Deck Post & Footing Calculator
Work out how many posts a deck needs, how far apart they land, the tributary area each footing carries, and how many bags of concrete go in the holes.
Post spacing comes from the beam, not from taste
On a fence you can put the posts wherever you like the look of them. On a deck you cannot. The posts hold up a beam, and the beam has a maximum span for its species, grade and size — so the post spacing is whatever that span allows. Put the posts further apart than the beam can reach and the deck sags between them, however much concrete you pour underneath.
Those beam spans are prescriptive, and they live in the tables in IRC Section R507. Read the span for your beam off the table, put that number in the post spacing box, and this page will lay the posts out under it.
Whatever number you settle on, the posts get divided evenly along the length rather than set at that exact figure with a leftover bay at the end. Twenty feet at a maximum of eight comes out as three bays of 6 ft 8 in, not two bays of 8 ft and a stub. Even bays are what make the deck look right from the house, and the beam is happiest on even spacing too. And the post count is one more than the number of bays, because there is a post at each end as well as between every pair — three bays, four posts.
What each footing actually carries
A footing is not sized by the post on top of it. It is sized by the patch of deck that post is holding up, and that patch has a name: the tributary area.
An inside post takes half a bay from the left and half a bay from the right, which adds up to one full bay width. Multiply that by the joist span — the distance from the house to the beam — and you have the area. On a deck with 6 ft 8 in bays and a 12 ft joist span, each inside footing carries 80 sq ft of deck.
The two end posts are different. An end post has deck on one side only, so it carries half of that: 40 sq ft. That is why the end footings are often the same size as the rest even though they are doing less work — the saving from shrinking them is not worth the extra trip for a different tube.
Tributary area is the number that matters, because it is the number the prescriptive footing table is indexed by. Work it out here, take it to the table, and you get a footing size back.
Sizing the footing
The footing exists to hand the load to more soil than the post would touch on its own. A post is a few square inches of bearing; a footing is a couple of square feet of it. That is the entire trick — spread the same weight over more ground and the ground stops sinking under it.
The prescriptive tables in IRC Section R507 give the footing size from two inputs: the tributary area you just worked out, and the height of the post. Take both to the table and it returns a diameter. Deep frost country often pushes that up a size, and a deck carrying something heavy — a hot tub, a masonry outdoor kitchen — is outside the tables altogether and wants an engineer.
Two things the table assumes, and both of them matter more than the diameter does:
- The footing bears on undisturbed soil. Not fill, not the loose material that fell back into the hole, not a puddle of mud at the bottom. A footing sitting on disturbed ground settles, and the deck goes down with it.
- The hole is wider than the footing. The concrete needs room around it, and the tube has to come back out. Digging the hole a size up is normal; what is not normal is counting the loose soil around the tube as bearing surface, because it is not.
Where a footing is not needed at all: on solid rock, a footing is often just a levelled bearing surface. And on a deck where a poured footing is awkward to reach, a precast concrete pier or a helical ground screw does the same job without mixing anything — check what your building department accepts before you commit to either.
Concrete: how much goes in the hole
A footing is a cylinder standing on end, so the volume is π × radius² × depth. The measurements go in as inches, because that is how footings are described, and the answer is divided by 1728 — twelve cubed — to come back in cubic feet.
Then divide by the yield of one bag. Those yields are printed on the bag, and across the ordinary concrete mixes they do not vary:
| Bag | Yield | Bags to a cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Round the bag count up, always. You cannot buy two thirds of a bag, and running short part way through a footing leaves a cold joint down a hole where you cannot see it.
Concrete chart for common footings
Real outputs from the calculator, for an 80 lb bag. Diameter and depth are the two numbers you control; the last two columns are what they cost you in mixing.
| Footing | Concrete | 80 lb bags | What it is for |
|---|---|---|---|
| 10 in dia × 36 in | 1.64 cu ft | 3 | A small deck on good ground, above the frost line |
| 12 in dia × 36 in | 2.36 cu ft | 4 | The default in a lot of footing tables |
| 12 in dia × 42 in | 2.75 cu ft | 5 | The same footing, dug below a moderate frost line |
| 12 in dia × 48 in | 3.14 cu ft | 6 | Deep frost — 4 ft is a common northern figure |
| 16 in dia × 48 in | 5.59 cu ft | 10 | A bigger tributary area, or softer ground |
| 18 in dia × 48 in | 7.07 cu ft | 12 | Heavy load, or a deck carrying more than people |
That difference decides where your money goes, so it is worth sitting with. Widening a footing spreads the load over more soil, which is the whole job. Deepening it does not spread the load at all — it only buys frost protection. Dig deeper when the frost line says to and wider when the load says to, and do not pay for one when what you need is the other.
Worked example: a 20 ft deck
Take a 20 ft deck, a 12 ft joist span from the house to the beam, posts no more than 8 ft apart, 6 × 6 posts, 12 in footings 42 in deep, 80 lb bags and 10% waste.
Twenty feet at an 8 ft maximum comes out as 3 equal bays of 6 ft 8 in, so 4 posts and 4 footings. Each inside footing carries one bay width times the joist span — 6 ft 8 in × 12 ft — which is 80.0 sq ft. The two end footings carry 40.0 sq ft each.
Take that 80 sq ft and the post height to the footing table, and suppose it returns a 12 in footing. The hole has to go below the frost line, so 42 in deep. The volume is π × 6² × 42, which is 4,750 cu in or 2.75 cu ft — 5 bags of 80 lb mix per footing.
