Deck Footing Calculator Formula
Understand the math behind the deck footing calculator. Each variable explained with a worked example.
Formulas Used
Beam Rows
num_beam_rows = beam_rowsTotal Footings
footings_total = beam_rows * footings_per_rowConcrete per Footing
concrete_per_footing = 3.14159 * pow(footing_radius_ft, 2) * footing_depth_ft / 27Total Concrete
total_concrete = beam_rows * footings_per_row * 3.14159 * pow(footing_radius_ft, 2) * footing_depth_ft / 27Variables
| Variable | Description | Default |
|---|---|---|
deck_length | Deck Length(feet) | 16 |
deck_width | Deck Width(feet) | 12 |
footing_spacing | Footing Spacing(feet) | 8 |
beam_spacing | Beam Spacing (rows)(feet) | 6 |
footing_diameter | Footing Diameter(inches) | 12 |
footing_depth | Footing Depth(inches) | 42 |
beam_rows | Derived value= floor(deck_width / beam_spacing) + 1 | calculated |
footings_per_row | Derived value= floor(deck_length / footing_spacing) + 1 | calculated |
footing_radius_ft | Derived value= footing_diameter / 2 / 12 | calculated |
footing_depth_ft | Derived value= footing_depth / 12 | calculated |
How It Works
How to Calculate Deck Footings
Formula
Beam Rows = floor(Deck Width / Beam Spacing) + 1
Footings per Row = floor(Deck Length / Footing Spacing) + 1
Total Footings = Beam Rows × Footings per Row
Concrete per Footing = pi × r² × depth / 27
Footing spacing is typically 6-8 ft for 2×8 beams or 8-10 ft for doubled 2×10 or 2×12 beams. Footing depth must extend below the frost line per local building code.
Worked Example
A 16×12 ft deck with 8 ft footing spacing, 6 ft beam spacing, 12-inch diameter footings 42 inches deep.
deck_length = 16deck_width = 12footing_spacing = 8beam_spacing = 6footing_diameter = 12footing_depth = 42
- 01Beam rows = floor(12 / 6) + 1 = 3 rows
- 02Footings per row = floor(16 / 8) + 1 = 3 footings
- 03Total footings = 3 × 3 = 9 footings
- 04Footing radius = 12 / 2 / 12 = 0.5 ft
- 05Concrete per footing = 3.14159 × 0.5² × 3.5 / 27 = 0.102 cu yd
- 06Total concrete = 9 × 0.102 = 0.92 cu yd
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