How much concrete do I need for a sonotube?

Multiply pi by the radius squared by the depth to get cubic feet: V = 3.14159 x (D/2 / 12)² x depth in feet. For a 12-inch tube 48 inches (4 feet) deep: 3.14159 x (0.5)² x 4 = 3.14 cubic feet. Add 10 percent for the bottom: 3.45 cubic feet. At 0.60 cubic feet per 80 lb bag, you need 6 bags. Our calculator does this automatically.

What size sonotube do I need for a deck?

Deck pier diameter is determined by the load the pier carries and the soil bearing capacity. For most residential decks, 10 to 12-inch tubes are standard. Heavier decks, hot tubs, or stairs landing on the deck may require 12 to 16-inch piers. The span of the beam the pier supports and the tributary deck area it carries determine the actual required size. Consult the local deck code or a structural engineer for specific sizing.

How deep should a sonotube be for a deck?

The bottom of the pier must sit below the frost depth to prevent frost heave lifting the deck. Frost depths range from 0 inches in the deep south to 60 inches in northern states and Canada. Add 3 to 6 inches of gravel drainage at the bottom of each hole, then set the tube to the remaining depth. The top of the tube typically extends 6 to 12 inches above grade to keep the post above moisture and snow.

Can you leave sonotube forms in the ground?

Standard cardboard sonotube forms can be left in the ground, as they will eventually decompose. However, the cardboard portion below grade should ideally be stripped after the concrete cures to prevent it from retaining moisture against the cured concrete surface. Waxed-exterior sonotubes and fibre-reinforced tubes are designed to be left in place. Above grade, the tube must be stripped for a clean appearance.

How long does concrete take to cure in a sonotube?

Concrete in tube forms reaches approximately 70 percent of its design strength at 7 days and full strength at 28 days under normal curing conditions. For deck construction, the standard wait before loading the deck frame onto piers is 7 days minimum, and 28 days is preferred before full deck loading. In cold weather, curing time increases and the concrete must be protected from freezing for the first 24 to 48 hours.

How do you anchor a post to a sonotube?

Post bases and standoff connectors are the standard method. A J-bolt, post base anchor, or threaded rod is embedded in the wet concrete at the correct position and plumb before it sets. Alternatively, a saddle-type post base can be drilled and anchored into the cured concrete. Embedding the timber post directly in the concrete is not recommended as it traps moisture and accelerates rot.

How many 80 lb bags to fill a 10-inch sonotube 4 feet deep?

A 10-inch diameter sonotube 4 feet deep has a volume of 3.14159 x (0.417)² x 4 = 2.18 cubic feet. Adding 10 percent for waste gives 2.40 cubic feet. At 0.60 cubic feet per 80 lb bag, you need exactly 4 bags. Four bags of 80 lb Quikrete or Sakrete provides just enough material with a small margin.

Should concrete piers have rebar?

Rebar in concrete piers is required by most structural codes for deck piers, particularly in seismic zones and where lateral loads are significant. A single central rebar of No. 4 or No. 5 (1/2 or 5/8 inch diameter) run the full depth of the pier and extending into the post base above is the standard for residential deck piers. Check local code requirements before omitting rebar.

What is the bearing capacity of a concrete pier?

The bearing capacity of a concrete pier depends on the soil bearing capacity, the pier cross-section area, and the concrete compressive strength. For standard residential soil with a bearing capacity of 2,000 psf (the conservative default assumption), a 12-inch round pier has a bearing area of 0.785 square feet, giving a bearing capacity of approximately 1,570 pounds. A 16-inch pier has over 2,700 pounds. Actual capacity requires geotechnical investigation.

Can I pour concrete in a wet tube hole?

Pouring concrete into standing water significantly reduces the concrete's compressive strength by diluting the mix at the base. Remove standing water by bailing or pumping before pouring. If the hole continuously fills with groundwater, use a tremie tube to place concrete from the bottom up, which displaces the water rather than mixing with it. In high-groundwater conditions, consult a contractor about dewatering methods.