All four footings take 11.00 cu ft. Add 10% for spillage and for holes dug a size too wide and that is 12.10 cu ft, or 0.45 cu yd — 21 bags of 80 lb mix.
At that quantity, bags are the right buy. Twenty-one bags is a little over half a cubic yard, and a ready-mix truck either will not come out for that or will charge a short-load fee that costs more than the concrete does. You also get to pour each footing as you dig it, which is how a deck actually gets built: one hole at a time over a few weekends, not four at once with a truck waiting at the kerb.
Does the building code cover deck footings?
Yes — and that is the difference between a deck and a fence. A garden fence is not covered by the International Residential Code at all. A deck is. Posts, beams, joists, footings and the connections between them are all prescriptive in IRC Section R507, and the rule that footings bear below the frost line is in Section R403.
What that means in practice is that a deck has a right answer, and the inspector already knows what it is. The tables in R507 give you a beam span from your lumber, and a footing size from your tributary area and post height. Neither table is reproduced on this page, and that is deliberate — they are indexed by species, grade, beam size and post height, and reciting them from memory would be worse than not reciting them at all. Get them from your lumberyard, your building department, or the code book itself. All three are free to look at.
Two things on this page are requirements rather than trade practice, and both are worth knowing before you dig:
- Footings bear below the frost line. That is Section R403, and the depth is a local number your building department publishes — nothing on the Gulf coast, 48 in or more across the northern Plains and New England. This is not about strength, it is about heave: ground that freezes and thaws under a footing lifts the footing, and the deck goes up with it. No amount of concrete prevents that, because the whole footing is moving.
- Posts are connected to their footings. A post standing on a footing is held down by gravity and nothing else, and a deck is a large flat sail. Uplift is what takes decks off their footings, and a post base is the only thing that stops it.
What is not in the code is the ground the footing sits on. Fill, soft clay, a seasonal watercourse, an old stump hole — none of that is in a table, and all of it decides whether the footing holds. If the soil is anything other than undisturbed native ground, say so at the counter when you pull the permit and let someone look at it. That conversation is free; the alternative is a deck that keeps settling.
Frequently asked questions
How far apart should deck posts be?
It is not a free choice — it is whatever your beam can span. Beam spans are tabulated by species, grade and size, in the tables in IRC Section R507. Look up the span for the beam you are actually using and treat that as your maximum post spacing, then divide the deck length into equal bays at or under it so the posts come out even. Plenty of decks land at 8 ft because 8 ft is a common beam span, but that is the result, not the rule.
How big should a deck footing be?
Take the tributary area — one bay width times the joist span — and the post height, and look both up in the prescriptive footing table in IRC Section R507. The table returns a diameter. A typical deck lands somewhere between 12 in and 18 in across, and deep frost country often pushes it up a size. If the deck is carrying something heavy, like a hot tub or a masonry fireplace, it is outside those tables and you want an engineer rather than a bigger number.
How many bags of concrete do I need for deck footings?
Work out one footing and multiply. A 12 in footing 42 in deep is 2.75 cu ft, which is 5 bags of 80 lb mix. An 80 lb bag yields about 0.60 cu ft, a 60 lb bag about 0.45 and a 40 lb bag about 0.30, and 45 bags of 80 lb make a cubic yard. Add 10% for spillage and for holes dug a size too wide, then round up: you cannot buy part of a bag, and running short part way through a footing leaves a cold joint you will never be able to inspect.
How deep should deck footings be?
Below the frost line, which is a local figure. It runs from essentially nothing on the Gulf coast to 48 in or more across the northern Plains and New England, and your building department publishes it. IRC Section R403 requires footings to bear below that line. The reason is heave rather than strength: ground that freezes under a footing lifts it, and the deck goes up with it. A footing above the frost line that has survived a few mild winters has not passed anything — it simply has not been tested yet.
Do I need concrete at all for a deck footing?
Not always. On solid rock a footing is often just a levelled bearing surface. And a precast concrete pier or a helical ground screw does the same job as a poured footing without mixing anything — the ground screw especially is fast, works in tight access and can be taken out again. What matters is that whatever bears on the soil is wide enough for the tributary area and deep enough for the frost. Check what your building department accepts before you commit to an approach.
How many posts does a deck need?
One more than the number of bays. Divide the deck length into equal bays at or under your maximum spacing — 20 ft at an 8 ft maximum gives three bays of 6 ft 8 in — and that is four posts, because there is a post at each end as well as between every pair of bays. It is the same fencepost problem as a fence, and it catches people out in exactly the same way.
What is tributary area?
The patch of deck that one post is holding up. An inside post takes half a bay from the left and half from the right, which comes back to one full bay width; multiply that by the joist span and you have the area. On 6 ft 8 in bays with a 12 ft joist span it is 80 sq ft. The two end posts have deck on one side only, so they carry half of that. Tributary area is the number the prescriptive footing table is indexed by, so it is worth working out before you go looking for a footing size.
Does a deck need a permit and an inspection?
Almost always, yes. A deck is a structure attached to a house with people standing on it, and it is covered prescriptively by IRC Section R507 — which is exactly the kind of thing permits exist for. Footings are the part inspectors look at hardest and the part you cannot fix afterwards, because they end up underground. Call your building department before you dig rather than after, and ask what they want to see. Many of them want to look at the holes before you pour, and that inspection costs you a day and saves you a deck.