Baluster Spacing

What it's for

You have a rail opening between two posts and need the balusters evenly spaced with every gap the same. Baluster Spacing gives you the count, the on-center spacing, the clear gap, a running mark-out to lay the rail out from one hook of the tape, and the number to buy. It checks the gap against the code's 4" sphere for a level guard, and against your own maximum when you set one, and says so in green or red.

Step by step

The example is the screen as it opens: a 6' 0" opening, 1-1/2" balusters, 4" maximum gap.

Baluster Spacing inputs: OPENING LENGTH 6' 0", BALUSTER WIDTH 1-1/2", MAX GAP 4", FORCE BALUSTER COUNT off, STAIR GUARD off, WASTE FACTOR 5%. The answer bar under the title reads 13 balusters, a green Under the 4" sphere limit — level guard (IRC 2021 R312.1.3), check local code, then OC 5-1/4" · Clear gap 3-3/4". RESULTS repeats them under three count chips: 12 @ ≈ 4-1/8" OVER 4" SPHERE, 13 @ 3-3/4" UNDER 4" (picked) and 14 @ ≈ 3-3/8" UNDER 4", then BASIS & WHAT WAS ASSUMED and the link Check before you cut · Measure twice. — Steps 1–5 are the inputs. Step 6 is the row of count chips in RESULTS. The STAIR GUARD row is covered under Stair guards, below.

OPENING LENGTH — the clear distance between the two posts, face to face, along the rail. Not center to center. The example is 6' 0".
BALUSTER WIDTH — the actual width of one baluster along the rail. Put a tape on it. The example is 1-1/2".
MAX GAP — the gap you want every opening kept under, between balusters and between a baluster and a post. It opens at 4", the code's sphere. Set it tighter if you like. A wider number changes the count, never the code check.
FORCE BALUSTER COUNT — leave it off and the calculator picks the fewest balusters that keep every gap under MAX GAP. Turn it on and a COUNT row appears, starting at the calculated count, for you to set your own. STAIR GUARD sits just below it. Leave it off for a level guard; it is covered under Stair guards.
WASTE FACTOR — extra balusters to buy, in steps of 5%. The example is 5%.
Read RESULTS: 13 balusters. The three chips under it are one fewer, the current count and one more, each with its gap: 12 @ ≈ 4-1/8" OVER 4" SPHERE, 13 @ 3-3/4" UNDER 4", 14 @ ≈ 3-3/8" UNDER 4". Tap a chip to use that count. It switches FORCE BALUSTER COUNT on for you.
The next line is the layout: OC 5-1/4" · Clear gap 3-3/4". The green line under it is the check.
Scroll down to the drawing and compare it with the rail in front of you.
Use RUNNING MARK-OUT to lay out the rail. Hook the tape on the face of the post at the start of the opening. Each row is the near and far face of one baluster from that hook: B1 is 3-3/4" → 5-1/4". Mark both, put an X on the far side, tap MARKED — NEXT and go on to B2.
Read MATERIALS for the order: 14 pcs in the example, 13 plus waste.

Baluster Spacing scrolled down. The answer bar keeps 13 balusters and the green check, cut off at (IR…. Under the link Check before you cut · Measure twice sits the rail elevation dimensioned 6' 0" with a post section at its left dimensioned 5-1/4" and 3-3/4" and a title block reading BALUSTER LAYOUT, 13 BALUSTERS, NOT TO SCALE. Below it the RUNNING MARK-OUT card, MARK 1 OF 13 with a speaker button, lists B1 3-3/4" → 5-1/4" down to B9 3' 9-3/4" → 3' 11-1/4". — Steps 8 and 9: the rail drawing, then the running mark-out.

The bottom of Baluster Spacing: mark-out rows B3 to B13, the note Hook the opening face — near/far face of every baluster. X on the far side., the MARKED — NEXT and RESET buttons, then MATERIALS: Balusters (1-1/2") — 5-1/4" OC, 3-3/4" clear gap, 14 pcs, with the guard-height note; Top rail — 6' 0" opening +5% waste, 7 lin ft; and Bottom rail, 7 lin ft. Under it a dimmed SHARE PDF with a list icon at its right end sits beside a lit SAVE. — Step 9 continues with MARKED — NEXT. Step 10 is the order quantity.

Reading the results

The headline is the number of balusters in the opening. sample beside it means you have not entered your own numbers yet.

The chips let you trade count against gap. A chip marked ≈ has a gap that does not land on a sixteenth. A chip reading OVER 4" SPHERE has a gap of 4" or more, which a 4" sphere goes through, or no gap left at all. 4" AT THE MARKS is under 4" by less than 1/16": built to the 1/16" marks it can come out exactly 4", so it fails too. OVER MAX is under the 4" sphere limit but not under your MAX GAP. A chip past the COUNT row's range of 0 to 200 is dimmed and does nothing when tapped.

OC is center to center of the balusters. Clear gap is the open space between them, and it is the same at both posts.

