How many balusters do I need per section?

Take the section length between the posts, subtract the largest gap, divide by the baluster width plus that gap, and round up. For a 6 ft (72 in) section of 2 × 2 balusters and a 4 in gap: (72 − 4) ÷ (1.5 + 4) = 12.4, so 13 balusters. Multiply by the number of sections for the total, then add spares. If the division comes out to a whole number, add one more, so that the gap is under the limit and not equal to it.

What is the maximum gap between balusters?

The International Residential Code (R312.1.3, Opening limitations) says required guards shall not have openings, from the walking surface to the required guard height, that allow passage of a 4 inch (102 mm) diameter sphere. So 4 in is a maximum, and a gap of exactly 4 in is too big; this calculator always works out a gap under the number you enter. The IRC also has two exceptions for stairs, below. Your local code governs: many places adopt the IRC with changes, and commercial buildings and pool barriers have their own rules. The calculator never says a guard is compliant; it does the arithmetic.

Is the limit different on stairs?

In the IRC, yes. Guards on the open side of stairs shall not have openings that allow passage of a 4 3/8 inch (111 mm) sphere, and the triangular opening formed by the riser, tread and bottom rail of a stair guard may be sized so that a 6 inch (153 mm) sphere cannot pass. To work a stair-side guard with the 4 3/8 in figure, type 4.375 in the largest gap field. A rail that follows the slope of a stair is laid out differently from the level sections this calculator handles, and the exceptions are written for stairs only: use 4 in for decks, balconies, porches and landings. Check the edition your jurisdiction has adopted.

Do I measure from the center of the posts or the inside faces?

The inside faces: the clear distance between the posts is the space the balusters and gaps fill. If you only know the post-to-post (center to center) distance, take off one post width (a 4 × 4 post is 3.5 in wide, so subtract 3.5 in). Then use that length for the section.

How do I lay out the balusters?

Cut a spacer block to the gap (for the 6 ft example, 3 3/4 in). Set the first baluster against the block placed against the post, fix it plumb, then use the block between each pair: gap, baluster, gap, baluster, and so on, ending with one gap before the other post. Or mark centres: the first centre is one gap plus half a baluster width from the post, then every on-center step (5 1/4 in in the example). Check the last gap before you fasten the last few balusters, and move all of them a little if it is off. Rounding to the nearest 1/16 in can add up over a long section, so the spacer block, not a tape, is the more reliable way.

Why does the gap come out smaller than my maximum?

Because balusters come in whole numbers. The calculator finds the fewest balusters that keep every gap under your limit, then spreads the leftover space evenly, so the real gap is usually a little under the maximum. A narrower section, or a wider baluster, changes how close it comes.

What width should I use for a 2 × 2?

A nominal 2 × 2 is about 1 1/2 × 1 1/2 in actual, so the width is 1.5 in. Some deck balusters are milled or rounded and a bit different: measure the one you are buying. For a round baluster use its diameter; for a metal bar use its width seen from the front (3/4 in for a 3/4 in square bar). If a baluster is turned or has a wider part, use its widest part; the gap there is what counts.

How many extra balusters should I buy?

It is your choice. The calculator offers none (0%), a few spares (5%) or 10%. Wood balusters have cull pieces and the odd bad cut, and a few spares are cheap to carry; metal balusters seldom need more than a spare or two. Rounding up is always done after the percentage is added.

What if my sections are different lengths?

Run the calculator once for each length and add up the totals: the number of sections field assumes they are all the same. Sections with unequal lengths each get their own equal gap, so the gaps are not the same across the rail, though each is under your limit.

Baluster Spacing Calculator: How Many Balusters Do I Need?

Four 6 ft rail sections of 2 × 2 wood balusters (1.5 in wide) with a 4 in largest gap need 55 balusters (13 per section, 52 in all, plus 5% spares). Each section has 13 balusters with an equal gap of 3 3/4 in (3.75 in, 95 mm), from the post to the first baluster, between balusters and from the last to the post, and the balusters sit 5 1/4 in on center. The code limit is a maximum and local code governs.

Figures based on the sources listed belowUpdated 2026-10-05How we calculate

Section length, inside the posts

Space between the rails

Extra balusters (your choice)

A few spares (5%)Extra for cuts and culls (10%)Exact (0%)Custom

BalusterThe width across the face, seen from the front. A nominal 2 × 2 is 1.5 in actual; a round baluster counts at its diameter.

Largest gap allowed (in)

A maximum, so the equal gap worked out here is always under it. The IRC says a 4 in sphere must not pass; for guards on the open side of stairs it is 4 3/8 in (4.375). Your local code governs.

Number of sections

Sections of the same length between posts. Sections of different lengths need a calculation each.

