Balustrade & railing tool
Stair Baluster Spacing Calculator
Work out baluster spacing that passes the 4in sphere rule, how many balusters per tread, and the pitch line and rail heights they hang from. Free, no account needed.
Finished floor to finished floor.
7-7.5in is comfortable; risers are equalised.
Horizontal depth, nosing excluded.
Plumb from the nosing line to the ceiling above.
Landings, stringers & handrail
Half-landings or quarter-turns in this flight.
16in suits most treads; 12in for thin or composite.
Measured above the nosing line.
4in (100mm) sphere rule.
Sets the bottom stringer drop.
Your flight
15 risers × 7.33in
14 treads at 10.50in / 26.7cm · 36.8 degree pitch
- Riser height (equal)
- 7.33in / 18.6cm
- Tread run (going)
- 10.50in / 26.7cm
- Total run
- 147.00in / 373.4cm
- Cut line (diagonal)
- 183.60in / 466.3cm
- Stringer stock length
- 195.60in / 496.8cm (16.3ft)
- Pitch
- 36.8 degrees
- 2R + G
- 25.17in / 63.9cm
- Stringers
- 4 at 12.0in centres (65.2ft stock)
- Handrail length
- 207.60in / 527.3cm (17.3ft)
- Balusters
- 39 at 5.2in centres
Code & comfort checks
IRC (US) allows a maximum 7.75in riser. Every riser in a flight must match within 3/8in.
Minimum going is 10.00in. Shallow treads are the single biggest cause of descending falls.
Comfortable domestic flights sit between 30 and 37 degrees; 42 degrees is the absolute ceiling.
Two risers plus one going should land between 24in and 25in (600-640mm) for a natural stride.
IRC (US) expects at least 36.0in of clear width above the handrail height.
Minimum headroom is 80in measured plumb from the tread nosing line.
IRC wants a graspable handrail 34-38in above the nosing line, continuous for the full flight, and no gap that passes a 4in sphere.
Guidance only — confirm against your local building control before you cut or pour.
Compare riser counts
| Risers | Riser | Going | Run | Pitch |
|---|---|---|---|---|
| 14 | 7.86in | 10.50in | 11.4ft | 38.9° |
| 15 | 7.33in | 10.50in | 12.3ft | 36.8° |
| 16 | 6.88in | 10.50in | 13.1ft | 34.9° |
The highlighted row is your current flight. Nudging the riser count is usually the cheapest way to bring a steep or shallow stair back inside code.
Advanced layout
Mark-out schedule, framing deductions & take-off
Quick answer
How do you calculate baluster spacing on stairs?
Baluster spacing is set by the 4in sphere rule: no gap in a stair guard may allow a 4in (100mm) sphere to pass through. Divide the clear run between newel posts by the maximum permitted gap plus one baluster width, then round up to a whole number of balusters and recalculate the gap so it lands under 4in. On a raking stair the spacing is measured along the pitch line, so each baluster is cut to a different length while the horizontal spacing stays constant.
- No gap may pass a 4in (100mm) sphere — US IRC and UK Approved Document K agree on this.
- Balusters = ceil(clear run / (4in + baluster width)); recalculate the gap after rounding up.
- Two balusters per tread is the usual result on treads of 10in or more.
- Handrail height is 34-38in in the US, 900-1000mm in the UK, measured vertically from the nosing.
- On a raking stair, spacing is set along the pitch line, not the floor.
- Cut balusters after the rail is fixed — each one on a stair is a different length.
The rule that decides every gap
Balustrade spacing is one of the few parts of a staircase where the design is dictated almost entirely by a single safety test: a 4in (100mm) sphere must not pass through any opening in the guard. That test governs the vertical gaps between balusters, the triangular gap formed where a raking rail meets a tread and riser, and the gap between the bottom baluster and the newel. Design to the sphere first, then make the spacing look regular — never the other way round.
| Baluster width | Max centres | Per 10in tread | Per 11in tread |
|---|---|---|---|
| 1 1/4in square | 5 1/4in | 2 balusters | 2 balusters |
| 1 1/2in square | 5 1/2in | 2 balusters | 2 balusters |
| 1 3/4in square | 5 3/4in | 2 balusters | 2 balusters |
| 32mm turned | 132mm | 2 balusters | 2 balusters |
| 41mm turned | 141mm | 2 balusters | 2-3 balusters |
Know your numbers in 30 seconds
Enter your step count, tread depth and riser height — the stair runner calculator gives you the exact length to order, in metres and feet, with waste included.
