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ConcreteVerified Formulas

Concrete Footing Calculator

Calculate volume and bag counts for round sonotubes and rectangular post footings.

Concrete Footing Calculator Interactive Tool

Interactive Calculator

Enter your project dimensions below. Results update instantly.

Standard forms: 8", 10", 12", 16"
Must extend below local frost line
Total pier/post locations
typical value, change to match your product10% accounts for rough auger hole walls

Bag Yield Specifications

typical value, change to match your productStandard 80-lb premix yields 0.60 cu ft
typical value, change to match your productStandard 60-lb premix yields 0.45 cu ft
typical value, change to match your productStandard 40-lb premix yields 0.30 cu ft

Results

Base Total Volume

0.93 cu yd

25.1 cu ft for all 8 piers
Total Volume to Order

1.02 cu yd

With +10% buffer (27.6 cu ft)
80-lb Bags (Premix)

47

At 0.6 cu ft/bag
60-lb Bags (Premix)

62

At 0.45 cu ft/bag
40-lb Bags (Premix)

93

At 0.3 cu ft/bag
Single Pier Volume:Each footing is 3.14 cu ft (~6 80-lb bags per pier).

Footing Geometry Formulas

Round Pier: π × (Radius (ft))² × Depth (ft) | Rectangular: Length (ft) × Width (ft) × Depth (ft) | Total cu yd = (Volume × Qty ÷ 27) × (1 + Waste %)

Formula Variables:

  • Sonotube Diameters:Standard tube forms are 8", 10", 12", and 16" interior diameter.
  • Frost Depth:Building codes require footing bases to extend 6" to 12" below the local regional frost line.

Structural footing sizing depends on roof tributary loads, deck live loads, and soil bearing capacity (psf). Consult local building codes.

Step-by-Step Worked Example

Example: 8 round sonotube deck footings (12" diameter, 48" deep)

Pouring 8 cylindrical frost footings for an elevated deck in a cold climate with a 42" frost line.

Type:Round Sonotube
Diameter:12 inches (1 ft)
Depth:48 inches (4 ft)
Quantity:8
Waste:10%
  1. Radius: 12 in ÷ 2 = 6 in = 0.5 ft.
  2. Volume per pier: π × (0.5)² × 4 ft = 3.1416 × 0.25 × 4 = 3.142 cu ft.
  3. Total volume for 8 piers: 3.142 × 8 = 25.13 cu ft.
  4. Add 10% buffer: 25.13 × 1.10 = 27.65 cu ft.
  5. Convert to cubic yards: 27.65 ÷ 27 = 1.02 cu yd.
  6. Convert to 80-lb bags: 27.65 ÷ 0.60 = 46.1 → 47 bags.
Total project requires 1.02 cubic yards (or 47 eighty-pound bags of concrete premix).

Input Explanations & Measurement Guidelines

Footing Shape

Typical: Round or Rectangular

Round cardboard tube form (sonotube) or square spread footing excavation.

Diameter or Width

Typical: 10 to 16 inches

Width of the footing column.

Depth

Typical: 36 to 48 inches

Total depth from ground grade down to load-bearing soil.

Quantity of Footings

Typical: 4 to 12

Total number of identical post support piers.

Assumptions & Limitations

  • Assumes straight vertical cylindrical or rectangular excavations.
  • If using flared plastic footing bases (BigFoot forms), additional base bell volume must be accounted for.

Safety & Professional Notes

  • Inspect open footing trenches immediately after rain for sidewall collapse before pouring concrete. Never step into unsupported deep excavations.
Recommended Guide · 8 min read

How to Calculate Concrete Volume: Slabs, Footings & Bag Counts

Concrete cannot be returned once ordered or mixed. Underestimating volume leads to disastrous cold joints, while overestimating wastes hundreds of dollars. Learn how to calculate precise concrete volume.

Read Guide

Frequently Asked Questions

What diameter sonotube should I use for a deck?

For standard single-story residential decks, 10-inch or 12-inch diameter sonotubes are most common. 8-inch tubes leave little margin for anchor bolt placement.

How deep must deck footings be?

Footings must extend below your local frost line to prevent frost heave from lifting your deck in freezing weather. In northern states, frost depth is often 36" to 48"; in southern states, 12" to 18".

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