Deck Footing & Load Calculator
Size deck footings for a US residential deck (IRC R507). Enter the joist span and post spacing to get the tributary area, load per footing, and the required footing diameter for your soil — with a soil comparison, standard-size table, and a downloadable PDF report.
| Soil | Req area | Round dia | Square |
|---|---|---|---|
| Enter your deck to compare soils | |||
| Size | Bearing area | Max load | Status |
|---|---|---|---|
| Enter your deck to size footings | |||
How to use the deck footing calculator
Four quick steps turn your deck framing into the load each footing carries and the footing size your soil needs — with a soil comparison, standard-size table, and a downloadable report.
Enter the framing
Give the joist span, post spacing, and any beam overhang. That sets the tributary area each footing carries.
Set the loads
Use the IRC minimum 40 psf live + 10 psf dead, or raise it for snow, a hot tub, or heavy decking.
Pick soil & shape
Choose the presumptive soil bearing and a round pier or square pad footing.
Read results & save a PDF
Get the load per footing, required bearing area, and recommended size, with a soil comparison — then download a report.
How the footing size is calculated
The load path runs from tributary area to pounds per footing to the bearing area the soil needs, then to a footing size. Here is the exact math the calculator runs.
Worked example — 12 ft joist span, 8 ft post spacing, no overhang, 50 psf, 1,500 psf clay, round
Tributary: 8 × (12/2 + 0) = 48 sq ft · Load: 48 × 50 = 2,400 lb
Required bearing: 2,400 / 1,500 = 1.6 sq ft
Round size: √(4 × 1.6×144 / π) ≈ 17.1″
Next standard footing: 18″ round (1.77 sq ft)
≈ 18″ round footing on clay. On 3,000 psf gravel the same load needs only ~12″. Verify with an engineer.
Deck footing reference charts
Handy lookups for soil bearing, standard round footing capacity, and load per footing by span and spacing. These use IRC presumptive values — confirm your soil and frost depth locally.
| Soil type | Bearing | Note |
|---|---|---|
| Clay / silt | 1,500 psf | Weakest — largest footing |
| Sand / sandy gravel mix | 2,000 psf | Mid-range |
| Sandy gravel / gravel | 3,000 psf | Strongest — smallest footing |
When you don’t know the soil, use the lowest value (1,500 psf) to stay conservative, or have it evaluated.
| Diameter | Bearing area | Max load @ 1,500 | @ 3,000 |
|---|---|---|---|
| 12″ | 0.79 sq ft | 1,178 lb | 2,356 lb |
| 16″ | 1.40 sq ft | 2,094 lb | 4,189 lb |
| 18″ | 1.77 sq ft | 2,651 lb | 5,301 lb |
| 24″ | 3.14 sq ft | 4,712 lb | 9,425 lb |
| 36″ | 7.07 sq ft | 10,603 lb | 21,206 lb |
Max load = bearing area × soil capacity. A footing works when its max load meets or exceeds the load per footing.
| Joist span | 6 ft posts | 8 ft posts | 10 ft posts |
|---|---|---|---|
| 10 ft | 1,500 lb | 2,000 lb | 2,500 lb |
| 12 ft | 1,800 lb | 2,400 lb | 3,000 lb |
| 14 ft | 2,100 lb | 2,800 lb | 3,500 lb |
| 16 ft | 2,400 lb | 3,200 lb | 4,000 lb |
Load = post spacing × ½ joist span × 50 psf. Wider spacing or longer spans put more on each footing.
Everything the calculator works out
The framing and soil give you the full load path — tributary area, load per footing, and the size your soil requires — with the whole chain shown, not a black box.
Key figures behind the footing
Built for a solid deck base
From a DIY deck to a contractor’s takeoff, the same calculator traces the load path to a footing size — then the building department confirms.
Building a deck and needs to know how big to dig and pour each footing before renting an auger.
- Enter span & post spacing
- Pick your soil
- Confirm frost depth locally
Wants a fast load-per-footing and size to plan concrete and check against the permit set.
- Set loads for snow / hot tub
- Compare round vs square
- Hand the PDF to review
Learning how tributary area, load, and soil bearing set the footing size.
- Watch the load path
- Flip soil to see the effect
- Read the standard-size table
7 things to know about deck footings
Footings carry the whole deck to the ground, so getting them right is a safety matter. These points keep the load path honest.
