Concrete Calculator
Enter your slab dimensions and get an instant estimate: cubic yards, 80 lb bag count, and 60 lb bag count. Free, no sign-up required.
How Much Concrete Do I Need?
Enter length, width, and thickness below. Results appear instantly.
Concrete needed
Enter a length and a width and the cubic yards appear here.
How to Measure Your Slab
Accurate measurements before you order save you a second trip to the concrete supplier. Here is how to get them right.
Length and width
Measure from the inside faces of your forms, not the outside. If you are planning a slab without forms yet, measure the finished dimensions you want. Use a tape measure and record in feet. Fractions are fine: a 12-foot 6-inch measurement enters as 12.5.
Thickness
Thickness is measured in inches, not feet. Common thicknesses by application:
- 4 inches: sidewalks, patios, basement floors, and most residential slabs
- 6 inches: driveways, garage floors, and farm shop slabs
- 8 inches or more: commercial floors, loading docks, and heavy equipment pads
If your engineer or contractor has specified a different thickness, use that number. Increasing thickness by just 2 inches raises the volume by 50% on the same footprint.
Irregular or L-shaped areas
Divide the area into rectangles, calculate each section separately using this calculator, then add the cubic yard totals together. For example, an L-shaped patio can be split into two overlapping rectangles. Subtract any overlap once to avoid double-counting.
For curved areas such as a circular patio, calculate the area of the circle (pi times radius squared) in square feet, then multiply by the depth in feet and divide by 27. Curved slabs with irregular radii are best left to a contractor to measure directly.
The 10% waste allowance
Always order 10% more concrete than your calculated volume. Spills during the pour, slight sub-base variation, and form flex all consume concrete you did not account for. A cold joint formed by running out mid-pour is a structural defect that cannot be fixed after the fact. The calculator shows both your base volume and the 10% waste-adjusted figure; order the higher number.
Frequently Asked Questions
How do I calculate how much concrete I need?
Multiply your slab length by the width to get square footage, then multiply by the depth in feet (inches divided by 12). Divide that result by 27 to convert cubic feet to cubic yards. For example, a 10x10 slab at 4 inches thick is 10 x 10 x (4/12) / 27 = 1.23 cubic yards. Always add 10% for waste and spillage.
How many bags of concrete do I need for a 10x10 slab?
A 10x10 slab at a standard 4-inch thickness requires about 1.24 cubic yards of concrete. That works out to roughly 56 bags of 80 lb pre-mix or 74 bags of 60 lb pre-mix. With a 10% waste factor, plan for 62 bags of 80 lb or 82 bags of 60 lb.
What thickness should a concrete slab be?
For sidewalks and residential patios, 4 inches is the standard. Driveways and garage floors typically call for 6 inches to handle vehicle weight. Commercial floors, shop slabs, and areas that see heavy equipment should be 8 inches or more. Increasing thickness uses significantly more concrete, so confirm the spec with your contractor before ordering.
Should I order extra concrete?
Yes. Order at least 10% more than your calculated volume. Spillage during the pour, slight variations in sub-base grade, and the reality that forms are rarely perfectly square all eat into your margin. Running short mid-pour creates cold joints that weaken the slab. The cost of a little extra concrete is far less than the cost of a bad pour.
Can this calculator be used for driveways or irregular shapes?
This calculator handles rectangular slabs. For a driveway with a consistent width, break it into rectangular sections, calculate each one separately, then add the totals. For L-shaped or irregular areas, divide the shape into rectangles, calculate each, and sum them. For curved or complex shapes, your concrete contractor can take field measurements and calculate the volume directly.
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Also see: Garage floor coatings