Footing formulas
| Type | Volume in cubic feet | Calculator setting |
|---|---|---|
| Continuous footing | length × width × depth | Rectangle |
| Square or rectangular pad | width × width × depth | Rectangle, How many = number of pads |
| Round pier or sonotube | 3.1416 × radius² × height | Circle, How many = number of piers |
| Post hole | hole volume − post volume | Circle for the hole; see the post table below |
Divide cubic feet by 27 for cubic yards, and by 0.60 or 0.45 for 80 lb or 60 lb bags.
Continuous footings
The calculator opens with a footing 40 ft long, 20 in wide and 10 in deep:
- 20 in = 1.667 ft and 10 in = 0.833 ft
- 40 × 1.667 × 0.833 = 55.6 cubic feet
- 55.6 ÷ 27 = 2.06 cubic yards, or 2.26 with 10 percent extra
| Footing size (width × depth) | Cubic yards per 10 ft | Cubic yards per 100 ft |
|---|---|---|
| 12 × 8 in | 0.25 | 2.47 |
| 16 × 8 in | 0.33 | 3.29 |
| 16 × 10 in | 0.41 | 4.12 |
| 20 × 10 in | 0.51 | 5.14 |
| 24 × 12 in | 0.74 | 7.41 |
For a building, measure the footing along the center line all the way around, so the corners are not counted twice or missed.
Sonotubes and round piers
Volume per foot of height, and bags for a 4 ft tube:
| Tube diameter | Cubic feet per ft | Cubic yards per ft | 4 ft tube: 80 lb bags | 4 ft tube: 60 lb bags |
|---|---|---|---|---|
| 6 in | 0.196 | 0.0073 | 2 | 2 |
| 8 in | 0.349 | 0.0129 | 3 | 4 |
| 10 in | 0.545 | 0.0202 | 4 | 5 |
| 12 in | 0.785 | 0.0291 | 6 | 7 |
| 14 in | 1.069 | 0.0396 | 8 | 10 |
| 16 in | 1.396 | 0.0517 | 10 | 13 |
| 18 in | 1.767 | 0.0654 | 12 | 16 |
| 24 in | 3.142 | 0.1164 | 21 | 28 |
Example: ten 12 in piers, each 4 ft deep. 0.785 × 4 = 3.14 cubic feet per pier, × 10 = 31.4 cubic feet = 1.16 cubic yards. In the calculator: Circle, diameter 12 in, depth 4 ft, How many 10.
If the piers sit on a wider bell or pad at the bottom, add that as another section.
Post holes
A post takes up part of the hole, so subtract it. Posts are smaller than their name: a 4 × 4 is 3.5 × 3.5 in and a 6 × 6 is 5.5 × 5.5 in.
| Post and hole | Concrete per post | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 × 4 post, 8 in hole, 24 in deep | 0.53 cu ft | 1 | 2 |
| 4 × 4 post, 10 in hole, 24 in deep | 0.92 cu ft | 2 | 3 |
| 4 × 4 post, 10 in hole, 36 in deep | 1.38 cu ft | 3 | 4 |
| 6 × 6 post, 12 in hole, 36 in deep | 1.73 cu ft | 3 | 4 |
| 6 × 6 post, 12 in hole, 48 in deep | 2.30 cu ft | 4 | 6 |
To use the calculator for posts, enter the hole as a Circle and set How many to the number of posts. The result includes the space the post itself takes up, so it runs high, by roughly 20 to 35 percent for the holes above. That margin is often welcome on fence lines; for a tighter number, subtract one post’s volume (3.5 × 3.5 in × depth for a 4 × 4) per hole.
Hole width and depth for fences and decks depend on post height, soil, wind load and frost depth. A common starting point is a hole about three times the post width, but follow local code and the fence or deck plans.
Square pads
Isolated pads under posts or columns, for example 24 × 24 in and 12 in deep: 2 × 2 × 1 = 4 cubic feet each. Six pads are 24 cubic feet = 0.89 cubic yards. Enter one pad and set How many to 6.
Putting a foundation together
Most jobs mix shapes. A deck might be eight 10 in piers plus two 24 × 24 in pads; a garage might be a perimeter footing plus a slab. Enter each as its own section, and the calculator totals the yards and bags and applies your waste allowance once. For the slab part, see the concrete slab calculator.
This page calculates volume only. Footing size, depth and reinforcement are set by local building code, frost depth and the loads involved. Check with your building department or follow your engineer's drawings before you dig.
Questions people ask
- How do I calculate concrete for a footing?
- Length × width × depth in feet, divided by 27. A footing 40 ft long, 20 in wide and 10 in deep is 40 × 1.667 × 0.833 = 55.6 cubic feet, or 2.06 cubic yards.
- How much concrete is in a sonotube?
- 3.1416 × radius × radius × height. A 12 in tube holds 0.785 cubic feet per foot of height; an 8 in tube holds 0.349; a 10 in tube 0.545.
- How many bags of concrete fill a sonotube?
- A 4 ft section of 8 in tube takes about 3 bags of 80 lb; 10 in takes about 4; 12 in takes about 6. Use the table on this page for other sizes.
- How many bags of concrete for a post hole?
- Subtract the post from the hole volume. A 10 in hole 3 ft deep around a 4 × 4 needs about 1.4 cubic feet: 3 bags of 80 lb or 4 bags of 60 lb.
- How big should my footings be?
- Footing size and depth are set by your local building code, the frost depth in your area, the soil and the load. Check with your building department or follow your engineer's plans.