Deck Railing Calculator
Enter your railing length, posts and baluster type to get the exact number of balusters you need — checked against the 4-inch code gap.
Estimates only. Buy a few extra balusters for cuts and breakage. Stairs, corners and gate openings change the count. We build every railing to code.
Get a Free Railing QuoteHow the Baluster Calculator Works
Enter the total linear feet of railing, choose your post size and how many posts you have, then pick a baluster type and the gap you want between balusters. The calculator subtracts the post widths from your railing length to find the fillable space, then divides by the baluster width plus the gap to give the number of balusters — and it checks that the spacing meets the 4-inch code gap.
The 4-Inch Code Gap
California and the IRC require that a 4-inch sphere cannot pass between balusters, so the gap can never exceed 4 inches. With 2x2 balusters (1.5" wide) that puts them about every 5.5 inches on center — close to the old rule of three balusters per foot. Tighter gaps look better and are always safe; wider gaps fail inspection. Guards on decks 30 inches or more above grade must be at least 36 inches tall.
See our deck railing service for cable, glass, aluminum and wood options built to code.
Ordering Tips
Buy 5 to 10 percent extra balusters for miscuts and the odd damaged piece. Remember each railing section between posts is calculated on its own on a real deck, so long runs broken up by many posts use slightly fewer balusters than one continuous rail. Stairs need angled baluster cuts and usually a few more per flight — size those with our deck stairs calculator.
Frequently Asked Questions
How many balusters do I need per foot of railing?
With standard 2x2 balusters and a 4-inch gap you need about 2.2 balusters per foot, or roughly 3 per foot with tighter spacing. This calculator gives the exact count for your railing length, posts and gap.
What is the maximum gap between deck balusters?
Four inches. Building code requires that a 4-inch sphere cannot pass between any two balusters or below the bottom rail, which keeps children safe.
How tall does a deck railing need to be?
On decks more than 30 inches above the ground, guards must be at least 36 inches tall in California residential code, measured from the deck surface to the top of the rail.
How do posts affect the baluster count?
Posts take up space in the railing, so the calculator subtracts each post's width (3.5 inches for a 4x4, 5.5 inches for a 6x6) from the total length before spacing the balusters.