Sonotube Concrete Calculator: How Many Bags Do You Need?
Sonotubes make round concrete piers straightforward, but estimating bags is where most deck and pergola projects go sideways. Buy too few and you are back at the store mid-pour with a half-filled tube setting up. Buy too many and you are hauling unopened bags back. This guide gives you per-post volumes and bag counts for every common tube size, so you can load the truck once.
The volume formula for a round pier
A Sonotube is a cylinder, so the volume is the circular area times the depth. Work in feet throughout to keep the units clean:
Bag yields are the other half of the equation. An 80-pound bag of standard concrete mix yields approximately 0.60 cubic feet, a 60-pound bag about 0.45 cubic feet, and a 50-pound bag roughly 0.375 cubic feet. Fast-setting post mix yields slightly less per bag, so check the label.
Volume per foot of depth by tube size
Because depth is a simple multiplier, the most useful number is cubic feet per linear foot of tube. Multiply by your actual depth and you have the answer.
| Tube Diameter | Cu Ft per Foot of Depth | 80 lb Bags per Foot | 4 ft Pier (80 lb bags) | 4 ft Pier (60 lb bags) |
|---|---|---|---|---|
| 8 inch | 0.35 | 0.58 | 3 bags | 4 bags |
| 10 inch | 0.55 | 0.91 | 4 bags | 5 bags |
| 12 inch | 0.79 | 1.31 | 6 bags | 7 bags |
| 16 inch | 1.40 | 2.33 | 10 bags | 13 bags |
| 18 inch | 1.77 | 2.95 | 12 bags | 16 bags |
Bag counts are rounded up, because you cannot buy a partial bag and a short pier is not something you can fix later.
Worked example: six 12-inch piers at 4 feet deep
- Radius: 12 / 24 = 0.5 feet
- Volume per pier: 3.1416 x 0.5 x 0.5 x 4 = 3.14 cubic feet
- Bags per pier at 0.60 cu ft yield: 3.14 / 0.60 = 5.24, round up to 6 bags
- Six piers: 6 x 6 = 36 bags of 80 lb mix
- Total volume: 18.8 cubic feet, or about 0.70 cubic yards
At roughly $6.50 to $9.00 per 80-pound bag in 2026, that is $235 to $325 in material. Note that the total is well under one cubic yard, which is exactly why bagged mix wins here — a ready-mix truck would trigger short-load fees that exceed the material cost.
Bags vs. ready-mix for pier projects
The crossover point is around 1 cubic yard, or about 45 bags of 80-pound mix. Below that, bags are cheaper and far more flexible since you mix only what you need per hole. Above it, the labor of mixing 50-plus bags by hand becomes the limiting factor and ready-mix pays for itself.
A practical middle ground for 10 to 20 piers is renting a towable mixer for $60 to $100 per day. You still buy bags, but one person can keep a steady supply moving while another places and screeds.
Depth, frost line, and footing flares
Pier depth is usually set by the local frost line, which is typically 12 to 24 inches in the South, 36 to 42 inches across the Midwest and Northeast, and up to 48 inches or more in northern-tier states. Always confirm the required depth with your building department, since it also drives whether an inspection is needed before the pour.
If your design calls for a flared or belled base — a wider footing pad below the tube — calculate that separately and add it to the cylinder volume. A 24-inch diameter by 8-inch deep pad adds roughly 2.1 cubic feet, or about 4 extra bags per pier. That is a common oversight and the usual reason a bag count comes up short.
Practical pour tips
- Brace tubes plumb before filling; wet concrete will push an unbraced tube out of alignment.
- Fill in 12 to 18 inch lifts and rod each lift to release air pockets.
- Set anchor bolts or post bases while the concrete is still plastic, then check plumb again.
- Keep 2 to 3 spare bags on hand — the cost of returning them is far lower than the cost of a cold joint.
Piers rarely stand alone. If your project includes a block stem wall or masonry veneer, size those materials with the Block Calculator and the Brick Calculator, or browse every tool in our concrete and masonry collection.