Baluster Spacing Calculator

Depends on baluster width. For a 16 ft (192″) section between posts using 1.5″ wood 2×2 balusters with a 2.5″ post offset, the inner run is 187″. The math gives 35 balusters with a ~3.94″ equal gap — but at 3.94″ you're a hair over the 3 7/8″ safety target, so the calculator bumps to 36 balusters with a 3.78″ gap. For aluminum ¾″ pickets at the same dimensions, you'd use 47 balusters with a 3.30″ gap. The calculator solves this in one click.

Two reasons. First, inspector measurement variance — a slip-fit gauge can ride a hair high or low. Second, lumber tolerance — a 'nominal' 1.5″ baluster is actually 1.49-1.51″ depending on the mill. A 1/8″ safety margin (3 7/8″ target) absorbs both sources of variance. If you're working with metal pickets cut to factory tolerance, you can tighten to 4″ exactly; for wood balusters, keep the buffer.

Cable rail uses the same 4-inch sphere rule, but the rule is applied vertically (cable spacing) rather than horizontally (baluster spacing). IRC inspectors typically test by pressing a 3 1/8″ sphere against the cable — the cables stretch, so a smaller sphere is used in practice. Most cable systems use cables every 3″ on center with ~3 1/16″ clear gap. This calculator handles wood and metal balusters; for cable, use the 'cable infill' option to skip the baluster line and price the termination posts + tensioner kit.

Run the calculator once per unique section length and sum the baluster counts. Most decks have 2-3 unique section lengths (e.g. two long sides at 14 ft and two short sides at 8 ft). The calculator's section count assumes equal division — for unequal sections, treat each set as a separate calc run. The PDF export labels which section count corresponds to which length.

Yes — IRC R312.1.2 requires guards to be 'continuous' which inspectors interpret as plumb (vertical) for level rails and parallel-to-stringer for stair rails. Slanted balusters fail unless the spacing rule still holds in the slant plane. The calculator assumes plumb balusters; for stair rails, use the deck-stair-calculator with handrail option.

Yes for metal pickets — most aluminum picket systems use a U-channel baluster shoe screwed directly to the deck surface, eliminating the bottom rail. For wood balusters you should use a sub-rail (2×2 or 1×4 PT) for stability and to prevent water pooling on cut ends. The calculator's rail kit line includes a sub-rail by default; deduct 30% from that line if you're going shoe-only.

IRC 2021 R312.1.2 minimum is 36″ for one- and two-family residential. Some jurisdictions (and most commercial) require 42″, and many homeowners prefer 42 anyway for safety on tall decks. The rail height doesn't affect the baluster spacing math — it only affects the rail kit cost and the diagram's vertical extent. Switch to 42 in the inputs if you need it.

Wood 2×2 PT runs $4.85 each (36″ stock); cedar 1×1 is $3.40. Aluminum square pickets are $7.90 each, round are $8.80. Glass panels run $30-45/lf depending on cap material, no per-baluster cost. Cable systems skip balusters entirely but the termination posts are $80-120 each — typically only worth it for ocean views or modern-aesthetic builds. Toggle 'include cost' in the inputs to see your full BoM.

Most jurisdictions don't require submitted baluster spacing drawings on residential permits, but they will inspect the gap rule in person. Some inspectors carry the 4-inch slip-fit gauge; others eyeball it. Either way, having the calculator's PDF taped to the rail during inspection has prevented more than one re-do. Save it.

Yes. The action bar generates a clean printable document with the baluster count per section, the equal-gap dimension, the IRC reference, and the BoM. The CSV export gives you per-section data ready to paste into a spreadsheet. Copy Link gives you a shareable URL with all your inputs preserved.