Price per baluster (optional)

Buy 55 balusters

24 linear ft + 5% waste = 25.20 linear ft

Balusters per section: 13 balusters, 52 in 4 sections
Equal gap: 3 3/4 in (3.75 in, 95 mm)
On-center spacing: 5 1/4 in (5.25 in, 133 mm)
First baluster: edge 3 3/4 in from the post (centre 4 1/2 in), then every 5 1/4 in
Baluster length: 30 in (30 in, 762 mm) between the rails, plus any tenon or mounting allowance

How this number is calculated

24 linear ft + 5% waste = 25.20 linear ft ÷ 0.458 linear ft per baluster = 55 balusters

Shopping list

2 × 2 deck balusters (36 in)
Square metal balusters (3/4 in)
Baluster spacing jig
Baluster connectors
Exterior screws (2-1/2 in)
Tape measure with 1/16 in marks

Show diagram

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Balusters per section, 4 to 8 ft sections

Largest gap 4 in, no spares, worked by this calculator for one section between the posts (inside faces). Gap and on-center spacing to the nearest 1/16 in. 2 × 2 is 1.5 in wide; metal is 3/4 in square.
Section length	2 × 2 balusters	2 × 2 gap	2 × 2 on center	3/4 in balusters	3/4 in gap	3/4 in on center
4 ft	9	3 7/16 in	4 15/16 in	10	3 11/16 in	4 7/16 in
5 ft	11	3 5/8 in	5 1/8 in	12	3 15/16 in	4 11/16 in
6 ft	13	3 3/4 in	5 1/4 in	15	3 13/16 in	4 9/16 in
7 ft	15	3 7/8 in	5 3/8 in	17	3 15/16 in	4 11/16 in
8 ft	17	3 15/16 in	5 7/16 in	20	3 7/8 in	4 5/8 in

Methodology & assumptions

Let L be the section length between the posts (inside face to inside face), w the baluster width and g the largest gap you will allow. n balusters in a section leave n + 1 equal gaps (one at each post and one between each pair), so the equal gap is (L − n × w) ÷ (n + 1). Requiring that gap to be under g gives n > (L − g) ÷ (w + g), so n = (L − g) ÷ (w + g) rounded up, with one more when that division comes out exactly whole, because a gap equal to a limit still lets a sphere of that size pass. For a section shorter than g no baluster is needed, so n is 0. The on-center spacing is the gap plus w. The first baluster’s near edge sits one gap from the post (its centre one gap plus half a baluster), and each next baluster is one on-center step further; the last one then lands one gap from the other post. Total balusters = n × the number of sections, plus the spares you choose (0, 5 or 10%; a choice, not a rule), rounded up. Results are shown to the nearest 1/16 in, also as decimal inches and in millimetres; near the limit the 1/16 value can read as the limit itself, so work from the decimal. The gap is worked from the numbers you type: it is not a check of your guard. The IRC’s opening limitation applies to the whole guard from the walking surface up to the required height, including the space under the bottom rail, the posts and any other part of the infill, none of which this calculator looks at. This calculator is for level rail sections with vertical balusters; a rail that follows a stair’s slope is laid out differently. Different section lengths need a run each. The metric toggle shows lengths in m, baluster width and gap in cm.

International Residential Code (IRC), Section R312.1.3 Opening limitations (2018 and 2021 editions): required guards shall not have openings, from the walking surface to the required guard height, that allow passage of a sphere 4 inches (102 mm) in diameter; exceptions for stairs: the triangular opening formed by the riser, tread and bottom rail may be sized so a 6 inch (153 mm) sphere cannot pass, and guards on the open side of stairs shall not allow passage of a sphere 4 3/8 inches (111 mm)
Section R312 Guards and window fall protection, as published by UpCodes
4" Sphere Rule, Viewrail code compliance resources
Lumber Dimensions: Nominal vs Actual, Archtoolbox: a nominal 2 × 2 is 1 1/2 × 1 1/2 in
Mega Plain Baluster, 3/4" (square hollow iron baluster), Stair Warehouse: 3/4 in square is a common metal baluster size
Your local building department and any HOA rules: the code in force where you build, and any amendments, govern

How to space balusters evenly

Measure the section. Measure the clear distance between the inside faces of the two posts, and the baluster width. Check the largest gap your local code allows.
Count the balusters. Subtract the largest gap from the section length, divide by the baluster width plus the largest gap and round up (one more if it divides exactly).
Work out the equal gap. Gap = (section length − balusters × baluster width) ÷ (balusters + 1). On-center spacing = gap + baluster width.
Make a spacer block. Cut a scrap block to the gap. Use it against each post and between every pair of balusters.
Fasten and check. Fix each baluster plumb, check the last gap at the far post, and total the balusters for all sections plus any spares.

