How big should my deck footings be?

Size the diameter to your tributary load divided by your soil's bearing capacity. For a typical 12×16 ledger-attached deck on default 1500-psf soil, 4 footings at 12″ diameter pass with comfortable margin. Bigger decks, higher snow loads, or weaker soil push you to 16″ or 18″ sonotubes. The calculator auto-sizes — pick a manual size only if you want to compare alternatives or your inspector specified one.

How deep do deck footings need to be?

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.

How much concrete do I need for a deck footing?

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.

How many bags of concrete per footing?

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.

Sonotube vs square footing — which should I use?

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.

Do I need rebar in deck footings?

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.

What's the difference between presumptive soil bearing values and a soils test?

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.

How does frost heave damage a deck footing that's not deep enough?

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.

What about helical piles or screw piles instead of concrete?

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.

Can I pour deck footings myself?

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.

Do I need permits for deck footings?

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.

What if my soil is really wet or has a high water table?

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.

How do I tie the post to the footing?

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.

Does this calculator pass building-department inspection?

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.

Can I export this estimate as a PDF?

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.

Structural · IRC 2021 R507.3 + R403.1.4

Deck Footing Calculator

Footing diameter, total depth, and concrete volume sized to your tributary load + soil bearing capacity per IRC R507.3, with depth keyed to your state's frost line per R403.1.4. PASS/FAIL the moment you change any input — every standard sonotube size compared at a glance, with concrete bag count for both 60-lb and 80-lb.

IRC R507.3R403.1.4 frost5 soil typesAll 50 states60lb + 80lb bagsHelical altFree forever

R507.3·IRC sized

6·Sonotubes

Frost·By state

Helical·Alt compare

Inputs

Deck lengthft (parallel to ledger)

Deck depthft (perpendicular)

Footing countpost-line only

Attachmentledger carries half the deck

Soil typeIRC Table R401.4.1

Live loadStandard residential — IRC default

Statedrives frost depth + labor

Frost depth overrideleave 0 to use state default

0 uses your state's IRC default. Override when your local AHJ specifies a different depth.

Footing shapeMost common — concrete poured into a cardboard form tube

Diameterauto = smallest passing standard

Bag sizepre-mix concrete

Above gradeexposed sonotube above ground

IRC R507.3 footing sizing · PA

PASS4 × 16″ · 3′-4″ deep

24.0 sq ft tributary · 1200 lb / footing · 1500 psf bearing

Diameter

auto-sized

Depth

3′ frost

Concrete

0.69 cu yd

80lb bags

5% waste included

Compliance · IRC 2021

16″ footing passes — 1.75× safety factor

IRC R507.3.1

1.40 sq ft provided ≥ 0.80 sq ft required at 1500 psf bearing.

Footing depth 40″ extends below frost line

IRC R403.1.4

36″ frost depth + 4″ above-grade exposure. IRC R403.1.4 requires bearing below the frost line to prevent heave damage.

Footing cross-section · side elevation

16″ sonotube · 3′-4″ total depth

Frost 3′PASS

16″ sonotube provides 1.40 sq ft bearing — 1.75× the 0.80 sq ft required. Total 3′-4″ depth bears below the 3′ frost line per IRC R403.1.4.

All standard sonotube sizes

tap to switch

Green = passes your 1200-lb tributary load at 1500 psf bearing. Total cost includes concrete bags, sonotubes, rebar, standoff bases, and regional labor.

Bill of materials · $916

80-lb concrete pre-mix bags

9 bags × 4 footings · 18.6 cu ft (0.69 cu yd) · 5% waste included

36 bag

$268

16″ sonotube concrete form

3.3 lf each × 4 footings · 40″ depth

13.3 lf

$83

#4 rebar (1/2″ deformed)

3 bars × 3.3′ each × 4 footings

40.0 lf

$30

Simpson ABU44Z standoff base

Galvanized post-to-footing standoff (4×4). Upgrade to ABU66Z if your post is 6×6.

4 each

$72

Excavation + form + pour labor (US_NE)

95 USD/footing × 4 × 1.22 regional multiplier

4 footing

$464

Total project

$916

Concrete priced by bag at 2026-Q1 retail. Labor adjusted for PA.

Build the full deck plan

The Deck Material Calculator turns these footings into a full bill — joists, beams, posts, hangers, and decking — sized to your dimensions and validated against the IRC.

Open

Footing sizes pulled from IRC 2021 R507.3 / Table R401.4.1 presumptive bearing values. Frost depths from IRC R403.1.4 — your local AHJ may require deeper. This calculator is not a substitute for a licensed inspector or geotechnical engineer.

How to use

Three steps. Permit-ready output.

Enter deck size

Length (parallel to the house if ledger-attached) and depth (perpendicular). The calculator multiplies these to get the loaded area, then splits it across your footings.

