Use this deck footing size calculator to figure out how wide and how deep your concrete footings need to be, based on post spacing, joist span, soil bearing capacity, and the frost depth in your area.
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Enter your post spacing and joist span to establish the tributary (load) area each footing supports, then choose your soil bearing capacity and footing shape. Pick your region for a typical frost depth, or enter a custom value from your local code. Click Calculate to get the recommended footing diameter (or width) and the minimum dig depth.
Undersized or too-shallow footings are one of the most common (and dangerous) deck-building mistakes. If a footing is too small for the load and soil type, it can settle unevenly and cause the deck to sag or lean over a few seasons. If it's not dug below the frost line, seasonal freeze-thaw cycles heave the concrete up and down, cracking posts, popping fasteners, and eventually pulling the ledger away from the house. Building inspectors in cold-climate states like Minnesota, Wisconsin, and Maine routinely reject permits where footings sit above the required 42"–60" frost depth. Even in the South, a minimum 12" embedment is standard to reach stable soil. Whether you're building a 10x12 ground-level platform or a two-story deck with a hot tub, getting footing size and depth right up front saves you from tearing out concrete later.
The calculator works in three steps:
1. Tributary area — the floor area each footing carries: Tributary Area = Post Spacing × (Joist Span ÷ 2). This formula assumes a simple ledger-to-beam deck where a single beam carries half the joist span; it does not directly apply to freestanding decks with two beams or interior posts that pick up two adjoining spans, which carry roughly double the load shown here.
2. Total load — Load (lbs) = Tributary Area × (Dead Load + Live Load)
3. Footing area — Footing Area (sq ft) = Load ÷ Soil Bearing Capacity, then converted to a round diameter (Diameter = √(4 × Area ÷ π)) or a square side (Side = √Area), rounded up to the next standard footing size (8", 10", 12", 14", 16", 18", 20", 24", 30", 36").
The minimum footing depth is simply the greater of your local frost depth or 12 inches, since code minimums typically require at least 12" of embedment regardless of frost conditions. If the calculated footing size exceeds 36" — common with hot tubs, second stories, or very soft soil — the tool flags it as an oversized load requiring an engineered footing design rather than a standard sonotube.
Say you're building a deck with posts spaced 8 ft apart, a 12 ft joist span, standard residential loads (10 psf dead + 40 psf live = 50 psf), sandy soil rated at 2,000 psf, round footings, and you're in a region with a 36" frost depth.
Tributary area = 8 × (12 ÷ 2) = 48 sq ft. Total load = 48 × 50 = 2,400 lbs. Footing area needed = 2,400 ÷ 2,000 = 1.2 sq ft. Diameter = √(4 × 1.2 ÷ π) × 12 ≈ 14.8", which is just over the standard 14" tube, so it rounds up to the next standard size of 16". Minimum dig depth = max(36", 12") = 36".
Footings should be dug so the bottom sits below your local frost depth, with a common minimum of 12" even in frost-free areas. In cold climates that can mean 36"–60" deep, so always check your local building code before digging.
It depends on the tributary load and soil bearing capacity, but most residential decks with 8 ft post spacing and average soil use 12"–16" diameter footings. Heavier loads (hot tubs, second-story decks) or weaker soil can push that up to 20"–24" or beyond, at which point an engineered design is safer than a standard tube.
Yes, cardboard concrete tubes (sonotubes) are the standard way to form round footings and are sized the same way — by diameter and depth calculated from load and soil capacity.
Yes, roofs and upper stories add significant dead and live load per post, so you should increase the dead/live load inputs in this calculator to reflect the added structure before sizing footings.
2,000 psf is a common conservative default for typical sandy loam soils, but a geotechnical report or your local building department can give you an exact number for your lot.
No — this tool gives you solid planning estimates, but your local building department has the final, code-required frost depth and footing size for your specific address and structure.