Along the house.
Ledger to beam.
Past the beam.
Use 1,500 unless you have a soil report.
Whichever is larger. 10 psf dead load is added.
Below frost line, minimum 12 in.
Advanced options
IRC assumes 10. Heavier for stone or built-ins.
0 = use the code-required size.
A flared base adds concrete at the bottom.
Holes are never perfectly cylindrical.
This deck footing calculator sizes footings from tributary area and soil bearing capacity following IRC R507.3.1, then returns depth, concrete volume and bag counts — with the tributary area worked out correctly, which is where most estimates go wrong.
This tool applies the IRC prescriptive method, which is a simplified code path for ordinary decks on ordinary soil. It is a planning aid, not a structural design.
Decks require a permit and a footing inspection in most jurisdictions. Local amendments to the IRC are common, frost depth is set locally, and poor or filled soil, unusual geometry or anything outside the prescriptive limits needs a licensed engineer. Confirm everything with your building department before you dig.
The short answer
Footing size comes from tributary area × total load ÷ soil bearing capacity. At the IRC default of 1,500 psf soil and 50 psf total load, a post carrying 48 ft² needs an 18 inch diameter footing.
Tributary area is post spacing × (joist span ÷ 2) — not deck area divided by post count. The house ledger carries half the deck.
On this page
How to use this deck footing calculator
Pick the deck type, enter the geometry, and set your soil and load. Three modes:
- Ledger-attached — bolted to the house, so the ledger carries half the joist span
- Freestanding — beams on both sides carrying everything between them
- Known tributary — you’ve already worked out the area per post
The calculator returns the required diameter, the actual bearing pressure against what the soil allows, concrete volume, and bag counts. The diameter override under Advanced options will warn you if you set something below the code requirement.
Tributary area — where most estimates go wrong
Tributary area is the slice of deck that one post actually carries. Get this wrong and every number downstream is wrong.
One published deck footing calculator defines it as “total deck area divided by the number of footings”. That is not how load travels through a deck.
Every joist spans from the ledger bolted to the house out to the beam. A simply-supported joist delivers half its load to each end — so the ledger takes half the deck and the beam takes the other half.
Dividing total deck area by post count assumes the posts carry all of it, which overstates the load on every footing.
Ledger-attached: Post spacing × (Joist span ÷ 2 + cantilever)
Freestanding: Post spacing × (deck depth ÷ number of beam lines)
Worked example — 16 × 12 ft deck, 3 posts on the beam
The “deck area ÷ posts” method would give 192 ÷ 3 = 64 ft², sizing a 20 inch footing. Bigger, more concrete, more digging — and not what the code asks for.
Erring large is safe, so this isn’t a danger. But it’s a third more concrete per hole for no structural benefit, and on a large deck that adds up.
The sizing formula
1. Load per post = Tributary area × (Live + Dead load)
2. Required bearing area = Load ÷ Soil bearing capacity
3. Diameter = 2 × √(Area ÷ π), converted to inches
Then rounded up to the next even inch, which is how IRC R507.3.1
tabulates it and what form sizes are actually sold in
The IRC baseline load is 40 psf live plus 10 psf dead = 50 psf under R301.5. Some sources use 60 psf, which assumes a 20 psf dead load — reasonable for heavy decking, wet lumber or built-in seating, but not the code default.
IRC footing size table
Minimum diameter in inches, at 40 psf live plus 10 psf dead. Twelve inches is the smallest practical form size.
| Tributary area | 1,500 psf | 2,000 psf | 2,500 psf | 3,000 psf |
|---|---|---|---|---|
| 20 ft² | 12 in | 12 in | 12 in | 12 in |
| 40 ft² | 16 in | 14 in | 14 in | 12 in |
| 60 ft² | 20 in | 18 in | 16 in | 14 in |
| 80 ft² | 24 in | 20 in | 18 in | 16 in |
| 100 ft² | 26 in | 22 in | 20 in | 18 in |
| 120 ft² | 28 in | 24 in | 22 in | 20 in |
| 140 ft² | 30 in | 26 in | 24 in | 22 in |
| 160 ft² | 32 in | 28 in | 26 in | 24 in |
If your tributary area falls between rows, round up. And note how much soil matters: the same 80 ft² post needs 24 inches on clay but 16 inches on gravel — that’s 2.25 times the bearing area, and a lot more concrete.
In snow country the live load rises. At 60 ft² tributary on 1,500 psf soil:
| Live or ground snow load | Total load | Diameter |
|---|---|---|
| 40 psf — standard | 50 psf | 20 in |
| 50 psf — moderate snow | 60 psf | 22 in |
| 60 psf — heavy snow | 70 psf | 24 in |
| 70 psf — severe snow | 80 psf | 26 in |
Use the live load or the ground snow load, whichever is larger — not both added together.
