Concrete Calculator Formulas
Choose the closest shape, calculate cubic feet, divide by 27 for cubic yards, then add waste.
Rectangle = length x width x thickness
Round = pi x radius squared x height
Tube = pi x (outer radius squared - inner radius squared) x height
Stairs approximate = steps x width x run x rise / 2
Example Calculation
A 20 ft by 12 ft slab at 4 inches thick is 80 cubic feet, or 2.96 cubic yards before waste. With 10% waste, estimate about 3.26 cubic yards.
Calculation Notes
- Bag counts use common dry bag yields: 40 lb about 0.30 cu ft, 60 lb about 0.45 cu ft, and 80 lb about 0.60 cu ft.
- Round and tube modes help estimate holes, columns, round footings and circular slabs.
- Stair mode is approximate because stair geometry and forms can vary.
- Curb and gutter shapes are deferred to keep this Phase 1 calculator simple.
FAQ
Should I order extra concrete?
Yes. A 5% to 10% waste factor is common for planning.
How many cubic feet are in a cubic yard?
One cubic yard contains 27 cubic feet.
Can I use this for footings?
Yes. Use round footing mode for circular holes or rectangle mode for square footings.
How many bags of concrete do I need?
Bag count depends on bag size and yield. Choose 40, 60 or 80 lb bags in the calculator.
Is the stairs mode exact?
No. It is a planning estimate for simple stair forms.