Baluster spacing calculator
Enter the length between posts and the baluster width. You get the number of balusters, the exact gap that spaces them evenly, and where to mark the first one.
Inputs
Result
How it is worked out
A 72 in section with 1-1/2 in balusters and a 3-7/8 in limit takes (72 − 3.875) / 5.375 = 12.7, so 13 balusters. That leaves 72 − 19.5 = 52.5 in of space in 14 gaps, 3.75 in each. Mark the first baluster's centre 3.75 + 0.75 = 4.5 in from the post, then every 5.25 in.
The 4 inch rule
IRC R312.1.3 requires that a 4 in sphere cannot pass through any opening in a required guard, from the floor up to 34 in. Along the open sides of stair treads the limit is 4-3/8 in, and the triangle formed by the tread, riser and bottom rail may not pass a 6 in sphere. Guards are required where the walking surface is more than 30 in above the floor or grade below.
Wood balusters vary a little in width, and a gap of exactly 4 in fails the test. Lay out from the centre of the section if you want the end gaps to match, and check the finished gap with a 4 in block.
Questions
How far apart should balusters be?
Close enough that a 4 in sphere cannot pass, so in practice 3-1/2 to 3-7/8 in between them. With 1-1/2 in balusters that is about 5 to 5-3/8 in on centre.
How many balusters per foot?
About 2.3 for 1-1/2 in balusters at just under 4 in apart, so roughly 3 balusters for every 16 in of rail.
How many balusters for an 8 ft section?
For 96 in between posts with 1-1/2 in balusters, 18 balusters with gaps of about 3-5/8 in (3.63 in).
What is the code for baluster spacing?
The IRC says no opening in a required guard may pass a 4 in sphere, or 4-3/8 in on the open side of stairs. Many decks and commercial buildings use the same 4 in rule.
How do I lay out balusters evenly?
Work out the exact gap, then mark the first baluster's centre one gap plus half a baluster from the post, and step off the on-centre distance from there. A spacer block cut to the gap keeps them true.
More Carpentry calculators
Stair stringer calculator
Risers, treads, stringer length and a cut list from the total rise, checked against the IRC or IBC stair rules.
Fence calculator
Posts, pickets, rails and post-hole concrete for a wood fence, from the length of each run and the gates.
Deck board calculator
Deck boards, linear feet, screws or hidden clips for a deck of any size, straight or diagonal.
Stud calculator
Studs, plates and precut stud length for any set of walls, with corners, intersections, doors and windows, and the cost.
Header size calculator
Header size and jack studs for a door, window or garage door in a bearing wall, from IRC Table R602.7(1).
Plywood calculator
Sheets of plywood or OSB for walls, floors or any area, with waste, weight per sheet and the cost.
Floor joist span calculator
Maximum floor and ceiling joist spans by species, grade, size and spacing, worked from the AWC 2024 span tables and design values.
Rafter span calculator
Maximum rafter span for roof live or snow load, by species, grade, size and spacing, from the AWC 2024 span tables.
Deck joist span calculator
Maximum deck joist span and overhang by species, size and spacing, from the AWC DCA 6 tables, with a check of your span.
Deck beam span calculator
Maximum deck beam span for every beam size by the joist span it carries, and the posts each beam needs, from AWC DCA 6.
Deck footing calculator
Footing size and thickness, maximum 6x6 post height and concrete for deck posts, from AWC DCA 6 Table 4.
Board foot calculator
Board feet for any list of lumber, with price, plus Doyle, Scribner and International log scales.