Deck Footing Calculator

Deck Footing Calculator lays out posts on each beam of your deck, sizes every footing from IRC Table R507.3.1 for your soil, and counts concrete bags, gravel and dig depths needed.

What the Deck Footing Calculator Sizes

Most footing tools stop at one number for one post. This deck footing calculator lays out the whole deck first. It places posts on each beam line, works out the load on every post, and sizes each footing from the IRC table for your soil.

Planning a new deck? Enter its length, width, beam layout, post spacing and soil, and the deck footing calculator counts the footings and sizes each one.

Checking one post on a deck you already have? Enter the deck area that post carries, or its load in pounds, kilograms or kilonewtons. The load option suits a hot tub, a stair landing or a figure from an engineer.

Deck Footing Size Math

A footing must spread the post load over enough soil that the soil does not sink. The first step is the post load, which is the deck area the post carries times the design load per square foot.

$$P=A_t\times (D+L)$$

A_t is the tributary area in square feet, D is the dead load and L is the live load or the ground snow load, whichever is higher. The IRC uses 10 psf dead and 40 psf live for decks, so the deck footing calculator starts every deck at 50 psf.

The footing area then comes from the allowable bearing pressure of the soil under the post, which the deck footing calculator takes from your soil choice.

$$A_f=P/q$$

Here q is the soil bearing value in psf. A round footing needs a diameter, in inches, that gives the same area.

$$d=12\times \sqrt{4A_f/\pi }$$

A 2,000 lb post on 1,500 psf clay needs 1.33 sq ft of bearing, which is a 15.6 in circle. The IRC table rounds that to a 16 in round or a 14 × 14 × 6 in square footing.

How the Tool Finds Each Post Load

The deck footing calculator works the post loads out from your layout. Each beam carries a strip of deck that runs halfway to the next support.

When joists run past the outer beam, the beam picks up more than half the span. The tool uses the support reaction for a span with an overhang, the same reaction you would get from a beam formula.

$$R=w\times (s/2+c+c^2/(2s))$$

Here s is the joist span, c is the overhang past the beam and w is the load per foot. The same rule applies along each beam, where the end posts carry the beam overhang.

That is why end posts often get a smaller footing than middle posts, and why the deck footing calculator reports how many footings need the largest size.

Deck Footing Size Chart From IRC Table R507.3.1

Table R507.3.1 of the IRC sets the minimum size for deck footings. The rows below are its 40 psf block, from the Town of Lancaster, NY copy of Section R507.

Each cell gives the round diameter first, then the square side and thickness of the pad option.

Tributary Area	1,500 psf Soil	2,000 psf Soil	2,500 psf Soil	3,000 psf or More
20 sq ft	14 in · 12 × 12 × 6	14 in · 12 × 12 × 6	14 in · 12 × 12 × 6	14 in · 12 × 12 × 6
40 sq ft	16 in · 14 × 14 × 6	14 in · 12 × 12 × 6	14 in · 12 × 12 × 6	14 in · 12 × 12 × 6
60 sq ft	19 in · 17 × 17 × 6	17 in · 15 × 15 × 6	15 in · 13 × 13 × 6	14 in · 12 × 12 × 6
80 sq ft	22 in · 20 × 20 × 7	19 in · 17 × 17 × 6	17 in · 15 × 15 × 6	16 in · 14 × 14 × 6
100 sq ft	25 in · 22 × 22 × 8	21 in · 19 × 19 × 6	19 in · 17 × 17 × 6	17 in · 15 × 15 × 6
120 sq ft	27 in · 24 × 24 × 9	23 in · 21 × 21 × 7	21 in · 19 × 19 × 6	19 in · 17 × 17 × 6
140 sq ft	29 in · 26 × 26 × 10	25 in · 22 × 22 × 8	23 in · 20 × 20 × 7	21 in · 18 × 18 × 6
160 sq ft	31 in · 28 × 28 × 11	27 in · 24 × 24 × 9	24 in · 21 × 21 × 8	22 in · 20 × 20 × 7

The table allows interpolation between rows and between soil columns, but not extrapolation past them. The deck footing calculator interpolates both ways and rounds up, so a post carrying 34.3 sq ft on clay gets a 16 in round footing.

