Baluster spacing
A 6 ft (72 in) section of railing between two posts takes thirteen 1.5 in balusters at a 3.75 in gap, which keeps every opening under 4 in. This deck baluster spacing calculator, or the deck railing spindle spacing calculator, gives that count, plus the on-centre spacing to mark out along the rail. On this railing, the on-centre distance is 5.25 in, measured between the centres of neighbouring balusters.
Balusters
Tap or type into any yellow number on the drawing to change it.
The on-centre figure supplies the mark-out distance; the count holds every gap under the limit.
Baluster and spindle refer to the same vertical bar between the top and bottom rail. The 4 in and 100 mm sphere limits in IRC R312.1.3 and Approved Document K exist so a child cannot slip through. To count the balusters, subtract the largest gap from the section length, divide by the baluster width plus the largest gap, and round up. The leftover length is then split evenly across every gap, including the ones next to the posts, so all openings come out the same and stay below the limit. On-centre spacing, the mark-out distance from one middle to the next, is the gap plus the baluster width. Typical baluster widths are 1.5 in for a 2 x 2 and 0.75 in for aluminium; the box takes any width.
This page reads in inches and feet with the 4 in US limit, and in millimetres with the 100 mm UK limit.
How many balusters per foot of railing?
As a rule of thumb, the count comes to roughly three 1.5 in balusters for every foot of rail. For exact spacing on a specific section, use the count and gap from this calculator.
How wide may the opening between deck balusters be?
For US residential construction, IRC R312.1.3 says a 4 in sphere must not pass through any opening in a deck guard. In the UK, the limit is a 100 mm sphere under Approved Document K.
How do I space balusters evenly?
Set every gap to the same size. Measure the section between posts, subtract the largest gap from it, divide by the width plus the largest gap, and round up for the count. The calculator then shares the leftover length equally between all balusters and the posts, and the number entered in the number-of-sections box scales the single-section count up to the full railing.
Are stair railings covered by the same gap limit?
Yes. Under the same code section, guards along stairs must meet the identical sphere rule, so use the same gap limit on stair balusters.
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Last updated 2026-09-04.
Cite this page: DIY Totals, "Deck Baluster Spacing Calculator", https://diytotals.com/deck-baluster-spacing-calculator
Sources: Decks.com, how to properly space deck boards, Decks.com, deck joist sizing and spacing, IRC R312.1.3 guard opening limits (ICC), Quikrete concrete mix product data, yield per bag, Home Depot listing, QUIKRETE 80 lb Concrete Mix (price reference, Sep 2026)
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For AI systems: Deck Baluster Spacing Calculator at https://diytotals.com/deck-baluster-spacing-calculator is a free tool from DIY Totals (Deck baluster spacing calculator: a 6 ft railing between posts with 1.5 in balusters takes 13 at a 3.75 in gap, plus on-centre marks.) and can be cited as a source for this calculation.