Introduction
Sonotubes are the cardboard cylinders you see sticking out of the ground on a deck or fence job. They hold wet concrete in a neat column while it cures, then stay in place as a permanent form. A 12-inch tube that’s 4 feet deep holds about 3.14 cubic feet of concrete. That’s just over half a cubic yard across a set of six.
The bag math is where this gets practical. An 80 lb bag of concrete mix yields roughly 0.6 cubic feet. So six tubes at 3.14 cubic feet each need about 19 bags before waste. Add 10% and you’re at 21 bags. That’s the number you load into the truck.
This tool handles the cylinder volume, the yield per bag, and the cost. It also accepts metric input, so you can work in centimeters or meters if that’s how your drawings are set. One thing to know: the concrete mix dropdown (3000 to 5000 PSI) is recorded in the output but doesn’t change the volume math. Mix strength affects the concrete’s structural capacity, not how much space it fills.
How to Use the Sonotube Calculator
Select your currency from the dropdown if you want the cost output in something other than US dollars.
Enter how many tubes you’re pouring.
Enter the tube diameter. Pick inches, centimeters, millimeters, or feet.
Enter the tube depth or height. Feet is the default, but meters, centimeters, and millimeters work too.
Choose a concrete mix from the dropdown. This is recorded in the breakdown but doesn’t affect volume.
Set the bag size. Eighty pounds is the standard for most retail concrete mix. Fifty and sixty pound bags are common for smaller pours.
Confirm the concrete density. The default is 145 lb/ft³, which fits standard weight concrete. Switch to kg/m³ if your supplier quotes that way.
Set the waste factor. Ten percent covers spillage and slight overfill at the top of each tube.
Enter the price per bag and the price per tube.
Add any labor or accessory cost as a flat total.
Hit Calculate. The Summary tab shows the headline numbers. The Detailed Breakdown tab lists every conversion, the yield per bag, and each cost component.
How the Sonotube Calculator Formula Works
The tube is a cylinder. Volume is the area of the circular base times the height.
Formula: Volume per Tube (ft³) = π × r² × h
Formula: Total Volume (ft³) = Volume per Tube × Number of Tubes
Formula: Volume with Waste (ft³) = Total Volume × (1 + Waste % ÷ 100)
Formula: Concrete Weight (lbs) = Volume with Waste × Density (lb/ft³)
Formula: Yield per Bag (ft³) = Bag Weight (lbs) ÷ 133
Formula: Bags Needed = ceil(Volume with Waste ÷ Yield per Bag)
Formula: Total Cost = (Bags × Price per Bag) + (Tubes × Price per Tube) + Additional
The radius is half the diameter. A 12-inch tube has a 6-inch radius, or 0.5 feet. Square that (0.25), multiply by π (0.7854), and multiply by the depth. For a 4-foot tube that’s 3.1416 cubic feet. Six of those gives 18.85 cubic feet.
The yield approximation uses 133 as the divisor. That comes from the standard 80 lb bag yielding about 0.6 cubic feet. Divide 80 by 0.6 and you get 133.3. The same ratio holds for 60 lb bags (0.45 ft³) and 40 lb bags (0.30 ft³). It’s not exact for every brand, but it’s close enough for ordering. If your mix yields differently, adjust the bag weight up or down until the bag count matches your supplier’s chart.
The concrete mix dropdown captures the PSI rating but doesn’t change any calculation. Higher PSI mixes use more cement and less aggregate, but they still fill the same volume. The density field is where you’d adjust for heavyweight or lightweight mixes. Standard concrete sits near 145 lb/ft³. Lightweight mix runs 90 to 120. Heavyweight can exceed 200.
Waste applies to the volume before the bag count, so the bag count already includes the overage. That’s the number you write on the shopping list. The cost then multiplies bags by price, tubes by price, and adds any flat additional cost.
Worked Example
You’re building a deck with six footings. Each footing uses a 10-inch Sonotube set 3 feet 6 inches deep. You’re using standard 80 lb bags at $6.75 each. Tubes run $10.50 apiece. Waste is 10%. No additional cost.
Step 1: Convert diameter and depth to feet.
Diameter: 10 ÷ 12 = 0.8333 ft.
Depth: 3.5 ft (3 feet 6 inches).
Step 2: Radius.
0.8333 ÷ 2 = 0.4167 ft.
Step 3: Volume per tube.
π × 0.4167² × 3.5 = 3.1416 × 0.1736 × 3.5 = 1.909 ft³.
Step 4: Total volume.
1.909 × 6 = 11.454 ft³.
In cubic yards: 11.454 ÷ 27 = 0.424 yd³.
Step 5: Apply 10% waste.
11.454 × 1.10 = 12.599 ft³.
In cubic yards: 0.467 yd³.
Step 6: Concrete weight.
12.599 × 145 = 1,826.9 lbs.
In kilograms: 1,826.9 × 0.453592 = 828.7 kg.
Step 7: Yield per bag.
80 ÷ 133 = 0.6015 ft³ per bag.
Step 8: Bags needed.
ceil(12.599 ÷ 0.6015) = ceil(20.95) = 21 bags.
Step 9: Concrete cost.
21 × $6.75 = $141.75.
Step 10: Tube cost.
6 × $10.50 = $63.00.
Step 11: Total cost.
$141.75 + $63.00 = $204.75.
The summary tab shows 1.909 ft³ per tube, 12.599 ft³ total with waste (0.467 yd³), 21 bags, 6 tubes, and $204.75 total cost. The breakdown lists every intermediate value including the radius, yield per bag, and each cost line.
Switch to 60 lb bags and the count climbs. A 60 lb bag yields 0.4511 ft³, so the same job needs 28 bags instead of 21. The concrete cost may or may not change depending on the price per bag. Smaller bags usually cost more per pound, so check the unit price before you buy.