Soil bearing capacity
| Soil | Presumptive value |
|---|---|
| Clay, sandy clay, silty clay | 1,500 psf |
| Sandy clay, clayey sand, silt | 2,000 psf |
| Sand, silty gravel, clayey gravel | 2,500 psf |
| Gravel, sandy gravel | 3,000 psf |
| Crystalline bedrock | 12,000 psf |
Use 1,500 psf unless you have a soil report. It’s the IRC presumptive value in the absence of testing and the conservative choice. Claiming a higher value to save concrete is exactly the kind of assumption that shows up years later as a settled corner.
Two conditions where the table doesn’t apply at all: fill, and organic soil. Both can be well under 1,000 psf, and neither is covered by the presumptive values. Footings must bear on undisturbed native soil or properly compacted engineered fill — if you over-excavate, backfill with compacted gravel or concrete, never loose soil.
Depth and the frost line
Footings must bear below the local frost line. Water in soil expands as it freezes, and anything above that line gets lifted — repeatedly, every winter, until the deck is visibly out of level and the ledger connection is stressed.
Northern states commonly require 48 to 66 inches. Mid-latitude states 42 to 54. Southern states 12 to 24. But these are ranges, and your jurisdiction publishes a specific figure that may be deeper than the state median.
Measure to the bottom of the footing, not the top of the concrete. Minimum 12 inches below grade regardless of climate.
Frost heave is why a deck that was level when built develops a lean. It isn’t settlement — it’s the footings being pushed up and not coming all the way back down.
Concrete per footing
Volume for a round tube footing, in cubic feet.
| Diameter | 24 in | 36 in | 42 in | 48 in | 60 in |
|---|---|---|---|---|---|
| 12 in | 1.57 | 2.36 | 2.75 | 3.14 | 3.93 |
| 14 in | 2.14 | 3.21 | 3.74 | 4.28 | 5.35 |
| 16 in | 2.79 | 4.19 | 4.89 | 5.59 | 6.98 |
| 18 in | 3.53 | 5.30 | 6.19 | 7.07 | 8.84 |
| 20 in | 4.36 | 6.54 | 7.64 | 8.73 | 10.91 |
| 24 in | 6.28 | 9.42 | 11.00 | 12.57 | 15.71 |
Diameter drives volume much harder than depth does, because area goes with the square of the radius. Going from 12 to 24 inches at the same depth is four times the concrete. Going from 24 to 48 inches deep is only double.
That’s worth knowing when you’re deciding whether to claim a higher soil bearing value — the concrete saving from a smaller diameter is real and substantial.
Bags or ready-mix
| Footing | 80 lb bags each | Footings per yd³ |
|---|---|---|
| 12 in × 42 in | 6 | 8.9 |
| 16 in × 42 in | 9 | 5.0 |
| 20 in × 42 in | 14 | 3.2 |
| 24 in × 42 in | 21 | 2.2 |
Including 10% waste, because a hand-dug hole is never a clean cylinder.
Above about a cubic yard, order ready-mix. Six 20 inch footings is 8.5 cubic feet each — over 80 bags to buy, haul and mix by hand, in a single day, before the first ones start setting. Ready-mix gets every footing identical concrete placed at the same time. Ask about short-load fees, which apply below roughly 3 cubic yards but are usually still worth paying.
Use minimum 2,500 psi concrete, or 3,000 psi in severe weathering regions.
Post connections
IRC R507.5 requires a mechanical connection between post and footing — a galvanised post base such as a Simpson ABU or ABW. It does two jobs: ties the post down against uplift, and holds it clear of the concrete surface so end grain can dry.
A post embedded in concrete wicks moisture and rots from the inside, and you won’t see it until the deck moves. A post sitting flat on concrete does the same thing more slowly.
Deck footing mistakes to avoid
- Dividing deck area by post count for tributary. The ledger carries half.
- Assuming better soil than you have. Use 1,500 psf without a report.
- Sizing for depth instead of frost. Frost line sets depth, load sets diameter.
- Bearing on fill or organic soil. Neither is covered by the presumptive values.
- Backfilling an over-dug hole with loose soil. Use compacted gravel or concrete.
- Post set directly in concrete. It rots. Use a post base.
- Skipping the permit. Decks need one and a footing inspection almost everywhere.
- Pouring below 40 °F. Concrete won’t cure properly.
Frequently asked questions
What size footings do I need for a deck?
It depends on tributary area and soil. On the IRC default 1,500 psf soil at 50 psf total load, a post carrying 20 ft² needs 12 inches, 48 ft² needs 18 inches, and 80 ft² needs 24 inches. Better soil allows smaller — the same 80 ft² post needs only 16 inches on gravel.