A soils report value such as 1,800 psf is read between the 1,500 and 2,000 psf columns. Past 160 sq ft, or below 1,500 psf, the deck footing calculator sizes the footing from A = P ÷ q and keeps the pad at least as thick as it projects past the post.

Soil values follow the presumptive bearing classes in the IRC. Clay, silt and unknown soil use 1,500 psf, sand and silty or clayey gravel use 2,000 psf, and sandy gravel or gravel uses 3,000 psf.

Sedimentary rock at 4,000 psf and crystalline bedrock at 12,000 psf both read the 3,000 psf or more column. Pick 2,500 psf in the deck footing calculator only when a soils test backs it up.

Deck Footing Sizes Under Snow Loads

The IRC table also has blocks for 50, 60 and 70 psf of live or ground snow load. A snowy region raises every post load, so the footings grow. Round diameters for 1,500 psf soil look like this.

Tributary Area	40 psf	50 psf	60 psf	70 psf
20 sq ft	14 in	14 in	14 in	14 in
40 sq ft	16 in	17 in	19 in	20 in
60 sq ft	19 in	21 in	23 in	24 in
80 sq ft	22 in	24 in	26 in	28 in
100 sq ft	25 in	27 in	29 in	31 in
120 sq ft	27 in	30 in	32 in	34 in
140 sq ft	29 in	32 in	35 in	37 in
160 sq ft	31 in	34 in	37 in	40 in

Enter your ground snow load under "Snow, Dead Load & Overhangs." With the standard 10 psf dead load, the deck footing calculator reads the next IRC block up from your snow load.

Snow from 41 to 50 psf uses the 50 psf block, 51 to 60 psf uses the 60 psf block, and 61 to 70 psf uses the 70 psf block. Rounding up to the next block keeps every footing on the safe side.

Above 70 psf, or with a dead load other than 10 psf, the IRC table no longer applies, so the deck footing calculator sizes footings by bearing area. Ask your building department for the ground snow load, since it is set locally.

How Many Footings a Deck Needs

In the deck footing calculator, the footing count is the number of beams times the posts on each beam. Posts per beam come from the beam length between the end posts and your largest allowed post spacing.

$$N=(\lceil (L-2c_b)/S\rceil +1)\times B$$

L is the deck length along the beam, c_b is the beam overhang past each end post, S is the max post spacing and B is the number of beams.

A deck on a house ledger usually needs one beam. A freestanding deck needs a beam on the house side too, which doubles the count.

The chart below uses the deck footing calculator defaults. That means an 8 ft max post spacing, a 1 ft beam overhang, a 1.5 ft joist overhang, 1,500 psf soil and 36 in deep round piers. The bag count is for the ledger layout with 80 lb bags and 10% waste.

Deck, Along Beam × Out	Footings, Ledger	Footings, Freestanding	Post Spacing	Largest Footing	80 lb Bags, Ledger
10 × 10 ft	2	4	8 ft	15 in round	17
10 × 12 ft	2	4	8 ft	16 in round	17
12 × 10 ft	3	6	5 ft	15 in round	25
12 × 12 ft	3	6	5 ft	16 in round	25
12 × 16 ft	3	6	5 ft	17 in round	27
16 × 12 ft	3	6	7 ft	18 in round	27
16 × 16 ft	3	6	7 ft	20 in round	34
20 × 12 ft	4	8	6 ft	17 in round	37
20 × 16 ft	4	8	6 ft	18 in round	37
20 × 20 ft	4	8	6 ft	20 in round	46

Fewer footings means more load on each one. On a 16 × 16 ft deck with an 8 ft max spacing, the beam gets three posts 7 ft apart. Each middle post carries about 62 sq ft and needs a 20 in footing.

Drop the max spacing to 6 ft and the beam gets four posts 4.67 ft apart. Each middle post then carries 41 sq ft and needs only a 17 in footing.

