Sonotubes (also known as concrete form tubes) are cylindrical cardboard forms used to create round concrete columns and footings for decks, porches, sign posts, and light poles. Calculating the exact amount of concrete needed to fill sonotubes prevents waste and ensures you have enough material for a continuous pour. Our free sonotube calculator makes this process quick and accurate.

Simply enter the tube diameter, height, and the number of tubes you need to fill, and our calculator instantly provides the total volume in cubic yards, cubic feet, and the number of 60-pound and 80-pound concrete bags required.

What Are Sonotubes?

Sonotubes are cylindrical cardboard forms manufactured by Sonoco and other companies. They are made from multiple layers of quality fiberboard, wound to create strong, moisture-resistant forms that hold their shape during pouring. After the concrete cures, the tubes can be left in place to deteriorate naturally over time, or cut away if desired.

Standard Sonotube Diameters

• 6-inch diameter: Small posts, mailbox posts, fence line posts
• 8-inch diameter: Standard 4x4 post footings, small deck piers
• 10-inch diameter: 6x6 post footings, medium deck piers
• 12-inch diameter: Large deck footings, porch columns, light pole bases
• 14-inch diameter: Structural columns, heavy deck footings
• 16-inch diameter: Porch supports, sign posts
• 18-inch diameter: Structural columns, medium building supports
• 24-inch diameter: Large structural columns, heavy load applications

Common Applications

Deck footings:

• 8 to 12-inch diameter typical
• Depth below frost line (varies by region)
• 4x4 posts use 8-10 inch tubes
• 6x6 posts use 10-12 inch tubes

Porch and column supports:

• 12 to 18-inch diameter common
• Height varies by application
• May require reinforcement

Light pole and sign bases:

• 12 to 24-inch diameter
• 3-6 feet deep typical
• Often reinforced with rebar

The Formula

The volume of a cylindrical sonotube is calculated using the formula for a cylinder:

Volume = pi x radius squared x height

Where:

• pi = 3.14159
• radius = diameter divided by 2
• height = depth of the tube

Converting Units

To convert to cubic yards, divide cubic feet by 27:

Cubic yards = Cubic feet / 27

Example Calculations

Single 12-inch diameter, 4-foot deep sonotube:

• Radius: 6 inches = 0.5 feet
• Volume: 3.14159 x 0.5 x 0.5 x 4 = 3.14 cubic feet
• Cubic yards: 0.12 cubic yards
• 80lb bags: 6 bags
• 60lb bags: 7 bags

Eight 10-inch diameter, 3-foot deep sonotubes:

• Radius: 5 inches = 0.417 feet
• Per tube: 3.14159 x 0.417 x 0.417 x 3 = 1.64 cubic feet
• Total: 1.64 x 8 = 13.12 cubic feet
• Cubic yards: 0.49 cubic yards
• 80lb bags: 22 bags (with 10% waste: 25 bags)

Four 18-inch diameter, 5-foot deep sonotubes:

• Radius: 9 inches = 0.75 feet
• Per tube: 3.14159 x 0.75 x 0.75 x 5 = 8.84 cubic feet
• Total: 8.84 x 4 = 35.34 cubic feet
• Cubic yards: 1.31 cubic yards
• 80lb bags: 59 bags (with 10% waste: 65 bags)

Site Preparation

Before placing sonotubes:

• Dig holes to required depth (below frost line)
• Add 4-6 inches of compacted gravel at the bottom for drainage
• Ensure holes are wide enough for the tube diameter plus working room
• Remove any loose soil or debris

Setting and Bracing Sonotubes

Proper installation steps:

• Cut tube to required height using a utility knife or handsaw
• Place tube in hole on compacted gravel base
• Brace tube at top and bottom to prevent shifting
• Use wooden stakes and cross-bracing for taller tubes
• Verify tube is plumb (perfectly vertical) using a level
• Backfill around outside of tube with soil for additional support
• Ensure top of tube is at desired height

Bracing Tips

Critical considerations:

