About this calculator
Round concrete columns — deck piers, porch posts, sign footings and pole-barn piers — are usually poured into cardboard form tubes such as Sonotube. This calculator finds the volume of each cylinder, multiplies by the number of columns and tells you how many bags of pre-mixed concrete to buy.
It reports bags per column as well as the total, which helps when you mix one tube at a time.
How to use it
- Enter the inside diameter of the form tube.
- Enter the height of concrete in each column — the full depth below grade plus any part above ground.
- Enter the number of columns.
- Choose a bag size. Keep 10% extra for spillage and flared hole bottoms.
The formula
Volume per column = π × (diameter ÷ 2)² × height
Total = volume per column × number of columns × (1 + extra%)
Bags = total (cu ft) ÷ bag yield, rounded up
A 12 in tube holds 0.785 cubic feet per foot of height, so a 4 ft pier needs about 3.14 cu ft — a little over five 80 lb bags.
Worked example
These are the calculator’s starting values and the results they produce:
| Column diameter | 12 in |
|---|---|
| Column height | 4 ft |
| Number of columns | 4 |
| Extra for waste | 10 % |
| Bag size | 80 lb bag (0.60 cu ft) |
Tips before you buy
- If you flare the bottom of the hole (a bell footing) or use a plastic footing base, add its volume separately — it can double the concrete in a short pier.
- Frost line depth sets the minimum pier depth in cold climates. Your local building department can tell you the required depth.
- Brace tubes plumb before pouring; wet concrete pushes them out of line.
- Set post anchors or J-bolts while the concrete is still wet.
Frequently asked questions
How many bags of concrete do I need for a 12 inch Sonotube?
A 12 in diameter tube holds about 0.785 cubic feet per foot of height. That is 1.3 bags of 80 lb mix per foot, so a 4 ft tube needs about 5.2 bags — buy 6.
How many bags for a 10 inch Sonotube?
A 10 in tube holds about 0.545 cubic feet per foot of height — roughly 0.9 bags of 80 lb mix per foot. A 4 ft pier takes about 3.6 bags, so buy 4.
What diameter should deck footings be?
Footing size depends on the load and your soil’s bearing capacity. Many deck footings are 10–16 in in diameter, but the correct size comes from your deck plans or local code tables, not from this calculator.