Deck footing calculator FAQ
The sizing, soil, and code questions people ask most when planning deck footings.
Work from the load each footing carries, then give it enough bearing area. Tributary area is post spacing × half the joist span (plus overhang); multiply by the total load (50 psf under the IRC) for pounds per footing; divide by soil bearing for the required area; then pick a footing size.
Example: 12-ft span at 8-ft spacing = 48 sq ft, × 50 = 2,400 lb. On 1,500 psf clay that needs 1.6 sq ft, about a 17″ round footing.
It’s a planning estimate — footings should be confirmed by your building department or an engineer.
Tributary area is the portion of the deck whose weight a single footing carries. For a beam on posts, it’s the post spacing times the joists’ tributary depth.
A ledger-attached deck sends about half the joist span to the house and half to the beam, so the beam’s tributary depth is half the joist span. Add any overhang past the beam. So tributary = post spacing × (½ joist span + overhang).
Multiplying that area by the design load in psf gives the pounds each footing transfers to the soil.
The IRC minimum is 40 psf live + 10 psf dead = 50 psf. Live load covers people, furniture, and use; dead load is the deck structure itself.
Raise it where needed: heavy-snow regions may require more, and a hot tub, planters, stone or concrete decking, or a roof adds substantial dead and concentrated loads this simple per-square-foot method doesn’t cover.
Always use at least the code minimum — under-designing the load path is what leads to failures.
It depends on the load and the soil. Divide the pounds per footing by the soil bearing value for the required bearing area, then size the footing to provide it.
A 2,400-lb footing on 1,500 psf clay needs 1.6 sq ft (~17″ round); on 3,000 psf gravel the same load needs only 0.8 sq ft (~12″ round). Weaker soil means a bigger footing.
Round up to a standard size that meets or exceeds the required area, and confirm frost depth and reinforcement separately.
Soil bearing capacity is how much pressure the ground supports without settling, in psf. The IRC gives presumptive values: ~1,500 psf clay/silt, 2,000 psf sandy mixes, 3,000 psf sandy gravel/gravel.
Stronger soil carries more per square foot, so it needs a smaller footing; weak clay needs a larger one for the same load. When unsure, use 1,500 psf to stay conservative, or have the soil evaluated.
Getting this wrong is a common cause of footings that settle or heave, so confirm the actual soil with your building department.
Deck footings generally must extend below the local frost line so freeze-thaw doesn’t push them up and heave the deck. Frost depth ranges from a few inches in warm climates to 4 feet or more up north — your building department sets it.
This calculator sizes the footing’s bearing area — its width or diameter — but does not determine how deep to dig. Frost depth is a separate, critical requirement that depends on your location.
A footing that’s the right width but too shallow will still fail from frost heave, so confirm depth locally.
Both work; the choice is mostly about forming. Round footings are usually poured piers or tube forms (like a Sonotube), quick to set and common for deck posts. Square footings are pads, sometimes with a pier on top, formed with lumber.
Structurally what matters is the bearing area, not the shape — either just has to provide enough square footage so soil pressure stays under capacity. For the same area, a square footing has a slightly smaller footprint dimension than a round one.
Pick whichever suits your method and site; this calculator sizes both to the same required bearing area.
Almost always. Decks are structural, load-bearing elements, and most jurisdictions require a permit and a footing inspection before you pour concrete.
The inspection checks footing size, depth to frost line, soil, and reinforcement before the holes are filled — once concrete is poured, mistakes are buried and expensive to fix. Skipping the permit can mean fines, forced removal, and resale problems.
Because a deck collapse is a serious safety risk, have the footing and framing design reviewed by your building department or a licensed engineer, and get the required inspections before backfilling.
Estimate only — not an engineered design. This calculator sizes deck footing bearing area from IRC R507 assumptions (40 psf live + 10 psf dead by default and presumptive soil bearing values), in US units. It does not size posts, beams, or joists, determine footing depth or frost protection, check reinforcement, or design for hot tubs, roofs, planters, heavy snow, or other concentrated loads, any of which can govern. A deck is a structural, load-bearing assembly and a collapse is a serious safety risk: have the footings and framing reviewed by your building department or a licensed engineer, obtain a permit, and get the required footing inspection before you pour. For planning purposes only.