Sonotube Calculator
Deck and post footings in round cardboard forms are cylinder math: diameter, depth, and tube count give cubic feet, cubic yards, and bag counts. Our rectangular-slab concrete calculator covers flatwork — this one covers round piers. Runs 100% in your browser.
Sonotube Calculator
About this sonotube calculator
A sonotube footing is a cylinder, so the volume formula is π × radius² × depth, with radius in feet: a 12-inch tube is 1 foot across, so the radius is 0.5 feet, and a 4-foot-deep tube holds π × 0.25 × 4 = 3.14 cubic feet. Multiply by tube count, add waste for spillage at the top of the pour, and divide by the bag yield (0.60 cu ft for an 80 lb bag, 0.45 for 60 lb) for the bag count. Our sonotube guide covers frost depth, tube sizing, and the ready-mix break-even.
Tube diameter follows the post, not the deck: the rule of thumb is a tube at least 3 times the post width — a 6 × 6 post wants a 12-inch tube at minimum, and heavier beams or multi-story decks push toward 14–16 inches. Depth follows the frost line, which runs 42–60 inches in the northern US and 24–30 inches in the South, plus about 6 inches of gravel at the bottom; always confirm the exact frost depth with your local building department, because a footing above the frost line heaves.
Bag yields vary slightly by brand — check the yield printed on your bag — and fast-setting mixes (which set in 20–40 minutes) yield the same volume per bag as standard mixes. For more than about a cubic yard total, compare ready-mix: a truck minimum is usually 1 yard, and mixing 40+ bags by hand for a big deck is a long day.
How to use
- Enter the tube diameter in inches and the depth of concrete in feet (to frost depth, not to grade).
- Enter the number of tubes from your beam layout.
- Choose your bag size and a waste allowance (10% covers pier-pour spillage).
- Optionally enter the price per bag, then press Calculate.
Good to know
- Straight cylinders only — belled or spread footings (wider bases) need their extra volume added by hand.
- Does not size the footing for its load: tube diameter and depth must come from your plans or local code, not from this calculator.
- Bag yields are planning figures (0.60 / 0.45 / 0.375 / 0.30 cu ft); check the yield printed on your specific bag.
Frequently asked questions
How many bags of concrete do I need for one sonotube?
A 12-inch tube at 4 feet deep holds 3.14 cubic feet — 6 bags of 80 lb mix (5.24 rounded up) before waste, or 8 bags of 60 lb mix. With 10% waste, buy 7 bags of 80 lb.
What size sonotube do I need for a deck footing?
At least 3 times the post width: a 6 × 6 post wants a 12-inch tube minimum. Heavier beams or multi-story decks need 14–16 inches. Confirm with local code.
How deep should sonotube footings be?
Below the local frost line: 42–60 inches in the northern US, 24–30 inches in the South, plus about 6 inches of gravel at the bottom. Check your building department for the exact depth.
How many bags for 6 sonotubes at 12 inches by 3.5 feet?
Six tubes hold 16.49 cubic feet; with 5% waste that is 17.32 cubic feet — 39 bags of 60 lb mix (0.64 cubic yards).
When should I order ready-mix instead of bags?
Beyond about 1 cubic yard total, ready-mix is usually easier and cheaper per yard than mixing bags — especially for decks with many footings.