Getting the numbers this calculator needs
A footing is only as good as the load estimate and soil value that go into it. Both take five minutes to get roughly right — get them wrong and the footing size on this page doesn't mean anything.
Find the load width and tributary run
Load Width is how much of the stair or landing this one post is holding up — usually the full width of a stair stringer set, or half the landing if two posts share it. Tributary Run is the horizontal distance the post carries in the other direction, typically half the span to the next post or to the ledger connection.
Confirm your live load
40 psf is the commonly used design live load for residential decks and exterior stairs. Snow load in your area, or a stair serving a commercial or high-occupancy use, can push this higher — check your local code before assuming 40 psf applies.
Get your real soil bearing value
The dropdown gives generic presumptive values by soil classification. These exist so you can plan before a geotech report exists — they are not a substitute for one. Your building department can often tell you the presumptive value they accept for your area, or require a soils report for anything non-trivial.
Get your local frost depth
Call your building department or check your jurisdiction's adopted frost-depth map. This number changes block by block in some regions and this calculator cannot know it for you — that's why it has no default.
The five formulas behind every result
Nothing on this page is a black box. Here's exactly what runs when you change a number.
Post Load (tributary method)
Design load is the combined live and dead load pressure multiplied by the tributary area this one post actually carries.
Required Footing Area
Post load divided by the soil's allowable bearing pressure. A weaker soil needs a proportionally bigger footprint to carry the same load.
Round Footing Diameter
Back-solving the circle-area formula for diameter, then rounded up to the next standard sonotube size — you can't buy a form in an arbitrary size.
Square Footing Side
The side length of a square footing with the same required bearing area, rounded up to the nearest whole inch.
Concrete Volume
Footprint area times footing depth gives the poured volume — converted to cubic yards and bag counts in the results.
Walking through a real deck stair post
Same formulas, filled in step by step, for a 4' wide deck stair post on clay soil in a 36" frost zone — the default this calculator loads with.
Post load
4' load width × 4' tributary run = 16 sq ft of tributary area. At 40 psf live load plus 10 psf dead load — 50 psf combined — the post load is 50 × 16 = 800 lb. This is the total weight this one post has to transfer into the ground.
Required footing area
800 lb of post load divided by a 1,500 psf clay soil bearing value: 800 ÷ 1,500 = 0.53 sq ft. This is a small footing — typical for a single deck stair post, which is why so many get built undersized without ever failing visibly.
Round footing diameter
Solving D = √(4 × 0.53 ÷ π) gives roughly 9.8", which rounds up to the next standard sonotube size of 10". Standard forms come in fixed increments — you always round up, never down, even if the math lands just under a size break.
Depth, and where frost overrides the code minimum
Code-minimum embedment is 12", but the local frost depth of 36" governs here since it's the deeper requirement — 36" wins. This is the single most common way a small, correctly-sized-diameter footing still fails: the diameter math and the depth requirement are two separate checks, and depth is driven entirely by frost, not by load.
This is exactly why a footing can pass the area/diameter check and still fail inspection — frost depth and code-minimum embedment are independent of load and soil bearing, and the deeper of the two always governs. Enter your own numbers above; this worked example uses the page's illustrative defaults, not universal values.
Presumptive soil bearing values, by classification
These are generic starting points, not site-specific facts. A real soils report always overrides a presumptive table value.
| Soil Classification | Description | Presumptive Bearing |
|---|---|---|
| Crystalline bedrock | Solid rock, minimal excavation | [VERIFY] 12,000 psf |
| Sedimentary rock | Layered rock, shale, sandstone | [VERIFY] 4,000 psf |
| Sandy gravel / gravel | Well-drained, coarse | [VERIFY] 3,000 psf |
| Sand, silty/clayey sand or gravel | Moderately drained | [VERIFY] 2,000 psf |
| Clay, sandy clay, silty clay | Poor drainage, frost-susceptible | [VERIFY] 1,500 psf |
| Unknown / uncompacted fill | No soils report available | Do not assume — get tested |
[VERIFY: table values are typical IRC-style presumptive load-bearing classifications and are provided for planning only. Confirm your soil classification and its accepted bearing value with your local building department or a geotechnical engineer before finalizing any footing design — presumptive tables are explicitly a fallback for when no soils report exists, not a substitute for one on anything beyond the smallest, lowest-risk structures.]
