Estimates only. Check measurements, product labels and local building codes before you buy or build.
How the Baluster Spacing Calculator works
The baluster calculator tells you how many balusters fit between two posts and the equal gap that makes the spacing the same everywhere, including at both posts. Enter the clear space between the post faces, the baluster width and the largest gap you will accept.
n balusters leave n + 1 gaps. The fewest balusters that keep every gap at or under your maximum is (space − max gap) ÷ (baluster width + max gap), rounded up. The equal gap is then (space − balusters × width) ÷ (balusters + 1), and the on-center spacing is gap + width.
Example: 72" (6 ft) between posts, 1 1/2" balusters, 3 7/8" max gap. (72 − 3.875) ÷ (1.5 + 3.875) = 68.125 ÷ 5.375 = 12.67, so 13 balusters. Gap = (72 − 13 × 1.5) ÷ 14 = 52.5 ÷ 14 = 3 3/4". On center = 3 3/4 + 1 1/2 = 5 1/4", and the first baluster center sits 3 3/4 + 3/4 = 4 1/2" from the post. Check: 13 × 1.5 + 14 × 3.75 = 19.5 + 52.5 = 72".
Measure at the rail, between the post faces, not the post centers. Mark the first baluster center from one post, then step off the on-center spacing; a scrap block cut to the gap makes a quick spacer. IRC R312.1.3 does not allow a 4" sphere to pass through a guard, so the gap must be under 4"; many builders aim for 3 1/2" to 3 3/4" so bowed lumber stays under the limit. The same rule covers the space under the bottom rail. Local codes may differ.
Frequently asked questions
What is the code for baluster spacing?
IRC R312.1.3 says a required guard must not have openings that let a 4" sphere pass, from the walking surface up to the required guard height. In practice that means a clear gap under 4"; many builders aim for 3 1/2" to 3 3/4". On the open sides of stairs the limit is 4 3/8". Local codes can be stricter.
How many balusters do I need for a 6 ft railing?
With 1 1/2" square balusters and a gap under 4", a 72" space between posts takes 13 balusters at a 3 3/4" gap, a little over 2 per foot. Thinner 3/4" metal balusters take 15 at a gap of about 3.8". Enter your exact space between posts, since the posts take up part of each section.
How do you space balusters evenly?
Measure the space between the posts, work out the count and the equal gap, then mark the first baluster center at the gap plus half a baluster from one post. From there, step off the on-center spacing to the other post; the last gap should match the first. A spacer block cut to the gap speeds up installation.
How high does a deck railing need to be?
Under IRC R312.1.1 a guard is required where the walking surface is more than 30" above the floor or grade below, and IRC R312.1.2 sets a 36" minimum height for a residential guard (34" along the open side of stairs, measured from the tread nosings). This calculator handles spacing only, so check height and post attachment against your local code.