Half the span each way, and nothing else
Tributary area is the one idea you need and it is simpler than it sounds. Any support carries the load from halfway to the next support on each side. That is all. A joist resting on a ledger and a beam gives half its load to each. A post in the middle of a beam takes half the span to its left and half the span to its right. A post at the end of a beam has a span on one side and nothing on the other, so it takes half of one span, plus whatever the beam runs past it.
Work a 16 by 12 deck with a 2 foot cantilever and three posts. The joists span 10 feet between ledger and beam, so each support takes 5 feet of that. The beam also takes the whole 2 foot overhang, because the joists past the beam are levering down on it, so the beam tributary depth is 7 feet, not 6. Sixteen feet of beam at 7 feet deep is 112 square feet, and at 55 pounds per square foot that is 6,160 pounds coming down the beam line.
With one foot of beam past each end post, the three posts sit 7 feet apart. The end posts take 3.5 feet of span plus the 1 foot overhang, so 4.5 feet each. The middle post takes 3.5 feet each way, so 7 feet. That is 4.5, 7 and 4.5, which adds to 16, which is the beam. The middle footing carries 2,695 pounds and each end footing carries 1,733. The interior one is working 1.56 times as hard.
Why the corner intuition is backwards
People assume corners are hard-working because corners are where a structure feels most exposed, and because in a rectangular building the corner is where two walls meet and everything is stiffened. On a deck beam it is the opposite geometry. The corner post is the end of the line. There is deck on one side of it and air on the other. The post next to it has deck on both sides.
The practical consequence is that decks built with identical footings everywhere are over-built at the ends and, if anything went wrong in the sizing, under-built in the middle. It also means that the footing worth paying attention to when a deck starts to settle is usually not the one people look at first. And when someone adds a hot tub, the load lands where the tub is, which is a fourth possibility again.
The overhang past the end post pushes back against this a little. Every foot the beam runs past the last post adds a full foot of tributary length to that post, because there is no other support out there to share it. Run a beam three feet past the end post and the corner footing catches up with the interior ones. That is a design choice with consequences at the other end of it, since the beam is now cantilevering.
Where the rest of the deck goes
| Path | Carries | Set by |
|---|---|---|
| Ledger, or inner beam | Half the clear joist span | The distance between supports |
| Outer beam | Half the clear span plus all of the cantilever | How far the joists run past it |
| End post | Half a beam span plus the beam overhang | Post spacing and overhang |
| Interior post | A full beam span, half from each side | Post spacing |
| Footing | Everything above it, plus its own weight | Soil, frost depth, the building department |
On a deck with a ledger, roughly 45 to 55 percent of the whole deck load goes into the house wall and never touches a footing. That is not a small share, and it is the reason the deck ledger fastener calculator exists and why an unflashed or badly attached ledger is the failure that kills people. A freestanding deck moves that load onto a second beam and a second row of footings, which is why the calculator offers it as an option and doubles the footing count when you pick it.
What this page will not do
It will not size a footing. Footing size is a function of the load, which is what this page gives you, and of the allowable bearing pressure of the soil at the depth you are founding on, which it cannot know. The same 2,700 pound load needs very different footings on dense gravel and on soft clay, and in some soils the answer is not a bigger footing at all but a different foundation. Depth is a separate question again, driven by frost, and a footing that does not reach below the frost line will lift and drop with the seasons no matter how large it is.
The bearing pressure figure here is provided so you have a number to bring to the conversation, not so you can conclude anything from it. Take it, the loads and the deck drawing to the building department. In most places a deck needs a permit and the footings are inspected in the open excavation before concrete, precisely because nobody can check them afterwards.
Before any of that, get the utilities located. Call 811 in the United States, or the equivalent service where you are, and wait for the marks. Once you know how much concrete the holes take, the concrete footing calculator and the concrete slab calculator handle the volume, and the deck post height calculator works out how long each post has to be once the ground turns out not to be level. For the beam sizing side of the same problem, the header and beam load calculator produces the loads and reactions a span table wants.
Questions people ask
Which deck footing carries the most weight?
An interior footing, not a corner one, on any beam with three or more posts. The interior post takes half a span from each side, so a full span in total, while an end post takes half a span plus whatever the beam runs past it. With no beam overhang the interior footing carries almost exactly twice what a corner one does. Add an overhang and the gap narrows, since every foot past the last post is a foot only that post can carry. The exception is a footing with a concentrated load on it, like a hot tub corner or a post supporting a roof, which can beat any of them.
Why does the cantilever add to the beam and not split?
Because there is no support beyond it. Load between two supports divides between them, but load out past the last support has only one place to go, and it goes there entirely. So the beam tributary depth is half the clear joist span plus the whole cantilever, while the ledger gets half the clear span and none of the overhang. On a 12 foot deck with a 2 foot cantilever the beam takes 7 feet of depth and the ledger 5, a 58 to 42 split rather than an even one. Long cantilevers shift a surprising amount of load outboard.
Can I use the bearing pressure to pick a footing size?
No, and that is a deliberate limit rather than caution for its own sake. The pressure a footing puts into the ground is arithmetic, but whether the ground will accept it is a soil property that varies by site, by depth and by season, and the allowable value is set by the building department or by a soils investigation. Frost depth is a separate constraint again and often governs how deep you go regardless of load. Bring the load and the pressure figure to the permit conversation and let the people with jurisdiction set the size.
Does this include the weight of the footing and post?
No. It reports what the deck structure delivers to the top of each post. The concrete in the footing, the post itself and any pier above grade add to what the soil under the footing sees, and a large pier can be a few hundred pounds on its own. Concrete runs about 150 pounds per cubic foot, so a 16 inch diameter footing 12 inches thick is roughly 210 pounds. Add that in yourself if you are doing a bearing check, and remember it works in your favour for uplift and against you for settlement.
What about a hot tub or an outdoor kitchen?
They are exactly the case where a general calculator stops being useful. A filled hot tub with people in it is commonly several thousand pounds concentrated over a small footprint, which is far beyond what a normal deck live load allowance covers, and it usually needs its own posts, its own footings and often its own frame. The extra point load field here lets you see the number, but seeing the number is where this page stops. Loads like that belong with an engineer and with the building department before anything is framed.