Concrete
How to Calculate Concrete for a Slab
A practical slab concrete calculation process using square footage, thickness, cubic yards, and bag yield.
Open Concrete CalculatorStart with slab area
Measure the length and width of the slab in feet, then multiply them to get square feet. For a patio, shed pad, walkway landing, or small driveway section, this is the base number every concrete estimate uses.
Convert thickness to feet
Slab thickness is usually described in inches. Divide the thickness by 12 before multiplying by area. A 4 inch slab is 0.333 feet thick, while a 6 inch slab is 0.5 feet thick.
Convert volume into yards or bags
Multiply area by thickness to get cubic feet, then divide by 27 for cubic yards. For bagged concrete, divide cubic feet by the bag yield on the product label and round up to a whole bag.
Use The Calculator With This Guide
This guide explains the measuring method, while the Concrete Calculator turns the same measurements into project quantities, waste-adjusted results, and a copyable shopping list. Use the guide first when you are deciding what to measure, then use the calculator when you are ready to compare material amounts.
Open Concrete CalculatorMeasurement Checks
- Using inches as feet for slab depth.
- Ordering exact volume with no allowance for uneven base.
- Assuming every bag size has the same yield.
Formula Reference
Cubic feet = area_sqft × depth_inches / 12
Cubic yards = cubic_feet / 27
Adjusted volume = volume × (1 + waste_factor)
Bags = adjusted_cubic_feet / bag_yield
FAQ
Why do concrete estimates use cubic yards?
Ready-mix and bulk concrete ordering commonly uses cubic yards. Bagged concrete is easier for small projects but still starts from volume.
How much waste should I add?
Five to ten percent is a common planning range for simple slabs. Uneven forms, hand mixing, and rough subgrade can need more.