Calculator
Deck Footing Size Calculator
Every deck post carries the load of the framing around it, not just its own weight — a post's share is its tributary area, roughly half the joist span on each side times the post spacing along the beam, and that area is what actually decides how big the footing under it needs to be. Two footings on the same deck, at the same depth, in the same soil, can need genuinely different sizes if the posts above them aren't evenly spaced.
This calculator sizes a footing from that tributary area, the design load per square foot, and the soil's bearing capacity, then rounds up to a standard form-tube diameter and reports the concrete each one takes. It's built around a specific, easy-to-miss failure: widening the spacing between two posts to route around an obstacle, or to make a layout come out even, quietly increases the load every post in that wider bay carries — and the footing sized for the original spacing doesn't automatically get bigger with it.
Calculate your quantity
Ledger to beam. Half of it lands on the beam.
psf. 40 is the residential deck standard.
psf. Heavier for a tile or paver surface.
Below the local frost line.
Footing diameter
20 in
Load per post
2,400 lb
- Tributary area per post
- 48 sq ft
- Design load
- 50 psf
- Required bearing area
- 1.6 sq ft
- Exact diameter required
- 17.13 in
- Bearing pressure at this size
- 1,101 psf
- Concrete per footing
- 7.636 cu ft
- Concrete total
- 1.131 cu yd
- Footing size follows the load, and the load follows the tributary area — doubling the post spacing doubles the load on each footing, which is why moving one post to tidy a layout can quietly overload it.
- 42 in is a frost-depth assumption, not a code figure. Your local frost line governs.
Shopping summary
- • 4 x 20 in footings, 42 in deep
- • 51 x 80 lb bags of concrete mix
This is an estimate — confirm structural work with a professional.
How this calculation works
Tributary area is post spacing along the beam times half the joist span, because a joist spanning between a ledger and a beam delivers roughly half its load to each end — the beam picks up half, the ledger the other half. Multiply that area by the combined live and dead load per square foot and you have the load one post actually has to carry down into its footing.
Required bearing area is that load divided by the soil's bearing capacity — a lower-capacity soil needs more footing area to spread the same load, which is why footing size is sensitive to both the post's load and what's actually under it. The exact diameter that area implies is then rounded up to the nearest standard form-tube size, and the concrete volume for the depth you enter follows directly from that rounded diameter.
- Doubling post spacing along a beam doubles the tributary area and therefore doubles the load on each footing at that spacing — a linear relationship, not a marginal one.
- Bearing pressure is reported at the actual rounded footing size, not just the exact required size, so you can see how much margin (or how little) the standard size you're pouring actually has.
- Footing depth is treated as a separate input from diameter — depth is set by frost line, diameter by load — and the two don't trade off against each other in this calculation.
- Concrete volume scales with the SQUARE of diameter, so rounding up from a 20 in to a 24 in footing adds noticeably more concrete than the diameter increase alone suggests.
The formula
tributaryArea = postSpacing x (joistSpan ÷ 2); postLoad = tributaryArea x (liveLoad + deadLoad); requiredArea = postLoad ÷ soilBearing
- tributaryArea
- Post spacing along the beam times half the joist span — the floor area whose weight actually funnels down through one post, assuming the joist splits its load evenly between the beam and the ledger at its two ends.
- liveLoad + deadLoad
- Combined design load per square foot: live load is people, furniture and snow the deck has to support in use; dead load is the weight of the deck's own framing and decking material. Both are entered separately since they're set by different considerations.
- requiredArea
- Post load divided by the soil's bearing capacity, in square feet — the minimum footing footprint that spreads the post's load thinly enough for the soil beneath it to support without settling.
- soilBearing
- How much load per square foot the soil under the footing can support without excessive settlement — a presumptive value that varies enormously by soil type, from soft clay up through sand, gravel and rock, and is the single biggest lever in how large a footing a given post load needs.
- footing diameter
- The exact diameter required area implies, rounded up to the nearest standard form-tube size — footings are poured in stock tube diameters, not custom-cut to an exact square-footage figure.
Footing depth does not enter the bearing-area calculation at all — it's set separately by frost line, and only affects how much concrete a footing of the required diameter actually takes to pour. A footing can be the correct diameter for its load and still be the wrong depth for its climate; the two checks are independent.
Where these numbers come from
- 40 psf live load as the common residential deck default
- A design live load figure commonly required for residential deck construction in US model building codes, used here as a working default rather than cited from a specific table section. Confirm the live load your local code actually requires, since some jurisdictions or deck uses (a hot tub, for instance) call for a higher figure.
- Presumptive soil bearing values from 1,500 to 4,000 psf
- Commonly used presumptive load-bearing values by general soil class — silty or sandy clay, sandy clay, sand and gravel, sedimentary rock — absent a geotechnical report, of the kind found in residential code bearing-capacity tables. A soil test on your specific site can justify a different figure, and always takes precedence over a presumptive table value where one is available.
- Standard footing form-tube diameters from 8 to 36 in
- Commonly stocked cylindrical concrete form tube diameters sold for footing and pier work. Not every diameter in this list is available everywhere — confirm what your local supplier stocks before finalizing an order against the exact size this page rounds to.
- 0.6 cu ft yield per 80 lb bag of concrete mix
- Manufacturer-published average yield across major bagged concrete mix brands, used here to convert a footing's cubic-foot volume into a bag count. Yield can vary slightly between specific products, and is usually printed on the bag itself.
