Footing Diameter Calculator

Usually yes for a code-compliant residential deck on average soil. 12″ Ø footing has 0.785 sqft bearing area × 2,000 psf clay = 1,570 lb allowable. A typical 12×16 deck with 4 posts at 55 psf design load = 2,640 lb per post — so 12″ FAILS this case (need 14″). At 4 posts on a 12×12 deck = 1,980 lb per post — 12″ TIGHT (94% utilized, no headroom). 12″ works comfortably for decks under ~150 sqft + average load. Bigger decks need 14-16″.

Almost always, yes — especially on weak soil. A Bigfoot BF-24 ($32) on a 12″ tube gives 3.14 sqft effective bearing area — same as a 24″ Ø straight tube but at 1/3 the digging effort and 1/3 the concrete. The math: 12″ tube + BF-24 bell uses ~9 bags of concrete vs 24″ straight tube uses ~25 bags. Material cost is nearly identical; labor cost (digging + handling) is dramatically lower with the bell. The only time straight beats bell is on very strong soil (4,000+ psf) where the small tube alone has enough capacity.

BELOW the local frost depth — minimum. North frost zone (Minnesota, Wisconsin, Maine, etc.): 48-60″ deep. Mid-Atlantic + Midwest: 36-42″. Southern states: 12-24″. Florida + Gulf Coast: as little as 12″. The Frost Depth Calculator has the exact number for your zip code. Footing depth is independent of diameter — the depth defeats frost heave, the diameter handles vertical load. A 12″ Ø footing at 48″ depth + a 16″ Ø footing at 48″ depth have the same vertical capacity (depth doesn't add capacity in a sonotube).

At standard 42″ depth: 8″ Ø needs ~3 bags, 10″ needs ~4 bags, 12″ needs 6-7 bags, 14″ needs 8-9 bags, 16″ needs 11 bags, 18″ needs 14 bags, 20″ needs 17 bags, 24″ needs 25 bags. Add 2-4 extra bags for Bigfoot bell. Each 60-lb bag of premix (Quikrete or Sakrete) makes 0.45 cu ft of cured concrete. Math: bags = ceil(tube_volume_cuft ÷ 0.45). For a typical 4-post deck with 14″ Ø sonotubes at 42″ depth, plan for 36 bags total + a few extra for spillage.

Recommended but not required for most residential decks. IRC R401.4.1 lets you presume 2,000 psf for sand / gravel / sandy clay (the most common urban soils) without a test — that's the 'default' soil bearing capacity. You need a soil test only if: (1) you have visible soft spots, organic matter, or recent fill on your site, (2) you're putting a hot tub or outdoor kitchen on the deck, (3) your local building department requires it for permit, or (4) you're seeing soil bearing values in load math that don't have headroom. A soil test is $400-800 from a geotechnical engineer — worth it for any deck over 300 sqft or any heavy point load.

Only for ground-level floating decks (under 200 sqft + under 30″ + no ledger to house). For any elevated deck above 30″, IRC R507.3 requires footings below frost depth — Dek-Blocks don't qualify. Ground-screw foundations (Pylex / Krinner) are an acceptable poured-footing substitute in most jurisdictions, but check with your building department. The Ground-Level Deck Calculator covers the Dek-Block alternative; this calculator is for elevated decks needing real footings.

Two separate jobs. DEPTH defeats frost heave — the soil at the bottom of a deep footing doesn't freeze in winter, so it doesn't lift the footing. DIAMETER handles vertical compression — bigger diameter = larger bearing area = more load capacity at the bottom. You need BOTH right: a 24″ Ø footing at 12″ depth fails frost in cold climates (footing heaves up); a 8″ Ø footing at 60″ depth fails load in soft soil (sinks down). The Footing Depth Calculator does the frost math; this one does the load math.

Differential settlement. The footing slowly sinks into the soil over years (sometimes decades), but unevenly — usually one corner settles faster than others. Results: deck-board gaps open up at one end, doors to the house don't latch, railings tilt, the ledger pulls away from the house wall. By the time it's visible, you typically need to dig out + replace 1-3 footings while the deck is supported on jacks — $2,500-6,000 per footing replaced. Spending an extra $30 per footing upfront to size correctly is the best deal in deck construction.

Sometimes. If the existing footings are at least 12″ Ø, at proper frost depth, in good visible condition (no cracking, no settlement), AND the new deck is the same size or smaller as the old one, you can reuse them. Have an inspector or structural engineer evaluate first — most building departments want a stamped letter confirming reusability. The catch: most pre-1990s decks have undersized footings (often 8-10″ Ø straight tubes) that don't meet modern IRC requirements. Plan to replace footings during any major rebuild.