Deck Post Spacing Calculator

About the Deck Post Spacing Calculator

A raised deck rests on beams, and the beams rest on posts. How far apart those posts can stand is set by how far the beam can span, which depends on the beam's size and species and on how much deck it carries. Once you know that maximum span, you still need to work out how many posts each beam needs, how to space them evenly, and whether the beam can overhang the end posts.

This deck post spacing calculator does that layout. Enter the beam length, the maximum span between posts from a beam span table, the overhang at each end and the number of beams. It gives the number of posts per beam, the even spacing between them, the total number of posts, and a check that the overhang meets the common rule of no more than a quarter of the span.

How to Use the Deck Post Spacing Calculator

Enter the beam length, usually the width of the deck along the house.

Enter the maximum span between posts for your beam, from IRC Table R507.5 or your local code.

Enter the overhang past the end posts and the number of beams.

The Formulas

  length between end posts = beam length − 2 × overhang
  spans                    = ⌈ length between ÷ maximum span ⌉
  spacing                  = length between ÷ spans
  posts per beam           = spans + 1
  overhang limit           = spacing ÷ 4

Step-by-Step Example

A 20 ft beam with a 1 ft overhang at each end and an 8 ft maximum span, two beams.

  between end posts   20 − 2 × 1 = 18 ft
  spans               ⌈18 ÷ 8⌉ = 3
  spacing             18 ÷ 3 = 6 ft
  posts               3 + 1 = 4 per beam, 8 in total
  overhang limit      6 ÷ 4 = 1 ft 6 in, so a 1 ft overhang is fine

Where the Maximum Span Comes From

Beam span tables list how far a beam of a given size can span between posts for a given joist span. The longer the joists the beam supports, the more load it carries and the shorter its allowable span. In the International Residential Code, Table R507.5 covers common beams made from two or three plies of 2×6 to 2×12 lumber in species such as southern pine and Douglas fir. Doubled or tripled 2×10 and 2×12 beams typically span about 6 to 9 feet for common joist spans. Engineered lumber such as LVL can span much further, following the manufacturer's tables.

Why Space Posts Evenly

Even spacing gives every span the same length, so no part of the beam is closer to its limit than the rest and the deck feels uniformly solid. The calculator uses the fewest spans that keep each one within your maximum, then shares the length equally. If you would rather line posts up with features below the deck, such as a patio edge, you can use a shorter maximum span to add a post.

Beam Overhangs

Letting the beam run a little past the end posts, called a cantilever, hides the posts slightly inboard and can reduce the number of posts. The IRC limits the cantilever to one quarter of the span between posts, because a long overhang levers load back onto the end post and lifts the far end. Joists can also cantilever past the beam, under a separate limit based on joist size and spacing.

Posts and Footings

Every post needs its own footing below the frost line, sized for the load it carries. Closer posts mean smaller loads per post but more footings to dig and pour, so most builders balance post count against beam size. Current code generally calls for 6×6 posts, notched or connected with approved hardware, and limits post heights for bracing. The deck footing calculator sizes the concrete for each post once the spacing is set.

Post Height and Bracing

Tall posts need more care than short ones. The IRC limits the height of 4×4 posts to about 8 feet and 6×6 posts to about 14 feet for typical decks, measured from the top of the footing to the underside of the beam. Taller decks, or decks on sloping ground where some posts are much longer, usually need diagonal knee bracing between posts and beams, or an engineered design. Bracing also stiffens a deck that feels wobbly even when the posts are strong enough.

Notching and Connections

Beams should bear fully on posts, either sitting in a notch cut into a 6×6 or held in a manufactured post cap. Notching a 4×4 to carry a beam is no longer allowed by most codes because it leaves too little wood. Through-bolts alone are not an approved way to hang a beam on the side of a post. Galvanized or stainless hardware rated for treated lumber prevents corrosion.

Understanding Your Result

The headline gives the number of posts per beam and the spacing between them.

The spacing line shows the length between end posts and how it divides.

The total posts line multiplies by the number of beams.

The overhang check line compares each end's overhang with the quarter-span limit.

When Should You Use This Calculator?

Use it when drawing a deck plan for a permit.

Use it to work out how many posts and footings to buy and dig.

Use it to compare a heavier beam with fewer posts against a lighter beam with more.

Common Mistakes

Guessing the maximum span. Look it up for your beam size, species and joist span.

Letting the beam overhang too far. Keep it within a quarter of the span.

Forgetting the second beam. Multiply posts by the number of beams.

Using 4×4 posts under beams. Most codes now require 6×6.