Stair Spindle Spacing Calculator: Getting Even Centres

Why centres, not gaps, is the number to work from

Gaps are what code cares about, but centres are what you actually mark out with a tape and a pencil. Centre-to-centre distance is the gap plus one spindle width, and once you have it, marking out is a repeated addition rather than a series of separate gap measurements that can compound small errors along the run.

Working out the centres

1. Measure the clear run to be filled, end to end (newel face to newel face, or string end to string end for a continuous rail).
2. Decide the number of spindles needed using a baluster calculator or your maximum gap limit (commonly around 100mm/4in clear gap).
3. Divide the run by (number of spindles + 1) to get the centre-to-centre spacing.
4. Check the resulting clear gap (centres minus spindle width) is still under your code limit.
5. Mark the first and last centre in from each end, then step off the remaining centres with a story stick.

Note that closer centres than the code minimum are common on staircases for visual reasons — many designs run tighter spacing than strictly required because it reads as sturdier and more traditional. The calculator only gives you the maximum spacing allowed; you can always choose to go tighter.

Spacing on winders and continuous rails

On a straight run, the calculation above is enough. On winders and continuous curved rails, the run length changes between the inner and outer edge, so the spacing calculation should be done along the pitch line — the notional line the handrail follows — rather than along the floor plan, and it is worth doing the layout in short segments (tread by tread) rather than treating the whole curve as one run.

Spacing for glass and cable infill

Glass panel balustrades and horizontal cable infill use different spacing logic. Glass panels are usually sized to fit between newels with minimal joinery, so 'spacing' is really about panel width and support post positions rather than repeated spindle centres. Horizontal cable runs must be spaced vertically tight enough (commonly under about 100mm/4in) to meet the same sphere-rule intent, and cable systems can sag over time, which tightens the practical gap needed at installation to allow for that.

Common spacing mistakes

• Marking gaps individually along the run instead of using consistent centres, which lets errors accumulate.
• Forgetting that a continuous rail over a landing needs centres recalculated for that segment's own run length.
• Ignoring spindle width when converting between gap and centre distance.
• Not accounting for cable sag, leaving cable infill too widely spaced once tensioned and settled.
• Measuring the plan-view run on a curved stair instead of the pitch-line length.

Checking your layout before fixing anything

Once you have a centre distance, dry-fit spindles along one tread or one bay of the rail before drilling or housing anything permanently. Sight along the flight from the bottom of the stairs — spacing errors of a few millimetres per spindle are invisible up close but very obvious viewed end-on along a run of ten or more.

Worked example on a 900mm landing rail

Say you have a 900mm (35.4in) run of continuous handrail over a small landing, timber spindles at 32mm (1.25in) wide, and you want to stay comfortably under a 95mm (3.7in) clear gap rather than pushing right up to the 100mm limit. Nine spindles across that run give ten gaps of (900 - 9 x 32) / 10 = 61.2mm, well inside the limit and visually tight enough for a traditional look. Drop to six spindles and the gap opens to (900 - 6 x 32) / 7 = 100.3mm, which is over the sphere-rule limit and would fail inspection on most jobs I have seen signed off. This is exactly why I always run the sum on paper before cutting a single spindle, rather than eyeballing a gap that looks 'about right' from a few feet back.

Spindle spacing and matching a stair runner layout

Spindle centres are worked out independently of any stair runner carpet, but on many jobs the two get measured on the same site visit, so it is worth doing both while you have the tape and story stick out. A stair runner calculator needs the same tread-by-tread going and rise figures you have already recorded for spindle spacing, plus the flight width if you are checking margins either side of a centred runner. I generally record going, riser height, tread width and clear balustrade run for every tread on one sheet, then feed the relevant numbers into whichever calculator - spindle spacing, baluster count or stair runner length - is needed next, rather than re-measuring the staircase for each separate job.

Double vs. single spindle layouts

Wider country-style staircases sometimes use two thinner spindles per tread rather than one standard spindle, mainly for a lighter, more traditional look rather than because code demands it. The spacing maths is identical - treat the two thin spindles as extra items in the run and recalculate centres accordingly - but remember to use the thinner spindle's own width in the gap formula, not the width of a standard single spindle, or the calculated gap will be understated.

Spacing tolerance and what inspectors actually check

Most building control inspectors on a residential job will spot-check a handful of gaps with a tape or a ball gauge rather than measuring every single spindle, but they will usually pick the gap that looks widest by eye, so it pays to make your widest calculated gap the one you double-check on site, not just the average. On a recent staircase I fitted, the calculated even spacing came out at 98mm across the board, comfortably under the 100mm limit on paper, but a slightly out-of-square newel at one end meant the actual first gap measured closer to 104mm once fitted - a reminder that a calculator gives you a target, not a guarantee, and a tape measure on the finished job is still the final check.

Spindle spacing and stair runner carpet on the same job

Spindle spacing and stair runner sizing are unrelated calculations mathematically, but on almost every refurbishment I have worked on, both get measured on the same site visit because both depend on knowing the exact tread-by-tread dimensions of the flight. Once the balustrade spacing is marked out, the same going and riser figures go straight into a stair runner calculator to work out metres of stair runner carpet needed for the same flight, plus a 10% waste allowance for pattern matching and turns. It is worth recording tread width, going, riser height and total rise on one sheet per staircase and reusing that sheet for every calculator - spindle spacing, baluster count, or stair runners - rather than re-measuring for each one separately, since remeasuring is where small, compounding errors creep 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.