Stair railing calculator

Enter the total rise and total run of the flight and the calculator returns three things: the stair angle in degrees, the straight-line handrail length along the slope, and a post count based on the spacing you set. All three come from the same right triangle you'd use to lay out stringers.

The rail angle is the same angle the stringer makes with the floor, because a sloped handrail is installed parallel to the nosing line. If your stringers run at 34 degrees, your rail runs at 34 degrees too, and every angled cut on the rail ends is referenced from that number.

Use the same total rise and total run you'd use for the stringers: rise is floor-to-floor height, run is the horizontal distance from the first riser face to the last. If the rail continues past the top or bottom newel with a level section, measure that separately and add it after the sloped length is known.

For an open-string rail that follows the nosings, measure from the outside edge of the bottom nosing to the outside edge of the top nosing rather than from the wall framing, since that's the line the rail actually parallels.

Rail angle = arctan(rise ÷ run), identical to stair angle. Sloped rail length (the stringer-line length) = √(rise² + run²). For a 108 in rise and 147 in run, that's √(108² + 147²) ≈ 182.4 in, at an angle of about 36.3°.

Handrail length for ordering is that sloped length plus extensions at both ends. A common approach adds roughly 12 in of horizontal run at the top for a level return into a wall or post, plus a shorter extension at the bottom, commonly cited as about one tread depth for some code-covered occupancies (hedged, and this varies by jurisdiction and whether the stair is residential or commercial).

Post count from spacing: number of posts ≈ round(run ÷ postSpacing) + 1. With a 147 in run and 72 in target spacing, that's 147 ÷ 72 ≈ 2.04, rounding to 2 intermediate posts plus the top and bottom newels.

Where a sloped rail meets a level rail (at a landing or the top of the stair), the two pieces are joined with a compound cut, sometimes called a gooseneck or volute fitting on stock stair parts, rather than a simple butt joint. Where a sloped rail dies into a newel post, the end is typically cut at the stair angle so the rail face sits flush against the post.

Mark the angle directly off the stringer or a bevel gauge set to the calculated angle rather than trusting a tape measure alone; a half-degree error compounds visibly over a long rail run.

Posts (balusters or newels supporting the rail itself, as opposed to the smaller balusters that fill the guard) are usually spaced by structural need and appearance rather than a fixed code number, though very long unsupported rail spans can feel bouncy under hand pressure.

A practical rule many builders use is a post at every third or fourth tread, or roughly every 60–72 in of horizontal run, then adjusting to land posts near landings and turns. The calculator's postSpacing input lets you test different targets and see how many posts that implies.

Handrail height is commonly cited in residential reference values as 34–38 in measured vertically from the nosing of the tread to the top of the rail, with continuous graspability (a round profile or one with a similar grip) required along the full run in most jurisdictions.

These are hedged, code-referenced figures, not guarantees, since local amendments and occupancy type change the numbers. Confirm the exact height and profile requirement with your local building department before installing.

• Ordering rail stock at the sloped length alone with no allowance for the top and bottom fittings, then coming up short.
• Cutting the rail-to-newel joint square instead of at the stair angle, leaving a visible gap on one side.
• Spacing posts evenly by eye instead of calculating from the actual run, so the last post lands awkwardly close to the newel.
• Measuring rail height from the tread surface instead of the nosing, which understates the code-referenced dimension.

If you haven't settled on riser and tread dimensions yet, start with the stair rise and run calculator or the stair stringer calculator to lock in the flight geometry first, then come back here for the rail. For the balusters that fill the space under the rail, use the baluster spacing calculator.