Concrete Tube Calculator – Sonotube Volume Estimator

What Is the Concrete Tube Calculator?

The Concrete Tube Calculator estimates the volume of concrete needed to fill a cylindrical tube form, commonly called a sonotube or tube form, used as the formwork for deck piers, fence post footings, and structural column bases. Deck builders, fence contractors, and homeowners use it to figure out how many bags of concrete to buy before digging, avoiding multiple trips to the hardware store mid-project. According to the American Wood Council's deck construction guide, properly sized and adequately deep concrete piers are the foundation of a safe, durable deck, making accurate pier volume calculation a critical first step in any deck project.

The sonotube is a round cardboard or fibreglass tube form that holds concrete in a cylindrical shape while it cures. Standard sizes range from 6 to 36 inches in diameter, with 10, 12, and 16-inch diameters being the most common for residential construction. The key calculation is the cylindrical volume formula, which requires the inner diameter rather than the outer tube dimension, as the cardboard wall has a meaningful thickness that reduces the interior volume by a small but measurable amount for smaller tube sizes.

Frost Depth: The Critical Dimension for Pier Length

The most structurally important dimension in a tube pier calculation is the total depth, which must place the pier base below the local frost line. Frost heave occurs when water in the soil freezes and expands, pushing upward with considerable force against anything in the frozen layer. A pier that does not extend below the frost line can be heaved upward by 1 to 3 inches over a single freeze-thaw cycle, racking the deck frame and damaging connections. As a result, determining the frost depth for the specific project location before calculating the tube depth is the first step, not an afterthought.

Frost depths in the United States range from zero in Florida and coastal California to 60 inches in Minnesota and Maine. The International Residential Code requires piers to bear on soil below the frost depth or on bedrock. Given that the required tube length increases significantly in cold climates, two identical-diameter piers can require very different volumes depending on location. On top of that, the bottom 3 to 6 inches of the hole is typically filled with compacted gravel for drainage, further increasing the tube length needed to reach below the frost line.

Concrete Volume by Tube Size and Depth

The table below shows the concrete volume per tube and equivalent 80 lb bag count for common residential pier sizes at standard frost-depth lengths. These figures include a 10 percent waste allowance for the irregular hole bottom. All data is consistent with NIST construction standards.

Tube Diameter	3-foot depth	4-foot depth	5-foot depth	80 lb Bags (5 ft)
8 inch	1.05 ft³	1.40 ft³	1.75 ft³	3 bags
10 inch	1.64 ft³	2.18 ft³	2.73 ft³	5 bags
12 inch	2.36 ft³	3.14 ft³	3.93 ft³	7 bags
16 inch	4.19 ft³	5.59 ft³	6.98 ft³	13 bags

Worked Example: Estimating Bags for a Deck Footing Project

The following example uses QUIKRETE's standard bag yield figures, which are the most widely used reference for DIY footing projects. A contractor is building a 12×16 ft deck requiring 8 footings. Local frost depth is 36 in; footings must extend 6 in below frost, so total pier depth = 42 in (3.5 ft). Pier diameter = 10 in (QUIKRETE Tube Form).

Step 1 : Volume per pier: V = π × (0.417 ft)² × 3.5 ft = π × 0.1736 × 3.5 = 1.909 ft³

Step 2 : Total 8 piers: 8 × 1.909 = 15.27 ft³

Step 3 : Convert to bags:

50 lb bag = 0.375 ft³ → 15.27 / 0.375 = 40.7 → 41 bags
60 lb bag = 0.45 ft³ → 15.27 / 0.45 = 33.9 → 34 bags
80 lb bag = 0.60 ft³ → 15.27 / 0.60 = 25.4 → 26 bags

Add 10% waste: Recommend purchasing 29 × 80 lb bags (1 extra pallet partial). At $7–8 per bag ≈ $200–230 in concrete.

Tube Form Size vs Bags per Linear Foot

Quick reference for QUIKRETE or Sakrete tube form sizes. Add 10% for waste.