The check. IRC R312.1.3 says a guard's openings must not let a 4" sphere through, so on a level guard every gap must be under 4". A gap of exactly 4" fails. The mark-out gives each baluster to the nearest 1/16", so a gap built to the marks can be up to 1/16" off the figure; the count keeps every gap at least 1/16" under the limit so that no gap you build to the marks reaches 4", and a gap closer than that reads red: FAILS 4" SPHERE RULE — A GAP WITHIN 1/16" OF 4" BUILDS TO 4" AT THE 1/16" MARKS (IRC R312.1.3). Green reads Under the 4" sphere limit — level guard (IRC 2021 R312.1.3), check local code. Red reads FAILS 4" SPHERE RULE — GAP … MUST BE UNDER 4" ON A LEVEL GUARD (IRC R312.1.3), with your gap filled in. The line is judged against the code's 4", never against MAX GAP. When MAX GAP is anything other than 4", a second line checks your own number: green Gap … under your … max, red GAP … NOT UNDER YOUR … MAX. A gap that is under your max but not under 4" is red too and says both: GAP … — UNDER YOUR … MAX, OVER THE 4" SPHERE LIMIT. The fix for either is more balusters: tap the chip with one more, or raise COUNT. A wider BALUSTER WIDTH closes the gap too. When a forced count is more than the opening can hold, the line reads NO GAP LEFT — TOO MANY BALUSTERS FOR THE OPENING.

BASIS & WHAT WAS ASSUMED — tap it to open. It says the check is for a level guard: on the open side of a stair R312.1.3 allows 4-3/8", and 6" in the triangle of riser, tread and bottom rail, and there the balusters go by the tread (see Stair guards). It names the code edition the check comes from and tells you to verify local amendments with your building department. Your local code and your inspector govern.

Stair guards

On a stair the balusters go by the tread, and "2 per tread" is not always enough: two 3/4" balusters on a 10-1/2" tread leave 4-1/2", over the stair's 4-3/8" (IRC 2021 R312.1.3 Exception 2). Turn on STAIR GUARD and enter TREAD RUN, the tread depth nosing to nosing (it starts at 10-1/2"). RESULTS then adds a line with the fewest balusters per tread that keep the gap under 4-3/8": Stair: 3 per tread, gap 2-3/4" — under 4-3/8" (IRC R312.1.3 Exc. 2) for 3/4" balusters on a 10-1/2" tread. Thin iron balusters need 3 on any legal tread; 1-1/2" square stock keeps 2 up to an 11-3/4" tread. The cut sheet and a saved result carry the tread run and the count per tread. With STAIR GUARD on, the whole screen is a stair guard: the count is the per-tread count times the treads the opening spans (a part tread still takes its balusters), the on-center spacing is the tread over that count, the drawing and mark-out lay it out in plan along the run, and the green or red line judges 4-3/8", reading Under the 4-3/8" sphere limit — stair guard (IRC 2021 R312.1.3 Exc. 2). The count chips are level alternatives, so they are not shown. From Stairs, SPACE THE BALUSTERS under the results opens this screen with the stair's total run as the opening and its tread as TREAD RUN, STAIR GUARD on; your width, MAX GAP and waste stay.

The drawing is the rail in elevation with the opening dimensioned, and a boxed detail of one baluster with the on-center and gap dimensions. Pinch to zoom, drag to move around, double-tap to reset, or tap the expand button at its top right for the whole screen. See Reading the drawing.

RUNNING MARK-OUT keeps your place as you mark, and the speaker button reads the marks aloud. RESET asks you to confirm, then goes back to the first mark. See Mark-out and speak.

MATERIALS starts with the balusters at your width, with the spacing and gap in the name and the quantity to buy with waste added and rounded up. Their row also says what length to buy: the guard height less your rails — a guard stands at least 36" above a level floor and 34" above the stair nosings (IRC 2021 R312.1.2). Then come the top rail and the bottom rail, each the opening long plus waste, in lineal feet; leave the bottom rail off if your balusters fasten straight into the framing.

The buttons wait for your own numbers: the export button and the list icon in it stay dimmed, and SAVE tells you to enter them first. After that SHARE PDF shares the drawing, the code check, the mark-out, your inputs and the materials as a PDF, the list icon at the right end of the same button shares the material list alone, and SAVE puts the result in History.

Baluster Spacing is a Pro calculator. The PDF, History and reading the marks aloud are Pro too. Pro is a one-time purchase. See Free and Pro.

Common mistakes

Measuring post center to post center. OPENING LENGTH is the clear opening, post face to post face. The mark-out hooks on that face.
Entering the nominal baluster size. A 2x2 is not 2" wide. Every gap is worked from BALUSTER WIDTH, so measure one.
Raising MAX GAP to get fewer balusters. The count follows MAX GAP, but the code check does not: past 4" it goes red. Leave it at 4" unless you want it tighter.
Putting 2 balusters on every stair tread without checking. With balusters 5/8" or thinner, 2 per tread fails 4-3/8" on every legal tread. Turn on STAIR GUARD and read the count per tread.