Concrete Volume Formula

For slabs, walls, and footings, use the following basic formula:

Volume = Length × Width × Thickness

Make sure all measurements use compatible units before multiplying. If you measure length and width in feet but thickness in inches, convert the thickness to feet first by dividing the number of inches by 12.

Concrete Calculation Example

Suppose you are pouring a concrete slab that is 20 feet long, 10 feet wide, and 4 inches thick.

First, convert the 4-inch thickness to feet: 4 ÷ 12 = 0.333 feet.

Then calculate the volume: 20 × 10 × 0.333 = 66.67 cubic feet.

Convert cubic feet to cubic yards by dividing by 27: 66.67 ÷ 27 = 2.47 cubic yards.

With a 10% waste allowance, the estimated amount becomes approximately 2.72 cubic yards of concrete.

How Many Bags of Concrete Do I Need?

The number of premixed concrete bags you need depends on the total concrete volume and the approximate yield of each bag size. Fahahu calculates the estimated number of 40 lb, 60 lb, and 80 lb bags automatically and rounds the result up to the next whole bag.

This calculator uses approximate yields of 0.30 cubic feet for a 40 lb bag, 0.45 cubic feet for a 60 lb bag, and 0.60 cubic feet for an 80 lb bag. Actual yield can vary by product and manufacturer, so check the coverage information printed on the bag before purchasing materials.

Bag Size	Approx. Yield	Approx. Bags per Cubic Yard
40 lb	0.30 ft³	90 bags
60 lb	0.45 ft³	60 bags
80 lb	0.60 ft³	45 bags

Note: Bag yields are approximate. Always check the manufacturer’s stated yield for the specific concrete mix you plan to use.

Common Concrete Thicknesses

Concrete thickness varies by project, load, soil conditions, reinforcement, and local building requirements. The values below are common starting points for estimating only and should not replace project specifications or local code requirements.

Project	Common Thickness	Notes
Sidewalks	4 in	Common for pedestrian use
Patios	4 in	Typical for residential patios
Driveways	4–6 in	Heavier vehicles may require more
Garage Floors	4–6 in	Depends on vehicle/load requirements
Concrete Walls	Varies	Follow structural design/specifications

Concrete Calculator FAQs

How much concrete do I need for a 10 × 10 slab?
For a 10 ft × 10 ft slab that is 4 inches thick, you need about 1.23 cubic yards before waste, or roughly 1.36 cubic yards with 10% extra.

How many 80 lb bags make a cubic yard?
Using an approximate yield of 0.60 cubic feet per 80 lb bag, one cubic yard requires about 45 bags.

Should I add extra concrete for waste?
A 5–10% extra allowance is commonly used for estimating to account for spillage, uneven excavation, and measurement variation. Complex pours may need more.

Does this calculator work for columns and footings?
Yes. Use the project-type buttons above to calculate slabs, walls, footings, and cylindrical columns.

Can I use metric measurements?
Yes. Switch to Metric to enter dimensions in meters and centimeters and receive the primary result in cubic meters.

Need Help With Your Concrete Project?

Have your concrete estimate but need help with pouring, forming, finishing, or other concrete work? Tell us about your project and location, and we may be able to help connect you with a suitable professional.

Related Calculators

Planning another project? Explore more free calculators from Fahahu to estimate materials and quantities quickly.