The reverse-check structural calc — answers 'how much weight can my deck actually hold?'. Describe your existing framing (joist size + spacing + species, beam configuration, post size + count, footing diameter + soil bearing) and the calculator runs the IRC R507 + AWC NDS allowables in reverse. Output: per-element headroom (joist + beam + post + footing), worst-performing bottleneck, remaining capacity in pounds and PSF. Built-in point-load presets for filled hot tubs (5,500 lb / 50 sqft), outdoor kitchens with stone-look cabinets (2,800 lb), large grills, planters, fire pit tables. Live + dead + snow + furniture + point load math per IRC R301.5. 4 species multipliers (SPF #2, SP #2, DF-L #2, HF #2). 5 soil-bearing tiers (1,500-4,000 psf).
Joist span 12 ft vs allowable 12 ft for IRC R507.6 — 2x10 at 16″ OC (spf-2) · snow region.
Applied 480 psf vs allowable 760 psf. 2-ply 2×10 (most common) at 8 ft span.
Applied 5,120 lb vs allowable 11,600 lb. 6x6 PT post · 3 posts · 64 sqft tributary each.
Applied 5,120 lb vs allowable 1,570 lb. 12″ Ø footing on 2000 psf soil.
Get the point-load check above plus a stamped structural engineer letter ($600-1,800). Or open the Hot Tub Deck Calculator for a deck designed-from-scratch around the tub.
IRC R301.5 + R507 + AWC NDS allowable spans. Not a substitute for a stamped structural engineer letter when the calculator flags amber or red, or when adding point loads.
How to use
Length × width in feet. If your deck has a cantilever beyond the beam (joists extending past the back beam), enter that separately — cantilever reduces joist allowable span.
Joist size (2×6, 2×8, 2×10, 2×12), joist spacing (12″, 16″, 24″ on-center), and lumber species. Most big-box stock is SPF #2 — pick that if you're not sure. Southern Pine #2 + Douglas Fir-Larch #2 carry slightly more load.
Beam config: single 2×10 (rare for decks), two-ply 2×10 (most common), three-ply 2×10, two-ply 2×12, three-ply 2×12, or LVL engineered beam. Beam span = distance between posts in feet. Post size: 4×4 (light decks under 8 ft tall) or 6×6 (standard). Post count = total number of posts supporting the beam.
Footing diameter in inches (8″, 10″, 12″, 14″, 16″, 18″, 20″, 24″ — common sonotube + Bigfoot sizes). Soil bearing capacity from your soil report or local default (clay 1,500 psf, gravel + clay mix 2,000 psf, sandy gravel 2,500 psf, dense gravel 3,000 psf, bedrock 4,000+ psf). If you've never had a soil test, 2,000 psf is the IRC default presumption.
IRC R301.5 default: 40 psf live + 10 psf dead. Furniture load (outdoor sectional, dining table, occupants standing for parties) adds 5-15 psf on top. Snow load from ASCE 7 ground-snow map (US default 25 psf, north heavy snow zones 35-50 psf, southern states 0).
Filled hot tub (5,500 lb / 50 sqft = 110 psf point load), outdoor kitchen with stone-look cabinets (2,800 lb / 25 sqft = 112 psf), fire pit table, large planters, built-in grill. Each point load is converted to a deck-wide PSF equivalent (conservative — actual point loads concentrate worse than a deck-wide average).
Top of results shows the bottleneck — the structural element that controls your deck's actual capacity. Green = pass with margin. Amber = tight (less than 15% headroom). Red = FAIL — the deck cannot safely hold the applied load + your point loads. Each failing element shows a specific fix (upsize joist, add a post, bigger footing). 'Remaining capacity' is how much more weight you can still add before any element fails.
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 Deck Load Calculator answers the question every deck owner asks before adding a hot tub, outdoor kitchen, or large planter: 'can my deck actually hold this?' Most deck calculators size your framing FROM a target load. This one works backwards — you describe your existing deck (joist size + spacing, lumber species, beam configuration, post size + count, footing diameter + soil bearing), and it reverse-checks every structural element against the applied live + dead + snow + point loads per IRC R301.5 and R507. Output is the headroom (allowable − applied) for each element, the worst-performing element that's the bottleneck, and the remaining capacity in pounds and PSF you can still add before something fails. Includes 5 preset point loads (6-person hot tub at 5,500 lb, outdoor kitchen at 2,800 lb, fire pit table, planters, large grill) so you can instantly check what your deck can handle. Designed for homeowners + DIYers + inspectors. Not a substitute for a stamped structural engineer letter when the project is borderline.
Joist span tables per IRC 2021 R507.6 (PT southern pine #2 baseline) with species multipliers from AWC NDS 2018. Beam allowable PLF derived from AWC NDS for 2-ply + 3-ply built-up + LVL. Post short-column capacity per NDS column buckling formula. Footing capacity = footing area × soil bearing psf per IRC R403.1.4. Point-load weights from manufacturer specs (Jacuzzi / Hot Spring hot tubs, Lynx + Coyote outdoor kitchens). Snow loads from ASCE 7-22 geographic ground snow zones.
IRC mandates 40 psf live + 10 psf dead minimum for residential decks. Snow load adds per ASCE 7 (zero in mild states, 35-50 in north heavy-snow zones). Furniture adds 5-15. Point loads (hot tub, kitchen) divide the lb weight by deck area for a worst-case PSF equivalent.
Base spans per IRC R507.6 Table for PT southern pine #2. Species multiplier: Southern Pine #2 (1.00×), Douglas Fir-Larch #2 (0.97×), SPF #2 (0.88×), Hem-Fir #2 (0.85×). 2×10 at 16″ OC = 14.8 ft for SP #2, 13.0 ft for SPF.
Beam carries half the joist length (one side) plus half cantilever. 12 ft deck = 6 ft tributary width per beam. 75 psf total × 6 ft = 450 plf — two-ply 2×10 at 8 ft span handles 760 plf, so margin is comfortable.
Tributary area = deck area ÷ post count. A 12×16 deck (192 sqft) with 3 posts = 64 sqft tributary each. 75 psf × 64 sqft = 4,800 lb per post. 6×6 PT post handles 11,600 lb up to 8 ft tall — plenty.
12″ Ø footing = 0.785 sqft area × 2,000 psf clay = 1,570 lb. 14″ footing = 1.07 sqft × 2,000 = 2,140 lb. Footing is often the controlling element on heavy decks with hot tubs — that's why hot tub spec usually demands 18″ Ø or larger footings.
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