Stair Railing Calculator: Handrail Length, Posts and Baluster Spacing

Why handrail length is not the same as the run

A common mistake when ordering handrail is measuring the horizontal distance the staircase covers and buying that length. Handrail follows the pitch line of the stair, which is always longer than the horizontal run because it rises as it goes. The calculator finds this by treating the flight as a right-angled triangle: the horizontal run is one side, the total rise is the other, and the handrail length is the hypotenuse. For a flight with a 2.6m (8ft 6in) horizontal run and a 2.6m (8ft 6in) total rise — a common 40-42 degree pitch for a straight domestic stair — the hypotenuse works out to about 3.68m (12ft 1in), roughly 41% longer than the horizontal run alone.

The formula is simple trigonometry: handrail length = square root of (run squared + rise squared), or equivalently run divided by the cosine of the pitch angle. Most domestic staircases pitch between 33 and 42 degrees; the UK's Approved Document K sets a maximum pitch of 42 degrees for a private stair, and the US IRC caps the maximum riser-to-going ratio in a way that typically lands stairs in a similar 30-37 degree range. Steeper stairs need proportionally more handrail per metre of horizontal run, which matters when you are pricing hardwood handrail sold by the linear metre or foot.

Newel posts and where they land

Newel posts sit at the top and bottom of every flight and at every change of direction on a landing or winder turn. Between those fixed points, intermediate posts are only needed on long straight runs where the handrail would otherwise sag or flex — typically every 1.2-1.8m (4-6ft) for a timber rail, closer to 1m (3ft 3in) for a lighter section or a glass balustrade panel that needs a post at every panel joint. The calculator treats the top and bottom newels as fixed and then divides any remaining straight run into equal bays no wider than your chosen maximum post spacing, rounding the bay count up so no single bay is oversized.

On a turned staircase, a half-newel or full newel sits at the landing, and the handrail is scribed and jointed to change direction there — either with a simple mitred joint for a 90-degree turn or a curved wreathed section for a sweeping turn, which is a specialist joinery job and typically adds 30-60% to the cost of that section of rail.

The sphere rule: spacing balusters correctly

Baluster (spindle) spacing is governed by a child-safety rule rather than an aesthetic one. UK Building Regs Part K requires that a 100mm sphere cannot pass through any gap in the balustrade, which in practice means a clear gap of 99mm or less between balusters, measured at the narrowest point. US IRC R312 uses a 4in (102mm) sphere rule with the same principle: a 4in sphere must not be able to pass through the guard. The two figures are close enough that hardware bought to one standard will almost always satisfy the other, but always check your local code before finalising an order.

Because balusters have real width, the gap you need to solve for is the clear space between them, not the centre-to-centre spacing. If your baluster is 32mm (1.25in) square and you want a maximum 99mm (3.9in) clear gap, the centre-to-centre spacing is 99mm + 32mm = 131mm (5.15in). The calculator takes the total bay width, subtracts one baluster width, divides by the target clear gap to find the minimum number of gaps needed, rounds up, then works backwards to give you the actual centre-to-centre spacing that fits the bay exactly with no leftover sliver at either end.

Worked example: a 1.8m bay

Take a straight bay 1.8m (5ft 11in) wide between two newel posts, using 32mm (1.25in) square timber balusters and the UK 99mm clear gap limit. Divide 1800mm by (99mm + 32mm) = 131mm to get 13.7, round up to 14 gaps, meaning 13 balusters. Recompute the actual spacing: 1800mm / 14 = 128.6mm centre-to-centre, giving a clear gap of 128.6 - 32 = 96.6mm, safely under the 99mm limit with a small margin for manufacturing tolerance.

Materials list for a typical straight-flight railing

• Handrail: hardwood (oak, ash) or softwood profile, ordered to the calculated pitch length plus 100-150mm (4-6in) waste for scribing at each end.
• Base rail: matching profile, same length as the handrail, sitting at the bottom of the balusters.
• Newel posts: 2 minimum on a straight flight (top and bottom), 90x90mm (3.5x3.5in) or larger for structural posts.
• Balusters/spindles: count from the spacing calculation, typically 32-41mm (1.25-1.6in) square or turned profile.
• Newel caps and fixing brackets or newel bolts for a rigid post-to-floor connection.
• Glass balustrade alternative: toughened laminated glass panels 10-12mm thick with a shoe channel, priced per panel rather than per baluster.

Cost per metre/foot: UK and US

A softwood staircase railing kit with handrail, base rail, newels and turned spindles typically costs £60-£120 per metre run in the UK, or $70-$140 per foot in the US for equivalent stock parts, fully installed. Hardwood oak or ash railing with bespoke turned or stopped-chamfer balusters runs £150-£300 per metre in the UK, or $180-$350 per foot in the US. Glass balustrade panels are priced per panel rather than per linear measure and typically add £200-£450 per panel in the UK or $250-$550 per panel in the US, fitted.

Handrail height and grip requirements

UK Approved Document K sets a handrail height of 900mm minimum measured vertically above the pitch line of the stair, with 900-1000mm being typical practice. US IRC R311.7.8 requires handrail height between 34in and 38in (864-965mm) measured the same way. Both codes also specify a graspable profile — broadly a circular section 32-50mm (1.25-2in) in diameter, or an equivalent non-circular profile with a similar hand-grip perimeter — because a rail that is too wide or too flat is genuinely harder to grip in a fall.

1. Measure horizontal run and total rise to find the pitch length with the trigonometric formula.
2. Decide newel post positions: fixed at top, bottom and every direction change, plus intermediate posts on runs over about 1.5m (5ft).
3. Choose your baluster width and the applicable sphere-rule clear gap for your jurisdiction.
4. Solve each bay for the number of balusters, always rounding up.
5. Order handrail to the pitch length plus scribing waste, not the horizontal run.

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.