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Concrete Calculator
Estimate concrete volume for a slab. Change the inputs below and the estimate updates immediately.
Input
Your numbers
Calculated result
Concrete volume
2.47 yd³
Cubic feet
66.67 ft³
With 10% waste
2.72 yd³
Estimate only. Verify important decisions with an appropriate professional or official source.
How to use it
A quick estimate, without the spreadsheet.
Inputs and units
Enter rectangular slab length and width in feet and thickness in inches.
Formula
Cubic feet = length × width × thickness / 12; cubic yards = cubic feet / 27; waste allowance = volume × 1.10.
Worked example
A 12 × 9 foot slab at 4 inches thick needs 36 cubic feet, or 1.33 cubic yards before waste.
Assumptions and limitations
Assumes uniform thickness. Footings, uneven subgrades, reinforcement displacement and delivery minimums are excluded; confirm the order with your supplier.
Understanding your result
The volume is the geometric amount for a rectangular slab of uniform thickness. The allowance result adds 10% for planning. Your supplier’s delivery increments, minimum order and site conditions determine the final order.
A practical comparison
A slab 12 feet long and 9 feet wide at 4 inches thick has a volume of 36 cubic feet. Dividing by 27 gives 1.333 cubic yards. Adding the tool’s 10% allowance gives about 1.467 cubic yards. Confirm delivery rounding with your supplier rather than ordering the displayed decimal blindly.
Common mistakes
- Length and width are in feet; thickness is in inches.
- Measure the prepared base at several points because deeper areas increase the actual volume.
- Calculate footings or thickened edges separately and add their volume.
Frequently asked questions
Can I calculate an irregular slab?
Divide it into rectangles, calculate each section and add the volumes. Avoid counting overlapping sections twice.
Does this determine structural thickness?
No. You enter thickness. Structural design, reinforcement, soil preparation and local building requirements need a suitable design for the project.