How do I calculate tributary area for a deck post?
Post spacing multiplied by half the joist span, plus any cantilever. A 16 ft wide deck with 3 posts has 8 ft spacing; with 12 ft joists that’s 8 × 6 = 48 ft² per post. Do not divide total deck area by post count — on a ledger-attached deck the house carries half the load.
How deep should deck footings be?
Below the local frost line, minimum 12 inches below grade. Northern states commonly require 48 to 66 inches, mid-latitude 42 to 54, southern 12 to 24. Measure to the bottom of the footing. Your building department publishes the figure for your jurisdiction — don’t rely on a regional map.
What soil bearing capacity should I use?
1,500 psf unless you have a geotechnical report. That’s the IRC presumptive value for clay and silt and the conservative default. Sand and gravel can be 2,500 to 3,000, but assuming a higher value without testing is how footings end up undersized.
How much concrete per deck footing?
A 12 inch footing 42 inches deep is 2.75 cubic feet — about 6 bags of 80 lb with waste. An 18 inch at the same depth is 6.19 cubic feet, and a 24 inch is 11.0. Diameter matters far more than depth, since area scales with the square of the radius.
What load should I design a deck for?
40 psf live plus 10 psf dead, so 50 psf total, per IRC R301.5. In snow regions substitute the ground snow load for the live load if it’s larger — use whichever is bigger, not the sum. Some sources use 60 psf, which assumes a heavier 20 psf dead load.
Should I use bags or ready-mix for deck footings?
Bags below about a cubic yard, ready-mix above it. Six 20 inch footings is over 80 bags to mix by hand in one day. Ready-mix gives every footing identical concrete placed at the same time. Short-load fees apply below roughly 3 cubic yards but are usually worth paying.
Can I set the post directly in the concrete?
No. IRC R507.5 requires a mechanical post base such as a Simpson ABU or ABW. It resists uplift and holds the post clear of the concrete so the end grain can dry. Wood embedded in or sitting on concrete wicks moisture and rots from the inside.
Do deck footings need rebar?
Not typically for residential footings under 24 inches diameter. Larger footings, engineered designs, or specific local amendments may require it per plan. Check with your building department — this is exactly the kind of detail that varies by jurisdiction.
Do I need a permit for a deck?
In most jurisdictions yes, along with a footing inspection before you pour. Some areas exempt small freestanding decks under a size threshold. The inspection happens with the holes open, so book it before mixing anything.
What if my soil is fill or organic?
The presumptive bearing values don’t apply. Both can be well under 1,000 psf, and this calculator’s output would be unsafe. You need either a geotechnical assessment, footings taken down to competent soil, or an alternative such as helical piles. This is engineer territory.
Is this calculator a substitute for an engineer?
No. It applies the IRC prescriptive method, which is a simplified path for ordinary decks on ordinary soil within defined limits. Anything outside that scope — unusual geometry, poor soil, elevated or multi-level decks, or any structure where you’re unsure — needs a licensed engineer and a permit.
Standards and sources
- IRC R507.3.1 — Minimum footing size for decks, by tributary area and soil bearing value
- IRC R507.5 — Post to footing connection requirements
- IRC R403.1.4 — Minimum depth of footings and frost protection
- IRC R301.5 — Residential live loads; R401.4.1 — presumptive soil bearing values
- AWC DCA-6 — Prescriptive residential wood deck construction guide
- Output reproduces published IRC table values at every data point checked
Related calculators
How this deck footing calculator works out its numbers
Tributary area is post spacing times half the joist span plus any cantilever for a ledger-attached deck, since the ledger carries the other half of every joist. For a freestanding deck with beams on both sides, it’s post spacing times the deck depth divided by the number of beam lines.
Load per post is tributary area times the total of live and dead load. Required bearing area is that load divided by the soil bearing capacity, and the diameter follows from the area of a circle, rounded up to the next even inch — which is how IRC R507.3.1 tabulates it and matches the form sizes actually sold. Twelve inches is applied as a floor, being the smallest practical size.
Concrete volume is the cylinder, plus a frustum for a flared base if selected, plus your waste allowance. The results also show actual bearing pressure against the allowable value, so you can see how much margin the rounded-up diameter gives you.
The IRC prescriptive method has limits. It assumes regular geometry, uniform undisturbed soil, and loads within the tabulated range. Fill, organic soil, steep sites, elevated decks and unusual framing are all outside it. So is any situation where you’re not confident the soil under the holes is what you think it is. In all of those cases the answer is a licensed engineer, not a calculator — and in every case, a permit and a footing inspection.