Deck Footing Calculator

Deep enough to bear below your local frost line — IRC R403.1.4. Florida is 0″ (no frost), most of the South is 12-18″, the Mid-Atlantic and Midwest run 30-42″, the Upper Midwest and Northern New England hit 48-60″, and Alaska tops out at 100″. The calculator pulls your state's standard depth automatically. Always confirm with your local building department — some jurisdictions have stricter local depths.

It depends on diameter and depth. A 12″ × 36″ sonotube takes 2.36 cubic feet of concrete (about 5 × 80-lb bags). A 16″ × 36″ takes 4.19 cubic feet (about 7 bags). Multiply by footing count and add 5% waste. The calculator gives you the exact number for both 60-lb and 80-lb bag sizes.

An 80-lb bag of pre-mix yields 0.60 cubic feet. A 60-lb bag yields 0.45. For a 12″ × 36″ footing: 2.36 ÷ 0.60 = ~4 bags of 80-lb (round up to 5 with waste), or ~6 bags of 60-lb. The 80-lb bags are cheaper per cubic foot and easier on bag count, but heavy — the 60-lb bags are gentler on your back. The calculator shows both totals.

Round sonotubes are the residential-deck standard: cheaper forms, faster to set up, and they shed soil pressure better than square corners. Square footings are formed with 2×8 or 2×10 lumber and used when the bearing capacity is borderline (you can build a wider square pad for the same volume of concrete) or when pouring against an existing foundation. For 95%+ of new decks, sonotubes are the right answer.

IRC R507.4 requires reinforcement only when the soil is unstable or the footing exceeds prescriptive thresholds. In practice, contractors add 3 vertical #4 bars (1/2″ deformed rebar) to every sonotube to reduce cracking — costs ~$2-3 per footing and noticeably improves frost-heave resistance. The calculator includes 3 bars × footing depth in the cost breakdown.

IRC Table R401.4.1 publishes conservative 'presumptive' values you can use without a soils test — 1500-4000 psf depending on soil type. A geotechnical engineer's soils test typically reveals 2-3× higher actual bearing capacity, which lets you use much smaller footings. For a $400-800 soils test on a deck under 400 sq ft, the savings rarely beat the test cost — but for large decks or unusual soil, a test can be worth it.

Water in soil freezes and expands ~9% by volume. If your footing bears within the frost zone, the freezing soil grips the sides of the sonotube and lifts the entire footing — sometimes by inches, often unevenly between footings. The result is a deck that tilts, racks, or pulls away from the house. A correctly-deep footing bears on stable soil below the frost line; the frost zone above it can move freely without lifting the footing.

Helical piles are 7-8 ft galvanized-steel screws with a contractor-installed hydraulic torque-tested capacity of 30+ kip per pile. Material + install is typically $400-500 per pile (vs $50-100 in concrete + bags + sonotube + rebar). Helicals are a great fit for tight access, sloped sites, or when you don't want to wait on a concrete cure — but they cost roughly 4-6× more for a typical deck. The calculator can show a side-by-side comparison.

Yes — DIY footing work is one of the most common owner-built deck stages. You'll need: post-hole digger or 2-person auger, sonotube and form-tube saw, mixing tub or wheelbarrow, hoe, level, and either bagged pre-mix or delivered ready-mix. Plan on ~30 minutes per footing for a 36″-deep × 12″ sonotube. Always check your permit requires a pre-pour inspection — most jurisdictions do.

Yes — every US jurisdiction requires a building permit for a structural deck, and the footing inspection is the first inspection. Permit fees are typically $75-300 depending on jurisdiction and deck size. The permit covers footings, framing, and final. Skipping the permit risks a stop-work order, a fine, and being told to dig up your concrete to expose the footings for inspection.

Wet clay or high water-table sites usually need wider footings or driven helical piles. The IRC presumptive values assume reasonably dry soil; saturated clay can have bearing capacity below 1000 psf. Signs you have a problem: standing water in the post hole an hour after digging, gray or blue-gray clay below 2 ft, or visible spring water seeping into the hole. When in doubt, get a soils engineer to do a hand auger test.

Use a Simpson ABU44Z standoff base for 4×4 posts or ABU66Z for 6×6 — a galvanized U-shaped bracket with 1″ standoff that keeps the wood out of standing water (per IRC R317.1.4). The base is set into wet concrete with a J-bolt or post-hole anchor, then the post sits in the bracket and is fastened with structural screws. The standoff is the second most-failed part of older decks — get it right and the post lasts decades.

The footing sizes are sourced from IRC 2021 R507.3 / R401.4.1 and frost depths from IRC R403.1.4 — exactly the references your inspector uses. Always confirm with your local Authority Having Jurisdiction (AHJ): some jurisdictions amend the IRC, require stamped plans, or use a different code edition. Use the printable PDF as supporting documentation with your permit application.

Yes. Use the action bar above the results — Save PDF generates a clean, permit-ready document with your inputs, the IRC compliance checks, the bill of materials, and the relevant code references. Print works too if you prefer browser-side. Copy Link gives you a shareable URL with all inputs preserved.