Beam size sets the real limit on spacing, since a longer span between posts needs a deeper beam under IRC Table R507.5. The Wood Beam Span Calculator finds how far a beam can reach between posts.

The Deck Post Spacing Calculator then lays out the posts along that beam. For joist size and spacing, the Floor Joist Calculator checks the span from ledger to beam.

The Beam Load Calculator then checks whether that beam can carry the deck load over the span between those posts.

Overhangs have code limits too. A joist may not reach past its beam by more than one-fourth of its span.

A beam may not run past its end post by more than one-fourth of the next beam span. The deck footing calculator warns you when either overhang goes over its limit.

Post height has code limits as well. IRC Table R507.4 lets a 4×4 post reach 8 ft under a one- or two-ply beam, but only 6 ft 9 in under a three-ply beam on a post cap.

A 4×6 post can reach 8 ft, and a 6×6 or 8×8 can reach 14 ft, all measured to the underside of the beam. Enter your post size and height, and the deck footing calculator flags any post over its limit.

Each post also needs lateral restraint where it meets the footing. That means a manufactured post base or at least 12 in of embedment in soil or a concrete pier.

Worked Example: a 12 × 12 ft Deck on a Ledger

The deck footing calculator opens with a 12 × 12 ft deck on a house ledger, one beam, 8 ft max post spacing and clay soil. The design load is 10 psf dead plus 40 psf live, which is 50 psf. The 30 psf default snow load is lower, so it does not govern.

The joists run 12 - 1.5 = 10.5 ft from the ledger to the beam, then overhang 1.5 ft. The beam carries 10.5 ÷ 2 + 1.5 + 1.5² ÷ (2 × 10.5) = 6.86 ft of deck. The beam runs 12 - 2 × 1 = 10 ft between end posts, so it takes three posts at 5 ft on center.

The middle post carries 5 × 6.86 = 34.3 sq ft, or 1,714 lb at 50 psf. On 1,500 psf soil it needs 1.14 sq ft of bearing.

The IRC table turns that into a 16 in round or a 14 × 14 × 6 in square footing. Each end post carries 24.7 sq ft and 1,234 lb, so it needs a 15 in round.

As full-depth piers 36 in deep with 2 in above grade, the three footings take 14.59 cu ft of concrete with 10% waste.

That is 25 bags at 80 lb, plus 1.4 cu ft of gravel for a 4 in base. Each hole is dug 40 in deep, and the deck corners should measure 16.97 ft on the diagonal.

Deck Footing Depth, Gravel and Concrete

Deck footings must sit at least 12 in below undisturbed ground and below the local frost depth. The frost depth is set by your building department, and in northern states it can reach 4 ft or more.

The deck footing calculator defaults to 36 in and flags anything under 12 in. The Fence Post Depth Calculator uses the same frost logic for fence posts.

Gravel is optional in the deck footing calculator. The IRC table sizes the concrete bearing on soil, so a gravel base does not shrink the footing.

A few inches of tamped crushed stone gives a firm, level bottom in a muddy hole. The calculator adds that depth to the dig depth and reports the gravel volume for all holes.

Concrete depends on the footing style. "Round Pier, Full Depth" fills a form tube from the hole bottom to the pier top.

The two pad styles pour a square pad at the IRC thickness, then a 10 in or 12 in tube up to grade, which uses less concrete on wide footings. The tool picks the next stock tube size, such as 16 in for a 15 in footing.

The deck footing calculator counts bags at 0.60 cu ft for an 80 lb bag, 0.45 cu ft for 60 lb and 0.375 cu ft for 50 lb. The waste allowance covers spillage and uneven holes.

For tube volume on its own, the Sonotube Calculator handles any tube size. The Concrete Footing Calculator covers wall and slab footings.

Squaring the Deck Footing Layout

The fourth result card in the deck footing calculator gives the square-up diagonal, which is the corner-to-corner length of the deck outline.

