Deck Footing Calculator

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 (3.6 × 4.9 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.

At least 12 inches (30 cm) deeper than your local frost line — that's the depth at which winter ground freeze stops, varies 0 inches in southern climates to 48+ inches (122 cm) in northern. 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). Always check your local building department's frost-line map. Footings shallower than frost line will heave annually and crack the deck framing.

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 — 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).

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 bags = 0.6 ft³ each (one 80 lb bag covers about one 10-inch × 24-inch footing); 60 lb = 0.45 ft³; 40 lb = 0.30 ft³. The calculator does all this automatically and rounds bags up.

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 frost line move every winter, cracking deck framing and lifting posts. Footings extending below frost line stay put because the ground around them never freezes. Local building codes specify minimum footing depth based on the frost line — never skip this check. For decks attached to a house, footing depth must usually match the house's foundation footing depth.

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.

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.

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. Permits typically require: foundation plan with footing size and depth, structural drawing showing post layout and beam span, frost-line documentation. The footing inspection is usually before pouring concrete. Skipping the permit creates resale issues and may force expensive retrofitting later. Always check with your local building department.

DIY for a 12 × 16 ft (3.6 × 4.9 m) ledger-attached deck with six 10 inch × 36 inch footings: 17 bags of 80 lb 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: $400-800. Renting a power auger (saves digging) adds $50-80/day. Larger footings (12-16 inch) and tall decks scale up linearly.

Yes — pick Metric in the unit selector at the top and inputs switch to meters and centimeters. Common conversions: 12 × 16 ft deck = 3.6 × 4.9 m; 10 inch footing diameter = 25 cm; 36 inch depth = 91 cm. Concrete volume outputs in m³ (1 ft³ = 0.0283 m³). Bag sizes display in both pounds and kg (80 lb = 36 kg standard). Your unit choice persists across pages and tabs via localStorage.