How to Calculate the Floor Space Needed to Install a Stairway
Troubleshoot, Fix and Repair
Measure and Calculate
By Kristy Robinson Updated December 29, 2018
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To calculate the space for your stairs, you'll need only basic math skills -- and if your middle-schooler needs extra math practice, make him do the calculations for you. California has strict building code regulations in relations to stairs and stairwells. This gives you a clear set of numbers to plug into a simple equation to figure out how many stairs you need and how much space needed to build them.
Measure the distance from the floor at the bottom of the stair location to the floor at the top of the stair location. This is the total rise of the stairs. Don't use the known height of the room. There could be small variances in the height or allowances for floor joists not included in the measurement.
Divide the total rise by the estimated unit rise height. The unit rise height is how tall you want your stairs. It must be between 4 to 8 inches tall, according to the 2010 California Building Code. If your equation equals a non-whole number, such as 12 1/2, then round up to the nearest whole number. In this case that would be 13. This number is the total number of risers.
Divide the total rise by the total number of risers. This will determine the exact riser height.
Multiply the number of risers by the width of your pre-selected treads. In the State of California, treads may not be shorter than 9 inches wide. The final number you equate will determine the total run length of the stairs.
Check the total run length to determine if any stairs will be located underneath the second floor. The 2010 California Building Code requires a minimum of 80 inches clearance between the top of the individual stair and the bottom of the floor. If 80-inches is not available, adjust the width of your stair tread and the height of your risers until you come up with a formula that provides that much clearance. This adjustment will affect the angle, or steepness, of the stairs.
Measure the width of the handrail. The space between the handrail and the opposite end of the stair tread must be at least 36 inches and the handrail must not overlap the tread by more than 3 1/2 inches. If your handrail is wide, add this length to the treads to determine how wide the stair area must be. If drywall has not been placed on the wall at the stairs, add 1/2 inch to the total area needed for stairs to accommodate this addition.
Check to verify the calculations meet with the International Building Code. Multiply the exact riser height by two and add the width of one tread. To comply with the code, this number must be between 24 and 25. This is not required by the State of California but will ensure you have the safest stairs possible.
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Based in southern Virginia, Kristy Robinson has been writing for various websites since 2008. Her work focuses on tutorials and self-help articles. Robinson holds a Bachelor of Science in criminal justice from American InterContinental University.
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