Footing depth & embedment, quick reference
This calculator checks your result against the two depth rules below — whichever requires more depth governs.
| Requirement | Typical Value | Notes |
|---|---|---|
| Code-minimum embedment | [VERIFY] 12" | Below undisturbed grade, applies even in frost-free climates. |
| Frost-depth embedment | Local — no default | Bottom of footing must sit below the frost line for your jurisdiction. |
| Minimum footing thickness | [VERIFY] 6"–8" | Below post base or pier, separate from total embedment depth. |
| Post-to-footing connection | Rated hardware required | Embedded posts and surface-mount brackets have different requirements — see deep-dive below. |
Reference only — always confirm against your local jurisdiction's adopted code before building. Footing depth requirements are one of the most frequently amended provisions at the local level.
Frost heave, soil variability & post attachment
A footing can pass every diameter and depth check on this page and still fail — for reasons the load math alone can't catch.
Why frost heave happens
Frost-susceptible soils (clay and silt especially) expand when the water in them freezes. A footing bottom sitting above the frost line gets pushed upward over repeated freeze-thaw cycles, which is why frost depth — not load — usually sets the minimum footing depth in cold climates.
Soil bearing isn't uniform
The same soil classification can vary meaningfully across a single lot, especially near fill, old foundations, or slopes. Presumptive tables assume undisturbed, native soil at a reasonable depth — disturbed or fill soil needs its own investigation, not a table lookup.
Embedded post vs. surface-mount base
A post set directly into the concrete carries lateral (sideways) load differently than a post sitting on a surface-mounted metal base bolted to the footing. Surface-mount bases generally need a rated connector; embedded posts need rot/decay-resistant treatment at the concrete interface. This calculator sizes the footing, not the connection — check hardware capacity separately.
Adjacent structure conflicts
A new stair footing dug next to an existing house foundation or another footing can undermine it if it's shallower and too close. As a general planning rule, keep footing separation at least equal to the difference in their depths — verify against your local code for the exact required setback.
These are planning-stage estimates, not a stamped structural design. Any footing supporting more than a simple deck stair, or built on marginal/unknown soil, should get a geotechnical and/or structural engineer's sign-off regardless of what this calculator shows.
Common mistakes when sizing stair footings
Most footing failures — heaving, cracking, or a failed inspection — trace back to one of these five planning errors, not to bad concrete work.
Assuming a presumptive soil value without checking
Table values are a fallback for planning, not a substitute for what your building department actually requires or what a soils report would show on marginal ground.
Sizing depth off the code minimum only
12" of code-minimum embedment means nothing if your frost line sits at 36". Frost depth and code minimum are separate checks — the deeper one always governs.
Rounding the footing diameter down
A calculated 9.8" diameter needs a 10" form, not an 8" one. Standard sonotube sizes come in fixed increments — always round up to the next available size, never down to save concrete.
Averaging load evenly across every post
Corner and end posts, or posts closer to a heavier span, often carry more than an interior post. Using one blended tributary number across all posts can undersize the ones actually doing more work.
Sizing the footing but ignoring the post connection
A correctly sized footing with an unrated post base or a rotting embedded post still fails structurally. The connection hardware has its own load rating, separate from the concrete size.
When to use a stair footing size calculator
Footing sizing shows up early in almost every exterior stair or deck project, before any lumber gets ordered.
Deck Stair Posts
Sizing footings for the posts that support a deck stair independent of the main deck frame.
Exterior Entry Stairs
Freestanding stairs at a side or back door not attached to an existing porch or slab.
Replacing Heaved Footings
Resizing and re-digging a footing that failed because it was originally poured above the frost line.
Permit & Plan Sets
Documenting footing dimensions and depth for a plan check or inspector before digging.
Freestanding Stair Structures
Stairs that aren't attached to a house wall and carry their full load through independent posts.
Sloped or Uneven Grade Sites
Estimating footing sizes at each post individually when grade varies across the stair's footprint.