Need posts and beams for your deck frame? Enter the deck length, the number of beams and the post spacing. You get the posts, the beam boards and the total lengths to buy.
Your shopping list
3Posts
2Beam boards to buy
16 ftTotal beam length
- Total post lengthAdd extra for the part in the ground9 ft
Beam size and post size depend on the load and span. Check a span table or ask your building office.
How to use the Deck Beam and Post Calculator
- Enter the deck length and pick one beam for a deck attached to the house or two for a free-standing deck.
- Pick the post spacing, the number of boards in each beam and the lumber length.
- Type the post height and read the posts and beam boards to buy.
The math behind it
Posts = beams x (deck length / post spacing + 1), rounded up
Beam boards = plies x boards needed per beam
Total post length = posts x post height
A worked example
What we typed in
- Deck length: 20 ft
- Support beams: 2 beams (free-standing deck)
- Post spacing along the beam: 6 ft apart
- Beam plies: Double (2 boards)
- Beam lumber length: 16 ft
- Post height: 3 ft
What we got
- Posts: 10
- Beam boards to buy: 6
- Total beam length: 40 ft
- Total post length: 30 ft
Tips from the job site
- Most decks keep posts 8 ft apart or closer. Your beam size sets the real limit.
- Each post sits on its own footing, so open the Deck Footing Calculator next.
Common questions
How many posts does a deck need?
Divide the beam length by the post spacing and add one. A 16 ft beam with posts 8 ft apart needs 3 posts.
How far apart should deck posts be?
Often 6 to 8 ft, but it depends on the beam size and the load. Check a span table.
See all Deck Calculators.
Last checked: (worked example and form display).