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Handrail Length Calculator

Enter the stair run, rise and any extensions.

How it is calculated

A stair rail follows the sloping line from the first nosing to the last, which is the hypotenuse of a right triangle made by the total run and total rise. slope length = √(run² + rise²), total handrail = slope length + top extension + bottom extension, and angle = arctan(rise ÷ run) in degrees. Run is the total horizontal distance of the stair, rise is the total vertical height, and both must be in the same unit. The result is in that same unit, with three decimals, since the tool does not convert anything. Top and bottom extensions are optional and count as 0 when empty; they are simply added to the slope length. Run must be above zero. The tool does not enforce any building code rule on extension length or rail height, so confirm those separately.

Worked example

A stair has a total run of 120 in and a total rise of 105 in, with 12 in extensions at the top and bottom. Slope length: √(120² + 105²) = √(14,400 + 11,025) = √25,425 = 159.452 in. Add the extensions: 159.452 + 12 + 12 = 183.452 in of rail. Angle: arctan(105 ÷ 120) = arctan(0.875) = 41.19°. Without the extensions the rail would be only 159.452 in, so a rail cut to the slope length alone would be 24 in short.

Try this example →

Common mistakes

  • Using one number for run in inches and another for rise in feet or millimetres. Both must be the same unit or the slope length is meaningless.
  • Entering the length of the stair run measured along the slope as the run. The run must be the horizontal distance.
  • Entering the riser height of one step instead of the total rise, or one tread depth instead of the total run.
  • Forgetting the extensions at either end, which makes the rail too short to reach the landing or the post.
  • Ordering material at exactly this length. Cuts at the angle of the slope, waste and fittings mean you should add some extra.

About this calculator

A sloped stair handrail's total length isn't just the horizontal run or vertical rise — it's the hypotenuse along the slope, found with the Pythagorean relationship √(run² + rise²), plus any horizontal extensions required at the top and bottom landings for code compliance and graspability. This calculator computes that sloped length, adds your specified top and bottom extensions, and reports the slope angle (arctan of rise over run) that the rail — and its wall brackets — will need to be cut and mounted at.

Stair builders, finish carpenters and code inspectors use this calculation because most building codes (like the IRC in the US) require handrail extensions of roughly one tread depth at the top and bottom of a stair run, specifically so a hand can find secure support before the first step and after the last one — getting the sloped length and angle right the first time avoids re-cutting an expensive hardwood or metal rail, since the cut angle at each end must match the slope precisely to seat flush against the newel post or wall bracket.

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