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Deck Footing Calculator

Updated How we build and test

Enter the joist span and post spacing to see how wide each deck footing needs to be for your soil, and how much concrete all of them take.

From the house ledger to the beam.

psf

40 live + 10 dead is the usual minimum. Use your snow load if it is higher than 40.

1,500 psf is the code default when the soil has not been tested.

in
footings

Round footing diameter

Enter the joist span and the post spacing.

How to use the deck footing calculator

  1. Enter the joist span from the ledger on the house to the beam, and the spacing between posts along the beam.
  2. Enter any joist overhang past the beam.
  3. Keep 50 psf unless your ground snow load is over 40 psf, then use snow load plus 10.
  4. Choose the soil. If you do not know it, use 1,500 psf.
  5. Enter the footing depth for your frost line and the number of footings.

Formulas

Deck area on a post = (joist span ÷ 2 + overhang) × post spacing
Load on the footing = area × design load
Bearing area needed = load ÷ soil bearing capacity
Round footing diameter = √(4 × area ÷ π), rounded up to an even inch
Concrete = π × (diameter ÷ 2)² × depth × number of footings

A beam post on a ledger-attached deck carries half the joist span, because the ledger carries the other half, times the length of beam it supports. The figures are for an interior post; end posts carry less, so using this size for every footing is on the safe side. A free-standing deck with two beams puts half the joist span on each beam in the same way.

Worked example: 12 ft joists with posts 6 ft apart

  1. Deck area on a post: (12 ÷ 2) × 6 = 36 ft².
  2. Load: 36 × 50 = 1,800 lb.
  3. Bearing area on 1,500 psf soil: 1,800 ÷ 1,500 = 1.2 ft².
  4. Diameter: √(4 × 1.2 ÷ π) = 1.24 ft = 14.8 in, rounded up to a 16 in footing.
  5. Concrete for four footings 36 in deep: 4 × 4.19 = 16.8 ft³, or 28 bags of 80 lb mix.

Questions

What size footing do I need for a 6×6 deck post?

The post size does not set the footing size; the load and the soil do. A post carrying 36 ft² of deck needs a 16 in round footing on 1,500 psf soil but only 12 in on 3,000 psf gravel.

Does this replace the code table?

No. The International Residential Code has a footing size table for decks (R507.3.1) and many building departments publish their own deck guide. This calculator shows the bearing calculation behind those tables so you can see what drives the size. Your permit drawing governs.

How deep should deck footings be?

Below the frost line for your area. An attached deck that heaves with frost can pull away from the house.

Sources

  1. 2021 International Residential Code, Table R401.4.1 (Presumptive load-bearing values of foundation materials), International Code Council. Used for: soil bearing capacities.
  2. 2021 International Residential Code, Section R507 (Exterior Decks), International Code Council. Used for: the 40 psf live load and 10 psf dead load.

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