Sonotube Calculator
Enter the diameter and depth of your sonotube form, the number of tubes, and choose your bag size to instantly see total concrete volume, number of bags, estimated weight, and a cost range. Toggle between imperial and metric units, add a waste allowance, and see the step-by-step math.
What is a sonotube?
A sonotube (also written Sonotube, the registered brand name of Sonoco Products) is a cylindrical cardboard tube used as a form for pouring concrete footings, piers, deck posts, and structural columns. The tube is placed in a dug hole, held plumb, and filled with concrete. Once the concrete cures, the cardboard peels away and leaves a clean, round column. Sizes range from 6 inches to 60 inches in diameter and from 4 feet to 20 feet in length; the most common residential sizes are 8, 10, 12, and 14 inches.
How to calculate concrete volume for a sonotube
The volume of concrete in a sonotube is the volume of a cylinder: V = pi x r squared x h, where r is the inside radius of the tube and h is the depth of the pour. Because concrete is sold by the cubic yard, divide the cubic-foot result by 27. For bagged concrete, divide the cubic feet by the yield printed on the bag (0.60 cu ft for an 80 lb bag, 0.45 cu ft for a 60 lb bag, 0.375 cu ft for a 50 lb bag, 0.30 cu ft for a 40 lb bag) and always round up to the nearest whole bag. Add a 10% waste allowance to account for spillage, uneven footing bottoms, and mixing losses.
Choosing between bagged concrete and ready-mix
For small jobs under roughly 1 cubic yard, bagged concrete from a home center is usually the easiest option. Above that threshold, a ready-mix delivery from a local plant becomes cost-competitive because you pay for the concrete itself rather than bags, mixing time, and labour. A standard ready-mix truck carries about 10 cubic yards, and most suppliers have a minimum order of 1 cubic yard with a short-load surcharge. If your project lands between 0.5 and 2 cubic yards, compare the cost of bags (factoring in mixing labour) against the per-yard ready-mix price plus any short-load fee in your area.
Frost depth, rebar, and installation tips
Building codes require footings to extend below the local frost depth to prevent heaving. The IRC maps frost depths from 0 inches (south Florida) to over 60 inches (northern Minnesota). Check your local jurisdiction for the required minimum. Most deck codes also require a minimum diameter for the sonotube based on the tributary load area: 8 or 10 inches for light loads, 12 or 14 inches for heavier spans. Adding rebar vertically inside the tube improves tensile strength; two or four pieces of 1/2-inch rebar centered in the tube and extending into an attached post anchor is typical. The rebar displaces a small amount of concrete (less than 2%), so this calculator does not subtract it, and the waste allowance more than covers the difference.
Quick-reference: concrete per sonotube (80 lb bags)
| Diameter (in) | Depth (ft) | Volume (cu ft) | Volume (cu yd) | 80 lb bags |
|---|---|---|---|---|
| 6 | 1 | 0.02 | 0.001 | 1 |
| 6 | 2 | 0.04 | 0.001 | 1 |
| 6 | 4 | 0.08 | 0.003 | 1 |
| 8 | 1 | 0.03 | 0.001 | 1 |
| 8 | 2 | 0.06 | 0.002 | 1 |
| 8 | 4 | 0.12 | 0.005 | 1 |
| 10 | 2 | 0.09 | 0.003 | 1 |
| 10 | 4 | 0.18 | 0.007 | 1 |
| 12 | 2 | 0.13 | 0.005 | 2 |
| 12 | 4 | 0.26 | 0.01 | 2 |
| 12 | 6 | 0.39 | 0.014 | 3 |
| 14 | 4 | 0.35 | 0.013 | 2 |
| 14 | 6 | 0.53 | 0.02 | 3 |
| 16 | 4 | 0.46 | 0.017 | 2 |
| 16 | 6 | 0.69 | 0.026 | 4 |
| 18 | 4 | 0.59 | 0.022 | 2 |
| 18 | 6 | 0.88 | 0.033 | 5 |
| 20 | 4 | 0.73 | 0.027 | 3 |
| 20 | 6 | 1.09 | 0.04 | 5 |
| 24 | 4 | 1.05 | 0.039 | 4 |
| 24 | 6 | 1.57 | 0.058 | 6 |
Volumes include a 10% waste allowance. Use this to spot-check your calculation or plan orders before measuring exactly.
Frequently asked questions
How many bags of concrete do I need for a 12-inch sonotube 4 feet deep?
A 12-inch diameter, 4-foot-deep sonotube holds about 0.26 cubic feet of concrete per tube before waste. With a 10% waste allowance, you need roughly 0.29 cubic feet, which rounds up to 1 bag of 80 lb concrete (0.60 cu ft yield) per tube. The calculator above will confirm this and adjust for multiple tubes or different bag sizes.
Can I use this calculator for other cylindrical concrete forms?
Yes. The formula is the same for any round form: volume equals pi times the radius squared times the height. Enter the inside diameter and depth of your form and the result is your concrete volume, regardless of whether it is a sonotube, a wax-coated tube, or a reusable plastic form.
How deep should I dig for a deck post sonotube?
The bottom of the footing must sit below the local frost depth, which is published by your municipality and ranges from under 12 inches in mild climates to over 60 inches in cold northern regions. Add 2 to 4 inches of gravel at the bottom for drainage, then measure the tube depth from the gravel surface to the top of the pour. The calculator uses the depth you enter, so measure the actual pour depth rather than the total hole depth.
What happens if I run short of concrete?
Stopping a pour partway and letting a cold joint form weakens the column significantly. Always order extra. A 10% waste allowance is the industry standard minimum; for irregular soil, rocky bottoms, or remote sites where getting more bags quickly is difficult, use 15%. The cost difference between 10 and 15 percent is usually only one or two bags.
How do I convert cubic feet to cubic yards for a ready-mix order?
Divide cubic feet by 27 (there are 27 cubic feet in one cubic yard). The calculator shows both values automatically. When calling a ready-mix plant, give them the cubic yard figure and round up to the nearest quarter yard.