Concrete Tube / Sonotube Calculator

Frequently Asked Questions

How many bags of concrete for a 10-inch tube at 36 inches deep?

Volume = pi x (5)^2 x 36 / 1728 = 2,827 cu in / 1728 = 1.635 cu ft. With 60-lb bags: ceil(1.635 / 0.45) = 4 bags.

What diameter sonotube do I need for a deck post?

Most residential deck posts use 8-12 inch diameter tubes. Your local building department or a structural engineer will specify the required diameter based on the load.

How deep should sonotubes be for a deck?

Extend below the local frost line plus 6 inches. Common depths are 36 inches in mid-climates and 48-60 inches in northern regions. Check local building codes.

How much does a sonotube concrete footing weigh?

Concrete weighs about 150 lb/cu ft. A 10-inch tube at 36 inches deep holds 1.64 cu ft x 150 = 245 lbs of concrete.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Each sonotube is a cylinder. Volume = pi x (diameter/2)^2 x depth in cubic inches, divided by 1728 to convert to cubic feet. A 60-lb bag of concrete mix yields approximately 0.45 cubic feet; an 80-lb bag yields approximately 0.60 cubic feet. The calculator rounds up to whole bags per tube, then multiplies by the number of tubes.

Formula: volume_cuft = pi x (d_in/2)^2 x depth_in / 1728. Bags per tube = ceil(volume_cuft / bag_yield).

Common Sonotube Sizes

Sonotubes are available in standard diameters. Common sizes for residential projects:

6-inch: fence post footings, light pergola posts

8-inch: deck posts, gate posts

10-inch: larger deck posts, arbor columns

12-inch: structural deck posts for large decks or heavy loads

16-inch: garage and carport posts, heavy structures

Required diameter and depth are typically specified by a structural engineer or local building code. Many jurisdictions require footings to extend below the frost line - often 36-48 inches in northern climates.

Frost Line and Footing Depth

Footings must extend below the local frost depth to prevent frost heave. Approximate frost depths by region:

Southern US (Florida, Gulf Coast): 0-6 inches

Mid-Atlantic, Pacific Coast: 12-24 inches

Midwest, Great Plains: 36-48 inches

Northern states, Canada: 48-72 inches

Check your local building department for the required frost depth in your area. Always add 6 inches below the frost line for the footing base. Use the tube depth input to include the full footing depth from grade to the bottom of the tube.

FAQ

How many bags of concrete for a 12-inch tube at 48 inches deep?

Volume = pi x (6)^2 x 48 / 1728 = 5,428 cu in / 1728 = 3.14 cu ft. With 80-lb bags (0.60 cu ft each): ceil(3.14 / 0.60) = 6 bags per tube.

How deep should deck footings be?

Footings must extend below the local frost line. Add 6 inches below frost depth for the footing pad. A minimum 12-inch diameter is common for deck posts; your building permit will specify the required size and depth.

Do I need rebar in sonotube footings?

For most residential deck footings, rebar is optional. For structural posts or where specified by an engineer, use 1-2 pieces of #4 rebar (1/2 inch diameter) centered in the tube, extending 12 inches above grade to anchor into the post base hardware.

Can I mix concrete by hand for sonotubes?

Yes. Up to about 2 cubic feet (4-5 bags of 60-lb mix) is manageable by hand. Above 4-5 bags per footing, rent a portable electric mixer. For many tubes, order a partial yard of ready-mix delivered with a pump truck for efficiency and consistency.

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