Worked examples
A whole deck at even 8 ft post spacing
| Joist span | 12 ft |
|---|---|
| Post spacing along the beam | 8 ft |
| Live load | 40 |
| Dead load | 10 |
| Soil bearing capacity | 1,500 psf (silt, clay - presumptive) |
| Posts | 4 |
| Footing depth | 42 in |
| Footing diameter | 20 in |
|---|---|
| Load per post | 2400 lb |
| Tributary area per post | 48 sq ft |
| Design load | 50 psf |
| Required bearing area | 1.6 sq ft |
| Exact diameter required | 17.13 in |
| Bearing pressure at this size | 1101 psf |
| Concrete per footing | 7.636 cu ft |
| Concrete total | 1.131 cu yd |
At 8 ft post spacing and a 12 ft joist span, each post carries 2,400 lb, and against 1,500 psf soil that works out to a 20 in footing — the exact required diameter was only 17.13 in, so this size carries a reasonable margin, landing the actual bearing pressure at about 1,101 psf against the soil's 1,500 psf capacity.
Four footings at this size and a 42 in depth take about 51 bags of concrete total. That per-post bag count is worth keeping in mind as a baseline for what an evenly spaced deck at this load and soil actually costs in footing concrete before the next example shows what happens when one bay gets widened.
One post moved to a 12 ft bay to clear an obstacle
| Joist span | 12 ft |
|---|---|
| Post spacing along the beam | 12 ft |
| Live load | 40 |
| Dead load | 10 |
| Soil bearing capacity | 1,500 psf (silt, clay - presumptive) |
| Posts | 1 |
| Footing depth | 42 in |
| Footing diameter | 24 in |
|---|---|
| Load per post | 3600 lb |
| Tributary area per post | 72 sq ft |
| Design load | 50 psf |
| Required bearing area | 2.4 sq ft |
| Exact diameter required | 20.98 in |
| Bearing pressure at this size | 1146 psf |
| Concrete per footing | 10.996 cu ft |
| Concrete total | 0.407 cu yd |
Widening just this one bay from 8 to 12 ft — a 50% increase in spacing to route around a tree or make a layout come out even — raises this post's own load to 3,600 lb, and the footing that requires jumps a full standard size, from 20 to 24 in. A builder who reuses a 20 in tube here because 'that's what the rest of the deck uses' would be pouring a footing rated for roughly a third less load than this specific post is about to carry.
That's precisely the quiet failure this page is built to catch: nothing about a widened bay looks different once the deck is built, but the footing underneath one post needs to be sized for that post's own tributary area, not for whatever spacing the rest of the deck happens to use. Running this page for every post whose spacing differs from the deck's typical bay is the reliable way to avoid it.
Common mistakes
- Sizing every footing on a deck to match the most common post spacing, without re-checking any post whose bay was widened or narrowed for layout reasons. That's exactly the scenario the second worked example on this page walks through.
- Using a presumptive soil bearing value without any real knowledge of the actual soil on site. Soft, wet, or fill soil can bear far less than a generic presumptive table assumes, and a geotechnical evaluation is the only way to know for certain on a site with any doubt about it.
- Confusing footing depth with footing diameter as if a deeper footing compensates for an undersized diameter. Depth is set by frost line, not by load — a footing poured to the correct frost depth in the wrong diameter is still undersized for its actual bearing area.
- Forgetting that dead load includes the decking material itself, not just the framing. A heavy paver or tile deck surface carries meaningfully more dead load than standard wood decking, and the field should reflect the actual finished surface, not just the joists and beams.
- Assuming footing concrete volume scales linearly with diameter. It scales with diameter squared, so a jump from a 20 to a 24 in footing takes noticeably more than 20% more concrete, even though the diameter itself only grew by that fraction.
Shopping summary
Order concrete by the bag count and footing tube size this page reports, per post — a deck with mixed post spacing genuinely needs mixed footing sizes, and ordering one size for the whole job risks under-sizing whichever post carries the widest bay.
Once footings are set, the deck board calculator sizes the decking itself, and if the deck includes a stair down to grade, the stair stringer calculator covers that separately structural piece.
FAQ
Why does moving one post to a wider spacing affect only that post's footing?
Because tributary area — and therefore load — is set by each post's own spacing to its neighbours, not by the deck's typical spacing. A post in a wider bay picks up more of the joist load on either side of it, while every other post's footing, still at the original spacing, doesn't change at all.
Should I always round up to the next standard footing diameter, or can I use the exact calculated size?
Round up — footings are formed with standard tube diameters, and pouring the exact calculated size (say, 17.13 in) isn't a real option most suppliers offer. Rounding up is also the conservative direction; it adds bearing margin rather than removing it.
How much does the design load matter compared to the soil bearing capacity?
Both matter proportionally — doubling either one roughly doubles the required footing area, since the formula divides load by bearing capacity directly. A site with poor soil and a heavy deck surface compounds both factors at once, which is worth checking explicitly rather than assuming either one alone is the binding constraint.
What if I don't know my soil's actual bearing capacity?
The presumptive values this page offers are a reasonable starting point absent a geotechnical report, chosen conservatively by general soil description. Where a project is borderline, unusually loaded, or on ground with visible signs of poor drainage or fill, a geotechnical evaluation is the reliable way to know rather than assume.
Does a bigger footing diameter always mean it's safer?
A bigger footing at the correct depth generally means more bearing margin, yes — but depth matters just as much for frost protection, and this page doesn't verify depth against your local frost line, only against the value you enter. A wide, shallow footing can still heave in a freeze-thaw climate regardless of how generously it's sized for load.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.