Set footing count + ledger

A typical 12×16 ledger-attached deck uses 4 footings (one beam, two posts per beam end + intermediate). A freestanding deck doubles to 8 (the ledger side becomes a second beam line). The calculator splits the load accordingly — ledger carries half the deck.

Pick your soil

If you don't know, leave the default 'Sand / silty' (1500 psf — the IRC default per Table R401.4.1). Clay is the same 1500 psf. Sandy gravel is 2000, crystalline gravel is 3000, bedrock is 4000+. A soils-engineer-tested value can be 2-3× higher, but the IRC's prescriptive table only allows the values shown.

Pick your state

Drives frost depth automatically — Florida 0″, North Carolina 18″, Pennsylvania 36″, Maine 54″, Minnesota 60″, Alaska 100″. IRC R403.1.4 requires the footing to bear below the frost line so frost heave doesn't push your posts up. Override the frost depth if your local AHJ uses a different value.

Read the result

Recommended sonotube diameter (auto-sized to the smallest standard size that passes), total depth, concrete volume per footing, total bag count, and a cost breakdown. PASS/FAIL the moment you change any input. Below the result, the alternatives panel shows every standard sonotube size with PASS/FAIL so you can up-size or down-size with one click.

How we calculate

The math, fully transparent.

The Deck Footing Calculator answers the three questions that decide whether your deck will pass inspection and last 50 years: how big should each footing be, how deep must it go to clear the frost line, and how much concrete will the project actually need? Inputs are deck dimensions, footing count, soil type, your state (for frost depth + regional pricing), and the live load you want to design for. Output: required diameter sized to the IRC 2021 R507.3 prescriptive table at your soil's bearing capacity, total footing depth keyed to your state's frost line per IRC R403.1.4, concrete volume by cubic foot and cubic yard, and bag count for both 60-lb and 80-lb pre-mix. Every result links back to the IRC code reference your inspector reads.

IRC references

IRC 2021 R507.3 — Deck footings (size, depth, reinforcement)
IRC 2021 R507.3.1 — Footing area sized to tributary load + soil bearing
IRC 2021 R403.1.4 — Footing depth below frost line
IRC 2021 Table R401.4.1 — Presumptive load-bearing values for soils
IRC 2021 R507.4 — Footing reinforcement (when required)

Footing sizes from IRC 2021 R507.3 / Table R401.4.1 presumptive bearing values. Frost depths from IRC R403.1.4. Concrete pricing reflects 2026-Q1 Home Depot / Lowe's national-median bag pricing. Always confirm with your local Authority Having Jurisdiction (AHJ).

Tributary load per footing

load_lb = tributary_area_sqft × (LL + DL)

Each footing carries one slice of the deck. For a ledger-attached deck the ledger carries half — so the footings see (deck_area / 2) / footing_count. Live load defaults to 40 psf per IRC, dead load is 10 psf for typical wood-frame decks. A 12×16 deck (192 sq ft) with 4 footings and a ledger sees 96 sq ft / 4 = 24 sq ft per footing → 24 × 50 = 1,200 lb each.

Required footing area

area_required_sqft = load_lb / soil_bearing_psf

Soil bearing capacity (psf) determines how much area each footing needs. At 1500 psf bearing (the IRC default), a 1,200-lb load needs 0.8 sq ft → an 8″ sonotube provides 0.35 sq ft (fails), 10″ provides 0.55 (fails), 12″ provides 0.79 (just barely fails), 16″ provides 1.40 (passes with 1.75× safety factor).

Concrete volume per footing

concrete_cuft = π × (diameter/2)² × depth / 1728

A round sonotube is a concrete column. Volume = πr² × height. For a 12″ × 36″ tube: π × 6² × 36 / 1728 = 2.36 cu ft. For 16″ × 36″ that becomes 4.19 cu ft — almost double. Multiply by your footing count and 5% waste for the project total.

Bag count

bags = ceil(volume_cuft / yield_per_bag × 1.05)

60-lb pre-mix yields about 0.45 cu ft per bag; 80-lb yields 0.60. We add 5% waste (form spillage, partial bags). For 2.36 cu ft you need 6 × 60-lb bags or 5 × 80-lb bags. Above 40 bags, ready-mix concrete delivery is usually faster and cheaper.

Footing depth

depth_in = frost_depth_in + above_grade_in

IRC R403.1.4 requires the footing to bear below the frost line — the depth at which water in the soil freezes in winter. Frost heave forces are huge (tens of thousands of pounds per footing) and will lift any footing that bears within the frost zone. Above-grade exposure is typically 2-4″ to keep the post off wet soil.

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Joist Span Calculator

Maximum joist span for any size, species, and spacing — directly from IRC 2021 R507.6 tables.

Deck Beam Span Calculator

Maximum beam span (post-to-post) by size, ply, species, and tributary load — straight from IRC R507.5 / DCA-6.

Deck Material Calculator

Calculate exact decking boards, joists, beams, posts, footings, and fasteners for your deck — IRC 2021 compliant.