Sonotube Calculator — Concrete Tube Form
A sonotube is a cylindrical cardboard form used as a mold when pouring concrete footings for fence posts, deck piers and structural columns. Enter the tube diameter, depth, and number of tubes to find the total concrete volume and how many bags to buy.
in
in
Bag size
%
Total bags including waste allowance, rounded up to the nearest whole bag
1.047 ft³
per tube- 1
Volume per tube
π × 4² × 36 ÷ 1728 = 1.0472Cylinder volume in cubic inches (π r² h) divided by 1728 (= 12³) to convert to cubic feet. - 2
Total volume (all tubes)
1.0472 × 1 = 1.0472 - 3
Volume with waste allowance
1.0472 × 1.1 = 1.1519 - 4
Bags needed (rounded up)
⌈1.1519 ÷ 0.6⌉ = 2Divided by the yield per bag and rounded up to the nearest whole bag.
How does this calculator work?
Concrete volume for a sonotube = π × r² × h (in cubic inches, ÷ 1728 for cubic feet). An 80 lb bag of ready-mix yields about 0.60 ft³. Add a 10% waste allowance and round up to the next whole bag. Check local frost-line requirements for the minimum required depth.
Formula
How this is calculated
A sonotube is a waxed-cardboard cylindrical form that holds wet concrete in a circular cross-section while it cures, producing a clean, round footing or column. The concrete volume for each tube is the cylinder formula π r² h, where r is the tube radius and h is the depth. With inputs in inches the result is in cubic inches; dividing by 1728 (= 12³) converts to cubic feet.
Bag yields are approximate manufacturer figures: an 80 lb bag of ready-mix (such as Quikrete) yields about 0.60 ft³, a 60 lb bag about 0.45 ft³, and a 40 lb bag about 0.30 ft³. These can vary slightly by brand and mixing water ratio. A waste allowance (10% is typical) accounts for uneven holes, ground irregularities and mixing losses; the bags total is rounded up to the nearest whole bag.
For frost-protection in cold climates, codes typically require footings to extend below the local frost line — enter the required depth (check your local building code). The sonotube diameter should match the post or beam it supports; common residential sizes are 8, 10 and 12 inches.
Frequently asked questions
Footings must reach below the local frost line to prevent frost heave. The required depth varies widely — from about 12 inches in mild climates to 48 inches or more in northern regions. Check your local building code for the required depth before digging.
No — the hole should be dry or pumped out before pouring, otherwise the water dilutes the concrete mix and weakens it. If groundwater is an issue, use a pump to remove it immediately before pouring.
A common rule of thumb is to use a tube diameter equal to about 1.5–2× the post width. An 8 inch tube typically suits a 4×4 post; a 10–12 inch tube suits a 6×6 post. Load, soil conditions and local codes ultimately determine the required size.
Also known as
TG we-Calculate Editorial Team. (2026). Sonotube Calculator — Concrete Tube Form [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/sonotube-calculator
TG we-Calculate Editorial Team. "Sonotube Calculator — Concrete Tube Form." TG we-Calculate. 2026. https://we-calculate.com/calculator/sonotube-calculator.
TG we-Calculate Editorial Team, "Sonotube Calculator — Concrete Tube Form," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/sonotube-calculator
@misc{wecalculate_sonotube_calculator, title = {Sonotube Calculator — Concrete Tube Form}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/sonotube-calculator}}, year = {2026}, note = {TG we-Calculate} }
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