Tube Diameter	ft³ per foot depth	50 lb bags/ft	60 lb bags/ft	80 lb bags/ft
6 in (150 mm)	0.196	0.52	0.44	0.33
8 in (200 mm)	0.349	0.93	0.78	0.58
10 in (250 mm)	0.545	1.45	1.21	0.91
12 in (300 mm)	0.785	2.09	1.74	1.31
14 in (350 mm)	1.069	2.85	2.38	1.78
16 in (400 mm)	1.396	3.72	3.10	2.33

Frost Depth by U.S. Climate Zone

Footings must extend below the frost depth to prevent frost heave from shifting the structure. The required depth varies significantly by region. Local building departments always have the final word, use this table as a planning guide only.

Region	Representative Cities	Typical Frost Depth
Southern states / mild climates	Miami, Houston, Phoenix, Los Angeles	0–12 in (0–300 mm)
Transition zone	Charlotte, Nashville, Dallas, Atlanta	12–24 in (300–600 mm)
Northern states	Chicago, Denver, Kansas City, Seattle	24–42 in (600–1,070 mm)
Northern tier / Great Lakes	Minneapolis, Detroit, Buffalo, Boston	42–60 in (1,070–1,520 mm)
Far north / mountain regions	Anchorage, Duluth, Fargo	60–100 in (1,520–2,540 mm)

Per the International Residential Code (IRC), footings must be placed below the frost depth to prevent frost heave. In Canada, frost depths range from 18 in (45 cm) in southern British Columbia to 72 in (183 cm) in the Prairie provinces. Always check the National Building Code of Canada or local authority for specific requirements.

Accuracy and Limitations

The calculator uses the pure cylindrical volume formula applied to the inner tube diameter. It does not account for the belled or flared bottom that some piers have when drilled with an underreamer, which can add 15 to 30 percent to the total volume. For straight-sided auger holes filled with a tube, the 10 percent waste allowance captures the bottom irregularity adequately. For hand-dug holes with irregular sides, increase the waste factor to 15 percent.

The calculator also does not include the volume of any above-grade exposed pier that extends above the ground surface. For an exposed pier finished above grade to serve as a decorative base or to elevate a post above snow and moisture, include the above-grade height in the total tube length entered. The concrete does not know whether it is below or above the ground surface, the total tube fill length is what determines the total volume. The American Concrete Institute (ACI) sets the professional standard for mix design tolerances and site testing protocols that apply when estimates will be used for structural or load-bearing applications. For above-grade column forms with non-standard shapes, our concrete column calculator handles structural piers and columns of any cylindrical dimension.

The Most Common Tube Calculation Mistake

The most common error is using the tube's outside diameter rather than the inside diameter for the volume calculation. A 12-inch nominal sonotube has a wall thickness of approximately 3/8 inch on each side, giving an inner diameter of approximately 11.25 inches rather than 12 inches. For a 4-foot deep pier, this difference reduces the actual volume by about 12 percent compared to using the nominal outer diameter. For a project with 10 piers, this error wastes approximately half a bag of concrete per pier, adding unnecessary cost. With that in mind, always check the product specification sheet for the inner bore diameter before calculating, particularly for heavy-duty fibre-reinforced tube forms where wall thickness is greater than standard cardboard tubes. This discrepancy turns up most often when ordering from a specification that lists the tube by its nominal outside dimension before anyone looks into the actual inner bore measurement. The National Ready Mixed Concrete Association (NRMCA) identifies material under-estimation as the leading cause of mid-project concrete shortages, recommending a 5-10% waste factor buffer on all project orders. If the tube sits inside a drilled excavation, use our hole volume calculator to account for the annular void around the tube before finalising the concrete order.

Concrete Estimator – Tube

Sonotube Concrete Estimator

Pre-Mix Bags Required

Density Constants

Standard Bag Yields

Related Expert Tools

Concrete Block Calculator

Cement Calculator

Concrete Calculator

Concrete Estimator – Tube Logic