Sonotube Concrete Calculator

What Is a Sonotube?

A sonotube — also called a tube form or column form — is a cylindrical cardboard form used to cast round concrete piers and columns. You set the tube in a hole, fill it with concrete, and once the concrete hardens you peel the cardboard away. The tube's job is to hold the wet concrete in a clean cylinder, which matters most in soft or sandy soil where an open hole would cave in. Tube forms are the standard way to pour deck footings, porch and pergola posts, mailbox bases, and other light structural columns.

This calculator estimates the concrete volume for a tube form or round pier — enter the diameter and depth to get cubic yards, cubic feet, and bag count for every standard size from 6 to 24 inches. For the footing base that goes under each tube, use the deck footing concrete calculator; for a round column poured without a cardboard form, try the concrete cylinder calculator.

What Size Sonotube for a Deck?

Diameter is the decision that drives everything else, because volume rises with the square of the radius. Pick the size from the load on the post and the bearing capacity of the soil under it:

• 8-inch diameter — light loads only: pergola posts, porch roof posts, mailbox bases, garden structures.
• 10-inch diameter — standard for a single post on a small to medium deck (roughly 200–400 sq ft).
• 12-inch diameter — the most common deck footing size; handles a post on a medium to large deck.
• 14–16-inch diameter — heavily loaded posts, multi-level decks, hot tub platforms, or soft soil with low bearing capacity.
• 18–24-inch diameter — large columns and commercial work, or residential piers in expansive clay needing more bearing area.

When in doubt, size up — a larger tube costs only a few extra bags but adds a real safety margin. Confirm footing size with your building department for multi-level decks or unusual loads.

How to Calculate Concrete for a Sonotube

A sonotube is a cylinder, so use the cylinder volume formula: Volume (yd³) = π × (Diameter ÷ 2)² × Height ÷ 27. All measurements must be in feet.

Example: 12-Inch Sonotube, 3 Feet Deep

1. Diameter = 12 inches = 1.0 ft; radius = 0.5 ft
2. Volume = π × 0.5² × 3 = 3.14 × 0.25 × 3 = 2.36 ft³
3. Convert to cubic yards: 2.36 ÷ 27 = 0.087 yd³
4. Add 10% waste: 0.087 × 1.10 = 0.096 yd³ — about 6 bags of 60 lb concrete

Common Sonotube Sizes and Concrete Volumes

Here are ready-to-use estimates for the most common sonotube sizes at typical deck footing depths:

• 6 in × 3 ft deep — 0.022 yd³ / 0.59 ft³ / ~2 bags (40 lb)
• 10 in × 3 ft deep — 0.061 yd³ / 1.64 ft³ / ~4 bags (60 lb)
• 12 in × 3 ft deep — 0.087 yd³ / 2.36 ft³ / ~6 bags (60 lb)
• 12 in × 4 ft deep — 0.116 yd³ / 3.14 ft³ / ~7 bags (60 lb)
• 16 in × 4 ft deep — 0.207 yd³ / 5.59 ft³ / ~13 bags (60 lb)
• 18 in × 4 ft deep — 0.262 yd³ / 7.07 ft³ / ~16 bags (60 lb)

Pairing a Sonotube With a Footing Base

A tube alone bears the post load on a small circle of soil, which can settle over time. Most decks need a wider footing base under the tube to spread that load:

• Poured spread footing — pour a flat, wider pad at the bottom of the hole first, then stand the tube on it. The pad spreads the load over more soil; the tube forms the column above. Common in firm soil and required by many inspectors.
• Manufactured footing form — a plastic, flared base that snaps onto the bottom of the tube, casting the footing and column in one continuous pour. Fast and consistent, and it sizes the bearing area for you.
• Bell-bottom pier — flare the bottom of the excavated hole into a wider bell, then set the tube above it. Adds bearing without a separate form, but needs firm soil that holds the flared shape.

Size the base to the post load and soil — when in doubt, run the numbers with the deck footing concrete calculator. Light pergola posts or floating deck-block setups in firm soil are the main cases where a bare tube is acceptable.

How to Brace, Plumb, and Finish a Sonotube

Most sonotube problems come from a tube that drifted out of plumb or floated up during the pour. Get the setup right before any concrete goes in:

• Set below the frost line — the bottom of the tube must reach below the local frost depth (about 36–48 inches in the North) so the pier cannot heave in winter and lift the deck.
• Extend 2–4 inches above grade — leave the top of the tube above finished grade so the post base sits above ground and stays dry, and so you can finish the exposed concrete cleanly.
• Brace and plumb — stake 2×4 braces to the tube on two sides and check it with a level on adjacent faces; backfill and tamp soil around the tube so it cannot shift or float as you fill it.
• Finish the visible portion — trowel the top smooth, then set an adjustable post base into the wet concrete rather than embedding a wood post directly; an above-grade base lets the wood breathe and lasts far longer.
• Strip the form on time — peel the cardboard once the concrete holds its shape: 24–48 hours for standard mix, as little as 4 hours for fast-setting. Stripping too early can slump or scar the exposed pier.

For a post set in a bare hole instead of a formed deck pier, see the post hole calculator.