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Deck Post Spacing Calculator

Updated How we build and test

Enter the length of the beam and the widest spacing your beam allows to see how many posts you need, where they go and how much concrete the footings take.

The allowable span for your beam size, from IRC Table R507.5 or your plans.

in

Each end. Use 0 if the posts sit at the ends.

beams
in
in

Posts per beam

Enter the beam length and the maximum spacing to lay out the posts.

How to use the deck post spacing calculator

  1. Enter the beam length. On most decks this is the width of the deck parallel to the house.
  2. Enter the maximum spacing between posts. This is the allowable beam span and it depends on the beam size, the wood species and how far the joists span. Look it up in IRC Table R507.5, your local deck guide or your plans; do not guess it.
  3. Enter how far the beam runs past the end posts, if at all.
  4. Enter the footing size from your plans to get the concrete.

Formulas

Supported length = beam length − 2 × overhang
Spans = supported length ÷ maximum spacing, rounded up
Posts = spans + 1
Actual spacing = supported length ÷ spans
Overhang limit = actual spacing ÷ 4
Footing concrete = π × (diameter ÷ 2)² × depth × posts

Rounding the number of spans up and then dividing evenly keeps every span at or under the limit and makes the layout symmetrical. The overhang check uses the rule in IRC R507.5 that a beam may cantilever past its end post by up to one quarter of the beam span.

Worked example: a 16 ft beam with posts up to 6 ft apart

  1. Supported length: 16 ft − 2 × 1 ft = 14 ft.
  2. Spans: 14 ÷ 6 = 2.33, rounded up to 3.
  3. Posts: 3 + 1 = 4 posts.
  4. Actual spacing: 14 ÷ 3 = 4.67 ft, or 4 ft 8 in on center.
  5. Overhang limit: 4 ft 8 in ÷ 4 = 14 in, so a 12 in overhang is fine.
  6. Footings: 12 in across and 36 in deep is 2.36 ft³ each, 9.4 ft³ for four, which is 16 bags of 80 lb mix.

Questions

How far apart should deck posts be?

It depends on the beam. As an illustration, many residential decks built with doubled 2×10 beams end up with posts 6 to 8 ft apart, but the real limit comes from the beam span table for your lumber and joist span. Posts under a smaller beam must be closer.

Can I use fewer, bigger posts?

The post size rarely sets the spacing; the beam does. To space posts further apart you need a deeper or thicker beam, and each footing then carries more load and may need to be larger.

How deep do deck footings need to be?

Below the frost line for your area and onto undisturbed soil. Your building department publishes the frost depth; 36 in is only the default in this calculator.

Sources

  1. 2021 International Residential Code, Chapter 5, Section R507 (Exterior Decks), International Code Council. Used for: the beam span table reference and the one-quarter-span cantilever limit.
  2. QUIKRETE Concrete Mix (No. 1101) product data sheet. Used for: bag yields.

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