The footing-sizing decision tool. Eight sonotube diameters (8″, 10″, 12″, 14″, 16″, 18″, 20″, 24″) compete head-to-head at your tributary load + soil bearing PSF — with applied load vs allowable load math per IRC R403.1.4, concrete bag count at your depth, and total material cost per footing. Plus 5 Bigfoot bell-base options (BF-20 through BF-36) — the flared bell increases effective bearing area 4-6× without increasing the tube diameter, the cheapest way to upgrade footing capacity on weak soil. The recommended-diameter tag flags the smallest size that passes with 20% headroom — the value sweet spot. 2026-Q1 retail.
12″ sonotube provides 0.785 sqft bearing × 2000 psf = 1,570 lb. Your tributary load is 2,640 lb per post — over capacity. Upsize footing.
Below 2,500 psf is the conservative range. A $400-800 geotechnical test may unlock 3,000+ psf bearing (50% capacity boost) or confirm worst-case (drive Bigfoot recommendation).
Per footing: $15 tube + $0 base + $41 concrete (7 bags) = $56.
Same load, same soil, same depth, eight sonotube diameters. Click any row to highlight. Green = pass with margin, amber = tight, red = fail.
Footing depth is independent of diameter — it defeats frost heave. Open the Footing Depth Calculator for your local frost zone, or the Frost Depth Calculator for your zip code.
2026-Q1 retail pricing for sonotube + Bigfoot Systems bases + Quikrete / Sakrete 60-lb premix. Not a substitute for stamped structural engineer letter on hot-tub-grade decks or weak-soil sites.
How to use
Length × width in feet, plus the number of posts supporting the deck. The calculator divides deck area by post count to get tributary area per post — the load each footing carries.
IRC R301.5 minimum is 50 psf (40 live + 10 dead). Add 25-35 psf if you're in a snow region. Add 5-15 psf for heavy furniture. Hot tub + outdoor kitchen point loads go up to 100-150 psf concentrated — for those, use the Deck Load Calculator instead (this one handles uniform loads).
From a soil test or IRC R401.4.1 presumptive table. Common urban soils: 1,500 psf (soft clay — most permissive case), 2,000 psf (clay + sand mix — the default presumption), 2,500 psf (sandy clay), 3,000 psf (dense gravel), 4,000+ psf (rock or bedrock). If you've never had a soil test, 2,000 psf is safe.
Frost depth requirement from your local building department. North frost zone (MN, MI, WI, NE) is 40-60″. Mid-Atlantic + Midwest is 30-42″. Southern states are 12-24″. Footing must reach BELOW the local frost depth to prevent heave damage. The Frost Depth Calculator has your local number.
Straight sonotube is cheapest — no flared base, bearing area = tube cross-section. Bigfoot bell bases (BF-20 on 8″ tube, BF-24 on 10/12″, BF-28 on 12″, BF-36 on 14″) give 4-6× the bearing area without increasing the tube diameter. The economic trade-off: a Bigfoot bell adds $24-64 per footing but lets you use a smaller tube diameter — often pays back at 4+ footings on weak soil.
Bottom panel shows all 8 sonotube diameters head-to-head: applied load vs allowable load (green/amber/red), concrete bags needed at your depth, total material cost per footing. The 'recommended' diameter is the smallest that passes with at least 20% headroom — the value sweet spot. Smaller wastes money on over-spec; larger wastes money on labor + concrete.
Material guide
Three honest paths. Composite wins the 25-year math for most homeowners, wood wins on upfront cost, and PVC is unbeatable around water. Each card below answers in one glance — recalculate the bill of materials by clicking a brand in the picker above.
How we calculate
The Footing Diameter Calculator answers the question every deck builder asks at the dig stage: 'do I really need a 14″ footing, or will a 10″ work here?' Sized for the actual tributary load (deck area ÷ post count × design PSF) and your local soil bearing capacity (1,500-4,000 psf per IRC R401.4.1). The output is an 8-diameter head-to-head comparison (8″ through 24″ sonotube) showing applied load vs allowable load vs cost-per-footing — so you can see exactly which diameter saves money without compromising safety. Includes Bigfoot bell-base options (BF-20, BF-24, BF-28, BF-36) — the flared bell increases effective bearing area 4-6× without increasing the tube diameter, which is the cheapest way to upgrade footing capacity on weak soil. Concrete bag count + total pour weight + material cost per footing. 2026-Q1 retail.
Footing area math from circular geometry (π × r²). Soil bearing presumptive values per IRC R401.4.1. Bigfoot bell-base effective areas from Bigfoot Systems published spec sheets (BF-20, BF-24, BF-28, BF-36). Concrete bag count = (footing area × depth) ÷ 0.45 cu ft per 60-lb bag. 2026-Q1 pricing: Quikrete + Sakrete premix $5.85/bag, sonotube tubes at Home Depot retail, Bigfoot bases at Bigfoot Systems online direct.
12×16 deck = 192 sqft. 4 posts = 48 sqft tributary each. At 55 psf design load = 2,640 lb per post. This is the load the footing must support — not the whole deck weight.
12″ Ø footing = 0.785 sqft area × 2,000 psf clay = 1,570 lb. 14″ Ø = 1.07 sqft × 2,000 = 2,140 lb. 16″ Ø = 1.40 sqft × 2,000 = 2,800 lb. Choose smallest diameter where allowable > applied with 20% margin.
BF-24 bell = 24″ diameter = 3.14 sqft (vs 0.785 for the 12″ tube alone). 4× the bearing area for $32 more per footing. BF-28 = 4.28 sqft. BF-36 = 7.07 sqft. Bell increases capacity ONLY at the bottom — tube sidewall still carries tube-diameter load.
12″ Ø sonotube: 1.75 bags of 60-lb premix per foot of depth. 14″ Ø: 2.38 bags/ft. 16″ Ø: 3.10 bags/ft. At 42″ depth (3.5 ft): 12″ = 7 bags, 14″ = 9 bags, 16″ = 11 bags. Each 60-lb bag covers 0.45 cu ft. Bigfoot bell needs an extra ~2-4 bags depending on size.
12″ tube at 42″ depth: $15 tube + 7 bags × $5.85 = $56. With BF-24 bell: $15 + $32 + 7 × $5.85 = $88. 16″ straight tube at same depth: $24 + 11 × $5.85 = $88. Often the bell on a smaller tube costs the SAME as upsizing to a bigger straight tube — pick based on dig effort + handling.
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