How to calculate concrete for a patio
A patio estimate starts with three measurements: length, width and slab thickness. Multiplying those dimensions gives the raw concrete volume. PrepMyBuild then keeps the raw quantity separate from the purchase quantity so you can see exactly what a waste or contingency allowance adds.
Choose patio thickness deliberately
Thickness has a large effect on concrete volume. A 16 × 20 patio at 4 inches uses one-third more concrete than the same footprint at 3 inches. Use the thickness specified for your project conditions rather than treating the default as a design recommendation.
Plan the order, not just the math
Forms are rarely perfect and real subgrades can vary. The calculator therefore shows raw volume and a recommended order quantity separately. You can change the allowance to match how precisely the area has been prepared.
What the estimate does not include
The concrete quantity is only one part of a patio project. Base material, reinforcement, joints, forms, finishing, drainage and local requirements can change the final material list and cost.
The raw volume is about 3.95 yd³. With a 10% allowance, the planning order is about 4.35 yd³.