Worked examples

Project	Result
Four 6 ft sections, 2 × 2 balusters, 4 in largest gap, 5% spares	24 linear ft of rail → 13 per section at a 3 3/4 in gap, 52 in all, 55 balusters to buy
Two 8 ft sections, 3/4 in square metal balusters, 4 in largest gap, 10% spares	16 linear ft of rail → 20 per section at a 3 7/8 in gap, 40 in all, 44 balusters to buy
One 5 ft 10 in section, 2 × 2 balusters, 4 in largest gap: the exact case	5.83 linear ft of rail → 13 per section at a 3 5/8 in gap (12 would leave 4 in gaps, too big), 13 balusters to buy
One 5 ft section, 2 × 2 balusters, 4 3/8 in largest gap (the IRC stair figure, 4.375), no spares	5 linear ft of rail → 10 per section at a 4 1/16 in gap, 10 balusters to buy

Frequently asked questions

How many balusters do I need per section?: Take the section length between the posts, subtract the largest gap, divide by the baluster width plus that gap, and round up. For a 6 ft (72 in) section of 2 × 2 balusters and a 4 in gap: (72 − 4) ÷ (1.5 + 4) = 12.4, so 13 balusters. Multiply by the number of sections for the total, then add spares. If the division comes out to a whole number, add one more, so that the gap is under the limit and not equal to it.
What is the maximum gap between balusters?: The International Residential Code (R312.1.3, Opening limitations) says required guards shall not have openings, from the walking surface to the required guard height, that allow passage of a 4 inch (102 mm) diameter sphere. So 4 in is a maximum, and a gap of exactly 4 in is too big; this calculator always works out a gap under the number you enter. The IRC also has two exceptions for stairs, below. Your local code governs: many places adopt the IRC with changes, and commercial buildings and pool barriers have their own rules. The calculator never says a guard is compliant; it does the arithmetic.
Is the limit different on stairs?: In the IRC, yes. Guards on the open side of stairs shall not have openings that allow passage of a 4 3/8 inch (111 mm) sphere, and the triangular opening formed by the riser, tread and bottom rail of a stair guard may be sized so that a 6 inch (153 mm) sphere cannot pass. To work a stair-side guard with the 4 3/8 in figure, type 4.375 in the largest gap field. A rail that follows the slope of a stair is laid out differently from the level sections this calculator handles, and the exceptions are written for stairs only: use 4 in for decks, balconies, porches and landings. Check the edition your jurisdiction has adopted.
Do I measure from the center of the posts or the inside faces?: The inside faces: the clear distance between the posts is the space the balusters and gaps fill. If you only know the post-to-post (center to center) distance, take off one post width (a 4 × 4 post is 3.5 in wide, so subtract 3.5 in). Then use that length for the section.
How do I lay out the balusters?: Cut a spacer block to the gap (for the 6 ft example, 3 3/4 in). Set the first baluster against the block placed against the post, fix it plumb, then use the block between each pair: gap, baluster, gap, baluster, and so on, ending with one gap before the other post. Or mark centres: the first centre is one gap plus half a baluster width from the post, then every on-center step (5 1/4 in in the example). Check the last gap before you fasten the last few balusters, and move all of them a little if it is off. Rounding to the nearest 1/16 in can add up over a long section, so the spacer block, not a tape, is the more reliable way.
Why does the gap come out smaller than my maximum?: Because balusters come in whole numbers. The calculator finds the fewest balusters that keep every gap under your limit, then spreads the leftover space evenly, so the real gap is usually a little under the maximum. A narrower section, or a wider baluster, changes how close it comes.
What width should I use for a 2 × 2?: A nominal 2 × 2 is about 1 1/2 × 1 1/2 in actual, so the width is 1.5 in. Some deck balusters are milled or rounded and a bit different: measure the one you are buying. For a round baluster use its diameter; for a metal bar use its width seen from the front (3/4 in for a 3/4 in square bar). If a baluster is turned or has a wider part, use its widest part; the gap there is what counts.
How many extra balusters should I buy?: It is your choice. The calculator offers none (0%), a few spares (5%) or 10%. Wood balusters have cull pieces and the odd bad cut, and a few spares are cheap to carry; metal balusters seldom need more than a spare or two. Rounding up is always done after the percentage is added.
What if my sections are different lengths?: Run the calculator once for each length and add up the totals: the number of sections field assumes they are all the same. Sections with unequal lengths each get their own equal gap, so the gaps are not the same across the rail, though each is under your limit.

Handy tools

Stair Calculator: Rail sections beside stairs follow the stair’s rise and run: work out the steps and stringers first.
Feet and Inches Calculator: Add up a section measured in feet, inches and fractions, and get decimal feet for the length field above.
Decimal to Fraction Converter: Turn a decimal gap such as 3.607 in into the nearest 1/16 on a tape measure.