Open the stair runner calculatorWorked example: a straight flight between two newels
Say the clear run between newel posts measures 118in along the pitch line and you are using 1 1/4in square balusters. Divide 118 by the maximum centre-to-centre distance of 5 1/4in and you get 22.5, so you need 23 gaps and therefore 22 balusters. Recalculate: 118in of run, less 22 balusters at 1 1/4in (27.5in), leaves 90.5in of gap spread over 23 openings — 3.93in each, safely under 4in with tolerance to spare for a slightly out-of-square post.
Why every baluster on a stair is a different length
On a level landing all balusters are identical. On a raking flight they are not: each one sits a fixed horizontal distance further along a rail that is climbing at the stair's pitch, so its length grows by the riser height divided by the number of balusters per tread. With a 7 1/2in riser and two balusters per tread, that is a 3 3/4in increase per baluster. Cut them in sequence after the rail is fixed, label them, and dry-fit the whole run before any glue goes near a mortice.
Get the geometry before the balustrade
Baluster schedules go wrong when the rise and run underneath them are still approximate. Fix the riser height, going and pitch first with the stair calculator, then set newels, then divide for balusters — the order saves a rebuild.
Handrail height and how it interacts with spacing
Rail height changes baluster length, not baluster spacing, but it changes the triangular gap at the foot of each baluster where the rake meets the tread. That triangle is the opening most likely to fail a gauge test on a shallow-pitch stair, so check it specifically: measure at the widest point, square to the rake, with the gauge pushed hard into the corner. If it fails, the fix is usually a shorter first gap or a slightly wider baluster rather than a wholesale respace.
Material take-off and cost
Once the count is settled, the take-off is straightforward: balusters, two rails (hand and base), newels, newel caps, fixings and the base fillet or spacer system if you are using one. Order 5-10% more baluster stock than the count if the flight is turned or the timber is figured, because a split at the mortice on a long baluster is a wasted piece. For a fitted runner going down the same flight, the runner width and the balustrade interact — the stair runner calculator and the stair runner width calculator will tell you how much timber margin you have left once the balusters are in.
Whatever you decide, get the numbers first: the stair runner calculator doubles as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length the first time.
Frequently asked questions
How far apart should stair balusters be?
Close enough that a 4in sphere cannot pass through any gap. With a 1 1/4in square baluster that means centres of no more than about 5 1/4in, which on a 10in or 11in going works out at two balusters per tread. Always calculate from the clear run between newels rather than assuming a spacing, because the last gap is the one that fails inspection.
How many balusters do I need per step?
Two per tread is standard on treads of 10in and deeper, and it is what most stair-part suppliers price to. Very deep treads, landings and bullnose steps often need three to keep every gap under 4in, and a tight winder can occasionally take one plus a post.
Is the 4in rule the same in the UK?
Effectively yes. UK Approved Document K uses a 100mm sphere, which is 3.94in, so a design that satisfies the UK figure also satisfies the US one. Building control in both places tests it the same way: with a gauge, at the widest gap they can find, including the triangular gap at the bottom of a raking rail.
How do I space balusters on a raking (sloped) stair?
Set the spacing horizontally along the pitch line — the imaginary line joining the tread nosings — and transfer that spacing up to the underside of the handrail. Horizontal spacing stays constant, so every baluster length differs by exactly one riser height divided by the number of balusters per tread.
What height should the handrail and guard be?
In the US, 34-38in measured vertically from the nosing to the top of the rail for a stair handrail, with guards on landings at 36in minimum (42in in many commercial settings). In the UK, 900mm on the flight and 1100mm on landings for a dwelling. Check your local amendments — this is one of the most commonly varied figures.
Does baluster spacing change for glass or cable infill?
The 4in sphere rule still applies to any opening, but the arithmetic changes: glass panels remove the gap problem entirely and shift the design onto panel thickness and fixing, while horizontal cable runs are tested for deflection as well as spacing — cables that pass a static gauge can still open past 4in when someone leans on them.
Can I use this calculator for the newel post positions too?
Yes. The tool gives you total rise, total run and the pitch line, which is what sets newel positions and the rail length between them. Get those numbers first, then divide the clear run for balusters — spacing calculated before the posts are located almost always has to be redone.
Is the stair baluster spacing calculator free?
Yes, the geometry, pitch line and code checks are free without an account. A free account saves the flight so the same spacing schedule can be reused across staircases in one project.
Related tools & guides
Start with the numbers: our stair runner calculator also works as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length first time.