Calculators
How Much Concrete Do I Need?
Volume is length times width times depth for a rectangle, or π times radius squared times depth for a circle. Post holes are cylinders times a count — not the same as a flat slab.
Worked example: 12 by 8 ft pad, 4 inches deep, 10% waste, 80 lb bags at roughly 0.6 cu ft yield.
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| Item | Area | Size | Buy |
|---|---|---|---|
| Concrete | — | 35.2 cu ft | 59 bags |
| Ready-mix equivalent | — | 1.3 cu yd | 1.5 cubic yards |
Assumptions
- Rectangle slab: 12 × 8 ft × 4 in
- Waste factor: 10%
- Volume: 32.00 cu ft
- Volume with waste: 35.20 cu ft (1.30 cu yd)
- Bag yield: 0.6 cu ft per bag (check bag label)
- Above about 1 cubic yard, ready-mix delivery is often more practical than bags — compare options locally.
- Estimates only. Check the coverage figure on your product before buying.
Supplies checklist
- Gravel or crushed stone base
- Wire mesh or rebar (slabs)
- Form boards and stakes
- Concrete mixer or mixing tub
- Float and edger
Estimates only. Check the coverage figure on your product before buying.
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