For a 12 × 12 ft deck it is 16.97 ft, or about 16 ft 11-5/8 in. When both diagonals of your string lines match that number, the layout is square and the footing holes will line up under the beam.

The 3-4-5 rule does the same check on one corner. Measure 3 ft along one line and 4 ft along the other, and the two marks should sit exactly 5 ft apart.

The 3-4-5 Rule Calculator works this out for any side lengths and shows how far a corner is out of square.

Deck Footing Answers by Size, Soil and Layout

The answers below use the deck footing calculator defaults unless stated. That means a ledger deck with one beam, 8 ft max post spacing, 1,500 psf soil and 50 psf design load.

How do you calculate deck footings?

Start with the deck area each post carries, which runs halfway to the next post and halfway to the next support. Multiply it by the design load to get the post load.

Divide the post load by the soil bearing value to get the footing area, then turn that area into a diameter. The deck footing calculator runs these steps for every post and checks each result against IRC Table R507.3.1.

How do you calculate the size of a footing?

Divide the load on the footing by the allowable soil pressure. A 3,000 lb load on 2,000 psf sand needs 1.5 sq ft, which is a 16.6 in circle.

Round that up to a 17 in round or a 15 in square footing. The IRC table gives the same 17 in round and 15 × 15 × 6 in square at that load.

How does the deck footing calculator find the number of footings?

It splits each beam into equal bays no longer than your max post spacing, then adds one post. That post count is multiplied by the number of beams in your layout.

A 20 ft deck with 1 ft beam overhangs has 18 ft between end posts. At an 8 ft max that makes three 6 ft bays and four posts, so a ledger deck gets four footings and a freestanding deck gets eight.

How many footings for a 12x12 deck?

A 12 × 12 ft deck on a ledger needs three footings on one beam, set 5 ft apart. A freestanding 12 × 12 ft deck needs six, because it also needs a beam on the house side.

In the deck footing calculator, the middle footing comes out at 16 in round and the two end footings at 15 in. The middle post carries the most deck, which is why it gets the larger size.

How many footings for a 20x20 deck?

A 20 × 20 ft deck on a ledger needs four footings on one beam at 6 ft on center, or eight if it is freestanding.

The middle posts each carry about 65 sq ft, which takes a 20 in round footing on clay. Picking Ledger + 2 Beams in the deck footing calculator halves each joist span. That adds four footings, but each one gets smaller.

How many footings for a 12x16 deck?

Either way, a 12 × 16 ft deck on a ledger needs three footings. With the 16 ft side along the house, posts sit 7 ft apart and the largest footing is 18 in round.

With the 12 ft side along the house, posts sit 5 ft apart and the largest is 17 in, because the joists span farther.

How many posts do I need for a 10x12 deck?

With the 10 ft side along the house, two posts at 8 ft apart carry the beam. With the 12 ft side along the house, the beam needs three posts at 5 ft apart. Choose a freestanding layout in the deck footing calculator and both counts double.

How many support posts for a 10x10 deck?

A 10 × 10 ft deck on a ledger needs two posts and two footings, set 8 ft apart with a 1 ft beam overhang at each end. Each footing carries about 30 sq ft and needs a 15 in round. A freestanding 10 × 10 ft deck needs four posts.

How deep should deck footings be?

At least 12 in below undisturbed ground, and deeper where the frost line is lower. In northern states such as Minnesota that can mean 4 ft or more.

The IRC exempts freestanding decks from the frost line rule, but many local codes do not, so check with your building department first.

Do I need footings for a ground-level deck?

A small freestanding deck can skip them. The IRC lets it rest on precast pier blocks at grade when it has no beams or posts and covers 200 sq ft or less.

Its walking surface also has to stay within 20 in of grade near every edge. Any deck attached to the house still needs real footings.

How far apart should deck footings be placed?

Footing spacing follows the beam, and the beam span table in IRC R507.5 sets the longest allowed span for each beam size and species. The deck footing calculator spreads posts evenly, so an 8 ft max on a 12 ft beam gives 5 ft spacing rather than 8 ft and 2 ft.

