01 Footing & Sonotube concrete calculator
Switch between round piers (Sonotubes), fence/deck post holes (automatically deducts buried post volume) and continuous strip footings in imperial or metric units.
Pouring a flat pad on top of your footings? Calculate it separately with our concrete slab calculator.
02 Cylinder & strip footing formulas
Round Pier / Sonotube (US)
ft³ = π × (D in ÷ 24)² × Depth ft
Round Pier (UK · AU · NZ)
m³ = π × (D mm ÷ 2000)² × Depth m
Divide total cubic feet by 27 to get cubic yards for a ready-mix order, or divide by 0.60 ft³ per 80-lb bag (0.45 ft³ per 60-lb bag).
03 Sonotube concrete chart (4-ft deep pier)
Doubling a tube’s diameter quadruples the concrete required because radius is squared in π × r² × h. Here is how much concrete a single 4-foot deep tube takes before and after 10% waste:
| Tube diameter | Net ft³ (4 ft deep) | Order yd³ (+10%) | 80-lb bags (+10%) | 60-lb bags (+10%) |
|---|---|---|---|---|
| 8 in | 1.40 ft³ | 0.06 yd³ | 3 bags | 4 bags |
| 10 in | 2.18 ft³ | 0.09 yd³ | 4 bags | 6 bags |
| 12 in | 3.14 ft³ | 0.13 yd³ | 6 bags | 8 bags |
| 14 in | 4.28 ft³ | 0.17 yd³ | 8 bags | 11 bags |
| 16 in | 5.59 ft³ | 0.23 yd³ | 11 bags | 14 bags |
| 18 in | 7.07 ft³ | 0.29 yd³ | 13 bags | 18 bags |
| 24 in | 12.57 ft³ | 0.51 yd³ | 24 bags | 31 bags |
04 Fence & deck post holes: accounting for the post
Standard rules of thumb for setting timber or steel posts in concrete:
- Hole diameter: Roughly 3 × post width (a 10–12 in hole for a 4×4 post; a 16–18 in hole for a 6×6 post).
- Hole depth: Bury ⅓ to ½ of the above-ground fence height (usually 24–36 in, or below local frost depth) plus 4–6 in of tamped gravel at the bottom for drainage.
- Subtract the buried post: Choose Fence or deck post hole in the calculator above so you don’t buy extra bags for space occupied by timber.
05 Continuous strip / trench footings
Strip footings carry CMU block walls, stem walls and brick veneer. Measure the total centreline perimeter in feet (or metres) and multiply by footing width and thickness:
yd³ = (Perimeter ft × Width in ÷ 12 × Depth in ÷ 12) ÷ 27
For example, 80 linear feet of a 16 in wide × 8 in deep footing equals 80 × 1.333 × 0.667 ÷ 27 = 2.63 yd³ net, or 2.90 yd³ with 10% trench-wall overage.
06 Frost line & code considerations
07 Common footing estimating mistakes
- Confusing diameter with radius. Entering diameter directly into
π × r²overestimates volume by 4×. - Earth-dug trench flare. Excavator buckets rarely cut razor-straight walls; allow 10–15% waste on unformed earth trenches vs 5–8% inside cardboard tubes.
- Belled pier bases. If your inspector requires flared bell bases under deck piers, add 0.5–1.5 extra bags per pier depending on bell diameter.
08 Frequently asked questions
How many 80-lb bags of concrete do I need for a 12-inch Sonotube 4 feet deep?
A 12-inch diameter tube that is 4 feet deep holds 3.14 cubic feet of concrete. Divided by 0.60 cubic feet per 80-lb bag, that is 5.24 bags—so buy 6 bags per pier (or 8 bags of 60-lb mix) to allow for waste and slight crowning at the top.
How do you calculate concrete for round Sonotube footings?
Use the cylinder formula V = π × r² × height. Convert the tube diameter in inches to radius in feet by dividing by 24, square it, multiply by 3.1416 and multiply by pier depth in feet. Divide cubic feet by 27 to convert to cubic yards.
How many bags of concrete do I need per fence post hole?
A typical 4×4 wood fence post set 2.5 to 3 feet deep in a 10-inch or 12-inch auger hole needs 2 to 3 bags of 80-lb concrete (or 3 to 4 bags of 50-lb fast-setting mix) once you subtract the volume of the buried post itself.
How wide should a footing be for an 8-inch block or concrete wall?
A traditional rule of thumb for light residential construction on stable soil is making the strip footing twice as wide as the wall thickness (16 inches wide for an 8-inch wall) and at least as thick as the wall width (8 inches deep). Always verify soil bearing capacity and local building codes.
Should I subtract the wood post volume when calculating post-hole concrete?
Yes. A nominal 4×4 post (3.5 × 3.5 in actual) buried 3 feet displaces 0.26 cubic feet, and a 6×6 post (5.5 × 5.5 in) displaces 0.63 cubic feet—more than a full 80-lb bag per hole. Over 20 fence posts, subtracting the post saves 10–20 bags.
When should I switch from bagged concrete to a ready-mix truck for footings?
Once your piers or strip trenches pass roughly 1 to 1.25 cubic yards total (45–55 bags of 80-lb mix), ordering ready-mix saves hours of heavy mixing and produces a monolithic pour without cold joints.
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