Deck Footing Calculator
Post count, concrete volume per footing, and bag count for any deck. Ledger-attached or free-standing with full post-grid support. Cylinder math, 40/60/80 lb bag yields, imperial and metric units.
Results
Estimates only. The post grid is a structural approximation. Actual post and footing requirements depend on beam configuration, cantilever, deck height, and local code. Verify with your local building department before pouring concrete.
How to use this calculator
- 01Choose Imperial or Metric so deck size, footing diameter, footing depth, and concrete volume use the right units.
- 02Pick Ledger-attached or Free-standing. Ledger-attached decks count posts along the outer rim; free-standing decks use a full post grid.
- 03Enter deck length, deck width, and maximum post spacing from your framing plan.
- 04Set footing diameter and depth. Depth should follow your local frost-line requirement before you pour concrete.
- 05Choose the bag size and optional price per bag, then use Bags Needed as the rounded-up order quantity.
If you are still sizing the deck surface and joist layout, start with the Deck Calculator, then return here for the post footing concrete.
Concrete per footing reference
Volume per single cylindrical footing at common diameter and depth combinations. Multiply by post count for total concrete, then divide by bag yield for bag count.
| Diameter | Depth 36″ | Depth 48″ | Depth 60″ |
|---|---|---|---|
| 8″ | 1.05 ft³ | 1.40 ft³ | 1.75 ft³ |
| 10″ | 1.64 ft³ | 2.18 ft³ | 2.73 ft³ |
| 12″ | 2.36 ft³ | 3.14 ft³ | 3.93 ft³ |
Frequently asked questions
How many footings do I need for my deck?
It depends on attachment type and post spacing. Ledger-attached decks (one side bolted to the house) need posts only along the outer rim. For a 12 × 16 ft (3.66 m × 4.88 m) deck at 8 ft (2.4 m) post spacing: (ceil(16/8) + 1) × 2 = 6 footings. Free-standing decks need a full grid: (ceil(16/8) + 1) × (ceil(12/8) + 1) = 9 footings. The calculator handles both. Add extra footings for cantilevered sections, stairs, and hot tubs.
How deep should deck footings be?
At least 12 inches (30 cm) deeper than your local frost line. That's the depth at which winter ground freeze stops, ranging from 0 inches in southern climates to 48+ inches (122 cm) in the north. Most northern US states are 36-48 inches (91-122 cm), central states 24-36 inches (61-91 cm), southern coastal states 0-12 inches (0-30 cm). Check your local building department's frost-line map. Footings shallower than the frost line will heave every year and crack the deck framing.
What size footings for my deck?
Diameter scales with post load. For 1-story residential decks under 4 ft (122 cm) high, 10 inch (25 cm) diameter is the workhorse choice. It handles ~6,000 lb (2,720 kg) per footing. Smaller decks: 8 in (20 cm). Tall decks (above 6 ft / 183 cm), heavily loaded decks (hot tubs, cantilevers), or decks supporting roof structures: 12-16 in (30-40 cm). Local code may require 12 in minimum regardless. The calculator computes concrete volume for any diameter from 6-24 in (15-60 cm).
How do I calculate concrete for deck footings?
Cylinder volume = π × r² × depth. For a 10 inch (25 cm) diameter footing 36 inches (91 cm) deep: π × 5² × 36 = 2,827 cubic inches = 1.64 cubic feet (0.046 m³) per footing. Multiply by post count for total. Convert to bags by yield: 80 lb (36 kg) bags = 0.6 ft³ (0.017 m³) each (one 80 lb bag covers about one 10-inch × 24-inch / 25 × 60 cm footing), 60 lb (27 kg) = 0.45 ft³ (0.013 m³), 40 lb (18 kg) = 0.30 ft³ (0.0085 m³). The calculator does this automatically and rounds bags up.
What's the frost line and why does it matter?
The frost line is the depth to which the ground freezes in winter. Wet soil expands when frozen, lifting anything above it (frost heave). Footings shallower than the frost line move every winter, cracking deck framing and lifting posts. Footings that extend below the frost line stay put because the ground around them never freezes. Local building codes specify minimum footing depth based on the frost line. Don't skip this check. For decks attached to a house, footing depth usually has to match the house's foundation footing depth.
Concrete footings vs deck blocks: which should I use?
Concrete footings (poured Sonotubes or precast piers below frost line) are required for any deck attached to a structure or built in frost-prone climates. They don't move. Deck blocks (precast concrete pyramids that sit on grade) only work for ground-level free-standing decks in non-frost climates, where seasonal movement is acceptable. Most building codes don't permit deck blocks for any deck above 30 inches (76 cm) above grade or attached to a house. When in doubt, use concrete footings.
What's the maximum spacing between deck posts?
Beam material determines the max post-to-post span. Doubled 2×8: 6 ft (1.8 m). Doubled 2×10: 8 ft (2.4 m). Doubled 2×12: 10 ft (3.0 m). Tripled 2× lumber spans roughly 25% farther. 4×6 and 4×8 solid posts can span similar distances. Joist size sets a separate constraint: 2×8 joists span 8 ft (2.4 m) at 16 in OC, 2×10 spans 10 ft (3.0 m), 2×12 spans 12 ft (3.7 m). Pick the more restrictive of the two limits.
Do I need a building permit for deck footings?
Almost always yes. Most US jurisdictions require a permit for any deck above 30 inches (76 cm) above grade, attached to a house, or larger than 100-200 sq ft (9.3-18.6 m²). Permits typically require a foundation plan with footing size and depth, a structural drawing showing post layout and beam span, and frost-line documentation. The footing inspection is usually before pouring concrete. Skipping the permit creates resale issues and may force expensive retrofitting later. Check with your local building department.
How much does it cost to pour deck footings?
DIY for a 12 × 16 ft (3.66 m × 4.88 m) ledger-attached deck with six 10 inch × 36 inch (25 × 91 cm) footings: 17 bags of 80 lb (36 kg) concrete at $5-8 each = $85-135. Plus Sonotube forms ($8-12 each × 6 = $50-70), rebar ($20-40), post anchors ($10-20 each × 6 = $60-120). Total materials roughly $215-365. Pro labor for the same job: $400-800. Renting a power auger (saves digging) adds $50-80/day. Larger footings (12-16 inch / 30-40 cm) and tall decks scale up linearly.
Can I use this calculator with metric measurements?
Yes. Pick Metric in the unit selector at the top and inputs switch to meters and centimeters. Common conversions: 12 × 16 ft deck = 3.66 m × 4.88 m, 10 inch footing diameter = 25 cm, 36 inch depth = 91 cm. Concrete volume outputs in m³ (1 ft³ = 0.0283 m³). Bag sizes show pounds and kg (80 lb = 36 kg standard). Your unit choice sticks across pages and tabs via localStorage.