Calculators/ Concrete/Concrete Slab Calculator
Concrete Slab Calculator
Cubic yards and bag counts for any rectangular slab.
A concrete slab is priced and ordered by the cubic yard, but it is measured in feet and inches. This calculator does the conversion and tells you how many bags to buy if you are mixing on site.
How this is calculated
- Multiply length × width × thickness to get the volume in cubic feet. Thickness is entered in inches, so it is divided by 12 first.
- Divide cubic feet by 27 to get cubic yards, which is how ready-mix is ordered.
- For bagged concrete, divide the volume by the yield of one bag: 0.60 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb and 0.30 ft³ for 40 lb.
- Add a waste allowance. 10% is normal for a slab on uneven ground; 5% is enough over a compacted, formed base.
Frequently asked questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet, which works out to 45 bags of 80 lb concrete, 60 bags of 60 lb, or 90 bags of 40 lb. Above roughly half a cubic yard it is usually cheaper and far less work to order ready-mix.
How thick should a concrete slab be?
Four inches is standard for patios, walkways and shed bases. Go to 5-6 inches if vehicles will drive on it, and check local code for anything structural.
Should I add a waste allowance?
Yes. Sub-grade is never perfectly level, and running short mid-pour leaves a cold joint. Ten percent is the usual allowance; never order under.