What is a Sonotube?
A Sonotube is a brand name for cardboard concrete forming tubes used to create cylindrical concrete piers and footings. Made from spirally wound layers of fiber, these tubes are strong enough to hold wet concrete in shape while it cures. They are widely used for deck footings, fence posts, sign posts, and structural pier foundations. After the concrete sets, the cardboard tube is typically stripped away or left to decompose naturally underground.
Sonotubes and similar concrete form tubes come in diameters ranging from 6 inches to 60 inches and lengths up to 12 feet. They can be cut to any length with a standard handsaw. For below-grade applications, the tube is placed in a drilled or excavated hole, leveled, and then filled with concrete. A post anchor or J-bolt is typically set into the wet concrete at the top.
Volume Formula
This gives volume in cubic inches. Divide by 1,728 to get cubic feet, or by 46,656 to get cubic yards.
Tube Size Chart
| Diameter | Cu Ft per Foot | Bags (80 lb) per 4 ft | Typical Use |
|---|---|---|---|
| 6" | 0.196 | 2 | Light posts, mailboxes |
| 8" | 0.349 | 3 | Fence posts, small decks |
| 10" | 0.545 | 4 | Standard deck piers |
| 12" | 0.785 | 6 | Large deck piers |
| 16" | 1.396 | 10 | Heavy structural piers |
| 24" | 3.142 | 21 | Bridge piers, heavy loads |
Installation Tips
- Dig below frost line: The bottom of the pier must be below the local frost depth to prevent frost heaving.
- Flare the base: For increased bearing capacity, dig the hole wider at the bottom and pour a 6-inch pad before placing the tube.
- Level and brace: Use cross bracing to keep the tube plumb during the pour. Check with a level from two perpendicular directions.
- Vibrate the concrete: Tap the outside of the tube with a hammer or use a piece of rebar to rod the concrete, eliminating air pockets.
Worked Example
4 tubes, 12-inch diameter, 48 inches deep:
Total = 4 × 3.14 = 12.57 ft³ = 0.47 yd³
80-lb bags = 21
Frequently Asked Questions
Should I leave the Sonotube in the ground?
For below-grade applications, you can leave the tube in the ground. It will eventually decompose. For above-grade portions, strip the tube after the concrete has cured (24-48 hours) to expose the finished concrete surface. Peel the tube or use a utility knife to cut it away.
How do I prevent the tube from floating?
When backfilling around the tube before pouring, tamp soil firmly against the tube. You can also drive stakes beside the tube and screw them to the tube wall. Some builders pour a few inches of concrete first, let it partially set, then fill the rest to anchor the tube.
Can I use a Sonotube above ground?
Yes, Sonotubes can be used as above-ground forms for columns and pillars. Brace them securely and pour in lifts (12-18 inches at a time) to prevent blowout. After curing, strip the cardboard to reveal the smooth concrete cylinder.
What size Sonotube for a 6x6 post?
A 12-inch diameter Sonotube is the minimum recommended size for a 6x6 post, providing about 3 inches of concrete on each side of the post. For better load distribution and stability, a 14 or 16-inch tube is preferable, especially for tall posts or heavy loads.