• Brace every tube independently
• Use 2x4 stakes driven into ground
• Cross-brace at two points (top and mid-height for tubes over 3 feet)
• Check plumb in two directions (front-to-back and side-to-side)
• Wet concrete exerts significant outward pressure

Rebar in Sonotubes

When to add rebar:

• Structural columns supporting significant loads
• Deck footings for elevated decks
• Posts in high-wind or seismic areas
• Local building code requirements

Typical reinforcement:

• 3-4 vertical #4 rebar bars for 10-12 inch tubes
• 4-6 vertical #4 or #5 bars for 14-18 inch tubes
• Rebar cage with horizontal ties for 18+ inch tubes
• Extend rebar above tube for column connection

Post Anchors

For connecting wood posts:

• Set metal post anchor in wet concrete at top of tube
• Use J-bolts or embedded post bases
• Verify alignment before concrete sets
• Provides elevated connection point to prevent wood rot

Pouring Technique

Best practices:

• Mix or order concrete before starting
• Pour from top in a steady, continuous stream
• Avoid dropping concrete more than 4 feet (causes segregation)
• Fill slightly above top of tube for finishing
• For deep tubes, pour in lifts of 2-3 feet and vibrate

Consolidation

Vibration requirements:

• Use a small diameter vibrator (1-1.5 inch)
• Insert at center of tube
• Withdraw slowly (2-3 inches per second)
• Vibrate until air bubbles stop rising
• Over-vibration causes aggregate separation

Alternative for small tubes:

• Rod with a piece of rebar
• Tap sides of tube with a hammer
• Less effective than vibration but acceptable for small footings

Concrete Mix Recommendations

For sonotube applications:

• Minimum 3000 psi concrete
• 3500-4000 psi for structural columns
• Standard slump: 4-5 inches
• Avoid very wet mixes (weakens concrete)
• Consider air-entrained concrete for freeze-thaw resistance

Recommended Waste Percentages

• Single tube: 10%
• Multiple tubes (same size): 10%
• Multiple tubes (various sizes): 10-15%
• Rocky or irregular soil conditions: 15%

Sources of Waste

• Spillage during pouring
• Tube not perfectly plumb (one side fills higher)
• Overfill for leveling and finishing
• Variations in hole depth
• Concrete left in mixing container

How many bags of concrete do I need for a 12-inch sonotube, 4 feet deep?

• Volume per tube: 3.14 cubic feet
• 80lb bags: 6 bags per tube
• 60lb bags: 7 bags per tube
• With 10% waste: 7 bags (80lb) or 8 bags (60lb)

Should I remove the sonotube after the concrete cures?

• Sonotubes are designed to be left in place
• They protect concrete during early curing
• They will naturally deteriorate over time when exposed
• If appearance matters, you can peel them away after 3-5 days
• Removing too early can damage the concrete

Do I need rebar in sonotube footings?

• Non-structural footings (small decks, fences): Often not required
• Structural columns and load-bearing footings: Yes, use rebar
• Elevated decks: Check local building code requirements
• High-wind areas: Reinforcement recommended
• Adding rebar is inexpensive insurance for any application

How deep should sonotubes be set?

• Below local frost line (varies from 12 inches in southern states to 60+ inches in northern states)
• Minimum 24 inches for most applications
• 30-36 inches for deck footings in moderate climates
• Check local building codes for specific depth requirements
• Deeper is always better for stability

Can I pour sonotubes in cold weather?

Yes, with proper precautions:

• Store tubes and concrete materials above 40 degrees F before use
• Use heated water for mixing if below 40 degrees F
• Cover poured tubes with insulating blankets
• Protect from freezing for at least 48 hours
• Consider using concrete with accelerating admixtures
• Extend curing time before applying loads

Accurate concrete calculation ensures you have enough material to fill all your sonotubes in one continuous pour. Use our calculator to estimate your needs, add 10% for waste, and order your materials with confidence.

Remember: Proper bracing is the most critical step in sonotube installation. A poorly braced tube can shift or bulge during pouring, resulting in a crooked or misshapen column. Take the time to brace each tube securely before adding concrete.