How to Calculate Risers on Spiral Stairs

Written by michael logan
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How to Calculate Risers on Spiral Stairs
A spiral stairway providing access to a platform. (stair in heaven image by astoria from

Calculating riser height for spiral stairs is no different from any other stairway. Building code enforcement inspectors are particularly strict regarding stair construction. Stairs must be constructed to withstand daily use and be easy to use by a variety of people with various physical abilities. People automatically expect each step to be the same height, since encountering a step that is not the same height can cause them to lose their balance and fall. For this reason, step riser heights must be easy to step up or down and be consistent in height within 1/4 inch.

Skill level:

Things you need

  • Calculator
  • Tape measure

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  1. 1

    Measure the total rise of the stairway from the upper-floor level to the bottom-floor level. Record the total rise in inches.

  2. 2

    Select an approximate riser height of 7 1/2 inches. A riser height of 7 1/2 inches is comfortable for most people. Divide the total rise by the approximate riser height to find the number of stairs. Round this number up or down.

  3. 3

    Divide the total rise by the number of stairs to find the actual riser height in inches. If riser heights fall outside the acceptable range, increase the number of stairs by 1 to reduce the riser height, or decrease the number of stairs by 1 to increase the riser height.

Tips and warnings

  • Check with your local building department for the acceptable range of stair riser heights. Building departments differ with some allowing a riser height of 4 to 8 inches, while others are less flexible and require a narrower range.

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