What is a good default post spacing?

The deck footing calculator starts at 8 ft, and you can set anything from 2 to 20 ft. A shorter spacing lets you use a smaller beam and smaller footings, while a wider one needs a deeper beam. Check your beam against the IRC span table before you widen it.

What is the 3/4/5 rule for a deck?

It is a way to make a corner exactly 90°. Mark 3 ft on one side and 4 ft on the other, and the diagonal between the marks must be 5 ft. It works because 3² + 4² = 5², the Pythagorean theorem.

What is the 6 8 10 rule?

It is the 3-4-5 rule doubled. Measure 6 ft and 8 ft along the two sides, and the diagonal should read 10 ft. The longer sides give a more accurate check on big decks, and 9-12-15 works the same way.

How much gravel do I need for deck footings?

Multiply the hole area by the gravel depth. A 4 in base under a 16 in round hole takes about 0.47 cu ft, so three holes need about 1.4 cu ft. The deck footing calculator works this out for every hole, using the pad size for square footings.

Do I need gravel under deck footings?

Gravel is not required. Footings must bear on undisturbed soil below the frost line, and the IRC table sizes them that way.

A tamped gravel base helps in wet or loose soil by giving the concrete a firm, level bottom. Set it to 0 in the deck footing calculator if you pour onto firm soil.

What type of gravel works best for deck footings?

Angular crushed stone works better than rounded pea gravel. The sharp edges lock together when tamped, while round stones roll under load. Keep the layer thin and tamp it hard so it does not settle.

Can I change the footing diameter?

You can always go bigger than the IRC minimum, but never smaller. The deck footing calculator rounds each footing up to the next stock tube size.

A 15 in footing, for example, pours in a 16 in tube. A square pad option sits beside every round size if you would rather form a pad.

What does soil bearing capacity mean?

It is the pressure the soil can carry without sinking, in pounds per square foot. If you do not know your soil type, pick 1,500 psf in the deck footing calculator, the IRC value for clay and silt.

The IRC gives 2,000 psf for sand and 3,000 psf for gravel. Rock rates higher, at 4,000 psf for sedimentary rock and 12,000 psf for crystalline bedrock.

What factors affect concrete for footings?

Footing diameter, hole depth and pier height above grade decide the volume. Deep frost lines add the most concrete, because a full-depth pier fills the whole hole.

A square pad with a 10 in tube uses about half the concrete of a full-depth pier at the same footing size, or less on wider footings.

Why do beam and joist sizes matter?

They set how far apart the posts can go and how much deck each post carries. Longer joist spans push more load onto the outer beam, and longer beam spans push more load onto each post. Both make the footings bigger.

Does the deck footing calculator follow IRC code?

It follows the IRC for footing size. It reads Table R507.3.1 at 10 psf dead load for live or snow loads up to 70 psf, and it checks the 12 in minimum depth, the cantilever limits and the post height limits.

It does not size beams or joists, which the span tools handle. Local codes can also add rules of their own, so check with your building department.

How do you build a 12x12 ft deck?

Start with the footings, sized in the deck footing calculator. Lay out the deck, check the 16.97 ft diagonals, and dig three holes for one beam about 5 ft apart.

Pour 15 to 16 in piers to frost depth, set the posts and beam, then hang the joists from a bolted and flashed ledger. The Decking Calculator counts the boards and screws.

What is the hardest part of building a deck?

For most builders it is the footings and the ledger. Holes must land on a square layout at full depth, and the ledger must be bolted and flashed to the house. Both are hidden once the deck is built, so mistakes there are the hardest to fix later.

Can I export deck footing calculator results to Excel or PDF?

Copy Results puts every result on its own line, so it pastes straight into Excel or Google Sheets. Print Results opens every chart in one page, and "Save as PDF" in the print window keeps a copy for your permit file.

Can I use the deck footing calculator in Australia or NSW?

The bearing math and metric mode work anywhere. The IRC table and its 50 psf design load are US rules, though. Australian decks follow the National Construction Code, so check your council's footing rules before you dig.