width (feet) × thickness (feet) ÷ 27
Extra allowance (cubic yards) = calculated cubic yards × entered percentage ÷ 100
Adjusted planning quantity = calculated volume + extra allowance
Worked example
A 30-foot by 20-foot slab at an entered thickness of 4 inches has a volume of 30 × 20 × (4 ÷ 12) ÷ 27 = 7.41 cubic yards. With an optional, user-selected 10% allowance, the extra allowance is 0.74 cubic yards and the adjusted planning quantity is 8.15 cubic yards. The allowance is calculated from the unrounded slab volume.
The 4-inch thickness and 10% allowance demonstrate the arithmetic; neither is a design or ordering recommendation.
Assumptions and limitations
This tool models one rectangular slab with uniform entered thickness. It does not include footings, thickened edges, beams, openings or other shapes. An extra allowance is included only if you enter one; the default is zero.
Common concrete slab questions
Do I have to add an allowance?
No. Leave the allowance at zero, or enter an extra percentage based on your own project information. The base volume, added allowance and adjusted quantity remain separate.
Can I use thickness in feet?
Yes. Select feet instead of inches. For example, 4 inches and one-third of a foot describe the same thickness; use sufficient decimal precision if entering feet.
Does this tell me how to build the slab?
No. It estimates material volume from your measurements. Structural thickness, reinforcement, concrete strength and other design requirements need appropriate project-specific guidance.
Discuss your project with JMEC
Use these figures to start a conversation about your actual site, material and scope.