How to calculate concrete for a slab
Multiply slab length by width and thickness. Because thickness is commonly measured in inches, convert it to feet before calculating cubic feet.
Concrete slab example
A 20-foot by 10-foot slab that is 4 inches thick contains about 2.47 cubic yards of concrete before adding any waste allowance.
Why add extra concrete?
Finished dimensions rarely match theoretical volume perfectly. Excavation variation, uneven bases, form movement and spillage can all increase the amount actually needed.
The calculator estimates volume only. Slab thickness, reinforcement and concrete specification should follow the project design and applicable building requirements.
Frequently asked questions
How many cubic yards of concrete do I need?
Multiply length, width and thickness to calculate volume, then convert cubic feet to cubic yards.
How many cubic feet are in one cubic yard?
One cubic yard contains 27 cubic feet.
Should I include extra concrete?
A small allowance is commonly used to account for site and placement variation.
Does the calculator determine slab thickness?
No. You enter the required thickness based on your project specification.
Using this calculator effectively
The slab calculator converts length, width and thickness into concrete volume. It is aimed at simple rectangular slabs such as pads and floors where cubic yards are the main ordering unit.
Common uses
- Estimating concrete for patios, pads and small floors.
- Comparing how different slab thicknesses change concrete volume.
- Preparing a preliminary quantity before contacting a ready-mix supplier.
Tips for a better estimate
- Measure the formed dimensions after excavation and preparation where possible.
- Add an allowance for uneven subgrade, over-excavation and placement loss.
- Verify structural thickness and reinforcement requirements independently.
Helpful guides
How Much Concrete Do I Need for a Slab? · How to Calculate Concrete
Related calculations
Concrete Calculator · Concrete Driveway Calculator · Concrete Footing Calculator