About the Deck Footing Calculator
This deck footing calculator sizes the concrete footings under the beam of a ledger-attached deck. From the deck width, joist length, overhangs and number of posts it works out the tributary area each post carries, the load on it, and the diameter (or square side) and thickness of footing needed so the soil is not overloaded. It also totals the concrete and 80 lb bags for all the footings.
The method follows the American Wood Council’s DCA 6 Prescriptive Residential Wood Deck Construction Guide (Appendix B): a 40 psf live load plus 10 psf dead load, 150 pcf concrete, and footing thickness at least 6 inches and no less than the distance the footing projects beyond the post. Results match the DCA 6 Table B3 footing sizes to within an inch (we always round up). You can change the loads, soil capacity and post size.
Use it to plan a deck, check a contractor’s drawings or prepare a permit sketch. Soil bearing capacity should come from your building department or a soils report — 1,500 psf is the presumptive value for clay soils in the IRC and is the safe default when unknown. Footings must also bear on undisturbed soil at least 12 inches deep or below the local frost line, whichever is deeper.
With the default inputs, the round footing diameter (largest) is 18 in. Change any value above to recalculate instantly.
How to use the deck footing calculator
- 1Enter the deck width along the beam and the joist length from the ledger to the centre of the beam.
- 2Add the joist overhang beyond the beam and the beam overhang past the end posts.
- 3Choose how many posts support the beam and whether the beam is one continuous piece.
- 4Pick your soil bearing capacity (use 1,500 psf if you do not know it) and post size.
- 5Read the footing diameter or square size, thickness, and the concrete to buy.
Formula and method
Each post carries the deck area that drains to it — its tributary area. Joists span from the ledger to the beam, so the beam picks up half the joist length plus any overhang beyond it; along the beam, a centre post takes one full beam span and a corner post half a span plus the beam overhang. These are equations B-1 and B-2 in the AWC DCA 6 appendix. If the beam runs continuously over interior posts, DCA 6 multiplies the centre-post area by 1.25.
The post load is the tributary area times the design load (40 psf live + 10 psf dead by default). The footing must spread that load so the soil pressure stays under its bearing capacity, after allowing for the weight of the footing itself (150 lb/ft³ of concrete). The calculator starts with a 6-inch thick footing, finds the area needed, then makes the footing at least as thick as it projects beyond the post face and repeats until the size is stable. Diameters and square sides are rounded up to the next whole inch.
- W
- Deck width along the beam (ft)
- JL, JO
- Joist length to beam centre and joist overhang (ft)
- BL, BO
- Beam span between posts and beam overhang (ft)
- LL, DL
- Live and dead load (psf)
- q
- Soil bearing capacity (psf)
- t
- Footing thickness (in)
Worked examples
16 × 12 ft deck on three 6x6 posts, clay soil
Posts are (16 − 2 × 1) ÷ 2 = 7 ft apart. The centre post carries (6 + 1) × 7 = 49 ft² × 50 psf = 2,450 lb; at 1,500 psf less the weight of a 7-in footing (87.5 psf) it needs 1.73 ft², an 18-in round or 16-in square footing. Each corner post carries 7 × 4.5 = 31.5 ft² (1,575 lb) and needs a 15-in round, 6 in thick. All three footings take about 2.26 cu ft of concrete, or 4 bags of 80 lb mix.
Same deck on two posts in gravel soil
With only two posts the beam spans 14 ft, so each post carries (6 + 1) × (7 + 1) = 56 ft² or 2,800 lb. At 3,000 psf less 75 psf for a 6-in footing, the area needed is 0.957 ft² — a 14-in round or 12-in square footing, 6 in thick.
Continuous beam with a heavier 60 psf live load
Load is 60 + 10 = 70 psf. The centre area becomes 49 × 1.25 = 61.25 ft², so the post carries 4,287.5 lb; at 2,000 psf less 100 psf for an 8-in footing it needs 2.26 ft² — a 21-in round. The corners carry 31.5 × 70 = 2,205 lb and need 15-in rounds.
Frequently asked questions
How big should deck footings be?+
It depends on the load each post carries and the soil. Under DCA 6 assumptions (50 psf total load), a post carrying 50 ft² of deck on 1,500 psf soil needs a round footing about 18–19 inches across and 7 inches thick (DCA 6 Table B3 lists 18), while 100 ft² needs about 26 inches and 11 inches thick. Stronger soil allows smaller footings; a 3,000 psf soil needs about 13 inches for 50 ft².
How deep do deck footings need to be?+
The IRC and DCA 6 require the bottom of footings to bear on undisturbed soil at least 12 inches below grade, or below the frost line set by your local building department if that is deeper. In cold climates that can be 36–48 inches or more.
How far apart should deck footings be?+
Post spacing is set by the beam size, not the footing. Typical DCA 6 beam spans for doubled or tripled 2x10 and 2x12 beams range from about 6 to 12 ft depending on joist length and species. Wider spacing puts more load on each footing, so recheck the footing size whenever you change it.
What soil bearing capacity should I use?+
If you do not have a soils report, use 1,500 psf — the presumptive value for clay and unknown soils in IRC Table R401.4.1. The same table gives 2,000 psf for sand, silty sand, clayey sand, silty gravel and clayey gravel, and 3,000 psf for sandy gravel and gravel; sandy and silty clays stay at 1,500 psf. Your building department may assign a value for your area.
Do I need footings at the house for an attached deck?+
Not usually. On a ledger-attached deck, the house side of the joists is carried by the ledger bolted to the rim joist, so only the outer beam needs posts and footings. A freestanding deck needs a second beam and its own row of footings near the house.