Understand the estimate
Formula and assumptions
Rectangular volume = length × width × depth. Cylindrical post-hole volume = π × radius² × depth. Step volume = tread × rise × width × count. Total volume receives the selected contingency before bag yields are compared.
Rectangular volume = length × width × depth. Cylindrical post-hole volume = π × radius² × depth. Step volume = tread × rise × width × count. Total volume receives the selected contingency before bag yields are compared.
Assumptions to confirm
- Dimensions describe the finished concrete volume.
- Post-hole calculation does not subtract the post volume, providing a conservative estimate.
- Bag yield must come from the manufacturer label.
- Large pours may be more practical as ready-mix delivery.
Label every measurement and keep exclusions visible. Add the shapes in the calculator instead of hiding them inside one total.
Worked example
Input
A 10 × 8 ft slab, 4 in thick, with 10% contingency.
Planning result
The purchase volume is about 29.3 cu ft or 1.09 cu yd, requiring roughly 49 bags at 0.60 cu ft yield.
Common package sizes
Package conventions vary by brand and region. Enter the usable coverage or yield shown on the actual product instead of relying on the name or weight alone.
- 20–25 kg small bags
- 30 kg / 60 lb medium bags
- 36–40 kg / 80 lb large bags
- Ready-mix by cubic yard or metre
Frequently asked questions
Should I subtract the post?
This planner intentionally does not. That small conservative margin helps cover irregular holes and spillage.
What contingency is reasonable?
Five to ten percent is common for measured forms; irregular excavation may need more.
Is this a structural design tool?
No. It estimates volume only. Footing size, reinforcement, mix strength, frost depth, and permits require local professional guidance.