Footing Depth Calculator

Diameter is driven by tributary load + soil bearing. Step 1: tributary area × (live load + dead load) = total load per post. Step 2: total load / (soil bearing / safety factor) = required bearing area in sqft. Step 3: convert to diameter: d_in = 24 × sqrt(area / π). Step 4: round up to next standard sonotube (10, 12, 14, 16, 18, 22, 24″). Typical residential corner-post footing on clay = 12-16″ diameter; on sand = 10-12″.

The footprint of deck that this single post supports — half the joist span × half the beam span. For a 16'×12' deck with 2 beams running parallel to the long side, each at 4 ft from edge: corner posts support 4 ft (half joist span to beam) × 6 ft (half deck length) = 24 sqft. Mid-span posts on the same beam support 4 ft × 8 ft (mid-section length) = 32 sqft. Compute per-post — corners typically have LESS tributary area than mid-spans.

Cylinder volume: π × radius² × depth, all in feet. 12″ diameter × 36″ deep = π × 0.5² × 3 = 2.36 cu ft. Quikrete 60 lb yields 0.45 cu ft → 6 bags. Sakrete 80 lb yields 0.60 cu ft → 4 bags. For 6 footings @ 12″ × 36″ = 14.2 cu ft total → 32 × 60 lb bags. Above ~1 cu yd (27 cu ft / ~60 bags), pre-mix truck delivery becomes cheaper than bags.

IRC R507.3.1: rebar NOT required for typical residential pier-and-bracket footings under 4 ft deck height + load <6,000 lb per post. RECOMMENDED for tall decks (>4 ft) or high loads. REQUIRED for spread footings (IRC R507.3.1.2 — 4× #4 vertical + #3 ties @ 8″ o.c.) and for engineered designs. For belt-and-suspenders insurance, a single #4 vertical bar centered in the sonotube adds ~$3/footing and significantly improves uplift + lateral capacity.

Per IRC Table R401.4.1 presumptive values: sand/gravel 3,000 psf · silt/loam 2,000 psf · clay 1,500 psf · rocky/bedrock 12,000+ psf · engineered fill 1,000 psf default. Apply a safety factor: sand/silt/rock 1.5×, clay 2.0×, fill 2.5×. Actual bearing requires a geotech test — soil-bearing test costs $400-800 in most areas and is worth it for any project where soil is uncertain, fill, or near the IRC threshold. Some jurisdictions REQUIRE a geotech report for permits.

Pier-and-bracket (Simpson ABU/PB) — strongly preferred. Post sits on the bracket above the concrete, never gets wet, replaceable, inspectable, doesn't void the PT warranty. Adds ~$15 per post for the bracket. Embedded post — pour concrete around a PT 6×6 buried 6-12″ deep in the footing. Cheaper (no bracket cost) but: post can never be replaced without demolishing the footing, traps moisture against the wood, requires UC4B heavy-duty PT (more expensive lumber), and most pros consider it lower-grade construction. Use pier-and-bracket every time you can.

Yes for difficult sites: sloped, wet, rocky-but-shallow, no equipment access, or you want to skip the 2-day concrete cure cycle. Helical pile = galvanized steel screw pile driven into ground in ~30 minutes per pile. Capacity 4,000-20,000+ lb depending on size. Cost ~$200-400 per pile (installed) vs concrete pier $80-120. Save the most when avoiding excavation in tight backyards, rocky New England soil, or wet/clay sites that pump water during excavation. Tested + rated under ICC-ES AC358.

Engineered design required per IRC R301.1.3. Northern states (MN, ND, ME) with 60″+ frost depth + clay soil OR snow-zone heavy loads frequently exceed the IRC prescriptive 48″ limit. Options: (1) Hire a PE for stamped foundation drawings (~$500-1,500 typical). (2) Switch to helical piles (no excavation depth limit). (3) Use frost-protected shallow foundation (FPSF, ASCE 32-01) — continuous perimeter insulation eliminates the deep excavation requirement. DeckMath flags this automatically when calculated depth > 48″.

For BEARING capacity yes — wider footings spread load over more soil, reducing per-sqft stress. For FROST protection no — depth must always exceed the frost line regardless of width. A footing twice as wide at the same depth doubles bearing capacity but provides ZERO additional frost-heave protection. The two requirements operate independently: depth handles frost, diameter handles load. The Footing Depth Calculator sizes them separately, then takes the controlling case for each.