Sonotube Calculator

Free sonotube calculator. Enter number of tubes, diameter and depth, get cubic yards plus 40, 60, 80 lb bag counts. Subtracts tube volume automatically. No sign-up.

A sonotube calculator figures how much concrete you need to fill a cylindrical sonotube. Sonotubes are heavy cardboard tubes used as forms for poured concrete piers, columns, and posts. They're the standard for deck footings, sign posts, and structural piers where you need a clean round column.

The calculator subtracts the sonotube's own volume (which is so small it's usually negligible) — the bigger saving is that sonotube pours don't need forming, so the labor is just the pour.

What size sonotube?

Sonotubes come in standard diameters from 6″ to 36″. The right size depends on the load:

| Use | Typical diameter | Typical depth | Concrete per tube | |---|---|---|---| | Deck footing (small) | 8″ | 36″ | 0.87 yd³ | | Deck footing (large / multi-story) | 12″ | 48″ | 2.33 yd³ | | Pergola post | 12″ | 36–48″ | 1.75–2.33 yd³ | | Sign post | 12″ | 48–60″ | 2.33–2.91 yd³ | | Structural pier | 18–24″ | 60–96″ | 6.5–17.4 yd³ | | Light pole base | 24–30″ | 72–96″ | 11.6–29.0 yd³ |

For most residential deck footings: a 12″ sonotube 48″ deep holds 2.33 yd³ of concrete, which is 105 × 80 lb bags or roughly 2 concrete trucks' worth of ready-mix.

How deep should a sonotube pier be?

In cold climates, sonotubes go below the frost line (typically 30–48″ in northern US) so frost heave doesn't push the pier up. In mild climates, the depth depends on the load — generally 36–48″ is enough for residential decks and pergolas. Local code often specifies; check before you dig.

Sonotube installation

1. Dig the hole to the right diameter (a few inches wider than the tube) and depth
2. Set the sonotube in the hole — it should extend a few inches above grade to keep water from pooling around the post
3. Brace the tube so it doesn't shift during the pour (the wet concrete is heavy and will push the tube out of plumb)
4. Drop in the rebar cage if the pier is structural (most code-required footings need vertical rebar tied into a cage)
5. Pour the concrete — sonotubes are usually filled with 4,000 psi ready-mix
6. Vibrate the tube (with a stick or vibrator) to consolidate and remove air pockets
7. Strip the tube after 24–48 hours (the cardboard peels off easily)

Sonotube vs big footing

For small deck footings, sonotubes are fast and clean. For larger structural work, a formed square footing is often cheaper (less concrete, same bearing area). A 16″ sonotube has 201 in² of bearing area; a 16″ square footing has 256 in² — same depth, 27% more bearing capacity, 27% more concrete.

Common mistakes

• Not bracing the tube: a sonotube full of wet concrete is heavy and will lean if not braced. The result is a plumb-but-not-vertical post.
• Pouring above the tube top: concrete gets on the outside of the tube and you can't strip it cleanly.
• No rebar in structural piers: most codes require #4 rebar in a 12″ sonotube and #5 in larger piers. Check local code.
• Pouring in freezing weather: concrete needs to stay above 50°F for the first 24 hours to cure properly. In cold weather, use blankets or heated enclosures.