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.
- 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
Frequently Asked Questions
How deep do deck footings need to be?
Footings must extend below the frost line in your area. This ranges from 12 inches in the southern US to 48+ inches in northern states. Check local building codes.
What size footing do I need for a deck?
A 12-inch diameter footing is standard for most residential decks. For heavy loads (hot tubs, roofed areas), use 16-18 inch footings. Check local codes for specifics.
Can I use deck blocks instead of concrete footings?
Deck blocks (precast pier blocks) can be used for freestanding, ground-level decks in some jurisdictions. Elevated or attached decks typically require poured concrete footings.
Ready to run the numbers?
Open Deck Footing Calculator