Enter the diameter and depth of the form tube, the size of the footing pad if there is one, and the number of footings. The calculator works out the concrete for each footing and for the whole job, adds extra for spillage and rounds up to whole bags.
For a flat patio, walkway or shed slab, use the concrete slab calculator instead, and for posts set straight into the ground, the post hole concrete calculator.
How the footing calculator works
- Pier = π × (diameter ÷ 2)² × depth, the volume of a round form tube.
- Pad = width × width × thickness for a square footing under the pier. Leave the pad fields empty if there is none.
- Bags = (pier + pad) × number of footings, plus the extra percentage, divided by the yield of one bag and rounded up. An 80 lb bag of pre-mixed concrete makes about 0.6 cu ft.
This calculator only estimates concrete. It does not size footings. The diameter, the pad size and how deep the footing goes below the frost line depend on the load, the soil and your local building code, so check with your building department before you dig.
Concrete per pier
| Tube diameter | 36 in deep | 48 in deep |
|---|---|---|
| 8 in | 1.05 cu ft (1.9 bags) | 1.40 cu ft (2.6 bags) |
| 10 in | 1.64 cu ft (3.0 bags) | 2.18 cu ft (4.0 bags) |
| 12 in | 2.36 cu ft (4.3 bags) | 3.14 cu ft (5.8 bags) |
| 16 in | 4.19 cu ft (7.7 bags) | 5.59 cu ft (10.2 bags) |
| 18 in | 5.30 cu ft (9.7 bags) | 7.07 cu ft (13.0 bags) |
Concrete per footing pad
| Pad size | Volume |
|---|---|
| 16 × 16 × 8 in | 1.19 cu ft |
| 20 × 20 × 8 in | 1.85 cu ft |
| 24 × 24 × 10 in | 3.33 cu ft |
Worked example
A small deck sits on 4 footings, each a 10 in tube 48 in deep on a 20 × 20 × 8 in pad. Each footing holds 2.18 + 1.85 = 4.03 cu ft. Four footings need 16.1 cu ft, or 17.7 cu ft with 10% extra. At 0.6 cu ft per 80 lb bag that is 29.6, so buy 30 bags.
Step by step: estimating concrete for footings
Tips before you buy
- Buy a bag or two extra; a short bag means a cold joint in the pier.
- For a cubic yard or more, compare the price with a short-load ready-mix delivery.
- For posts set straight into the ground, the post hole concrete calculator subtracts the post from the hole.
- For slabs and pads, use the concrete slab calculator and see the concrete bag yield chart.
