Deck Footing Calculator Guide: How Many Footings and How Deep
How to calculate deck footing sizes, spacing, depth, and concrete volume. Includes IRC tables and soil bearing capacity basics.
Deck footings are the most safety-critical part of any deck project. They must carry the deck load to stable soil below the frost line. Under-sized or under-depth footings cause deck settlement, tilting, and structural failure. This guide covers how to get them right.
How Many Footings Do You Need?
The number of footings depends on your post layout, which is driven by beam spans. A typical pattern:
- Posts every 8–10 feet along the beam run
- Beams spanning 8–12 feet between posts
- One footing per post
Example: A 12×20 ft deck with two beam lines, posts at 8 ft:
- Beam 1 (20 ft): 3 posts = 3 footings
- Beam 2 (20 ft): 3 posts = 3 footings
- Total: 6 footings
For attached decks, the ledger board at the house eliminates one beam line entirely — reducing footing count.
Footing Size Requirements
Use our Deck Footing Calculator or use these IRC-based guidelines for 1,500 psf soil bearing (conservative):
| Tributary Load Area | Footing Diameter |
|---|---|
| Up to 25 sq ft | 10” diameter |
| Up to 50 sq ft | 14” diameter |
| Up to 75 sq ft | 18” diameter |
| Up to 100 sq ft | 20” diameter |
Tributary area = the deck area supported by each post. For a simple deck with equal post spacing, divide total area by number of posts.
Footing Depth
Warning — Frost Heave: A footing that doesn’t reach below the frost line will heave upward every winter as the ground freezes and expand. Even a few inches of frost heave will rack your deck frame, crack ledger connections, and can pull lag bolts out of the house rim. This is non-negotiable — go to the local frost depth or deeper, every time.
Always extend below the frost line. Minimum depths by region:
| Region | Frost Depth |
|---|---|
| South (FL, TX, AZ) | 0–12 inches |
| Mid-South | 12–18 inches |
| Mid-Atlantic | 24–36 inches |
| Midwest | 36–48 inches |
| Northern states | 42–60 inches |
Verify with your local building department — they have exact required depths for your jurisdiction.
Concrete Volume Per Footing
For a round sonotube footing: Volume = π × (diameter ÷ 2)² × depth
| Diameter | 36” Deep | 42” Deep | 48” Deep |
|---|---|---|---|
| 10” | 1.63 ft³ (3 bags 80lb) | 1.91 ft³ (4 bags) | 2.18 ft³ (4 bags) |
| 12” | 2.36 ft³ (4 bags) | 2.75 ft³ (5 bags) | 3.14 ft³ (6 bags) |
| 14” | 3.21 ft³ (6 bags) | 3.75 ft³ (7 bags) | 4.28 ft³ (7 bags) |
Bags assume 0.60 cu ft yield per 80 lb bag. Use our Concrete Bags Calculator for totals across all footings.
Frequently Asked Questions
Do I need a permit for deck footings? Most jurisdictions require a permit for any attached deck or any deck over 30 inches above grade. The footing inspection is typically the first inspection — the inspector must see the footing holes before you pour concrete.
Can I use deck blocks instead of poured footings? For floating decks (not attached to the house, close to grade), deck blocks may be acceptable and avoid permit requirements in many jurisdictions. Check your local code — requirements vary significantly.
How long after pouring before I can put posts on the footings? Standard concrete reaches adequate strength for post installation in 24–48 hours for an unloaded post. Wait the full 7 days before applying significant beam and deck loads.
See also: Concrete Calculation Methodology — footing volume formulas, bag yield tables, and ACI 318 references behind ConstructCalc concrete calculators.
Browse more