Staircase Calculator
Calculate steps, risers, treads, and stringers for your staircase.
Example: 9 ft floor = 108 in. Measure from finished floor to finished floor.
IRC minimum: 10"
IRC minimum: 36"
IRC Code Compliance
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This calculator takes your total floor-to-floor rise, tread depth, and stair width, then computes the number of steps, riser height, total run, and stringer length. It also checks every result against IRC (International Residential Code) requirements and flags any issues. Here is how to get accurate results:
- Measure total rise. This is the vertical distance from the finished floor at the bottom to the finished floor at the top. A standard 9-foot ceiling with a typical subfloor and framing puts total rise around 108 to 112 inches. Always measure the actual dimension rather than assuming from ceiling height.
- Set tread depth. The IRC requires a minimum of 10 inches. The default of 10.5 inches gives a comfortable walking feel and keeps the riser-plus-tread sum near the ideal 17-18 inch ergonomic range. Deeper treads (11-11.5 inches) are common in custom residential builds.
- Enter stair width. The IRC minimum for residential stairs is 36 inches. Wider stairs (42-48 inches) are common in main entry halls. The width affects stringer layout but not the step count.
- Read the results. The calculator shows the number of steps, individual riser height, total horizontal run, and the stringer length (the diagonal board that supports the steps). The code compliance section gives a green or red indicator for each IRC requirement.
Results update as you type. Use Share to send your layout to a contractor, or Copy to paste the step count into a quote or permit application.
About the Staircase Calculator
Stair design is governed by the IRC, which sets limits on riser height (maximum 7.75 inches), tread depth (minimum 10 inches), and stair width (minimum 36 inches). Risers that are too tall are a major fall hazard, especially for older adults and children. The IRC also requires that the variation between the tallest and shortest riser in a single flight not exceed 3/8 inch, which is why the calculator distributes the total rise evenly across all risers rather than rounding to a convenient number.
The stringer length is the hypotenuse of the rise-run triangle. For a 9-foot rise (108 inches) and a 14-tread run at 10.5 inches each (147 inches total run), the stringer is approximately sqrt(108² + 147²) = 182 inches or about 15.2 feet. You will typically need two side stringers plus a center stringer for any stair wider than 30 inches. The riser-plus-tread comfort formula (sum of 17-18 inches) is not an IRC code requirement but is a widely used ergonomic guideline that produces stairs that feel natural to walk.
Frequently Asked Questions
How many steps do I need for a 9-foot ceiling?
A 9-foot floor-to-floor height is typically 108 to 112 inches when you account for subfloor, joists, and finished flooring. At 108 inches of total rise, you will need 14 to 15 risers, producing 13 to 14 treads (one less tread than risers because the top landing is not counted as a tread). The exact count depends on your finished dimensions. Enter your actual measured rise for the precise number.
What is the maximum riser height allowed by code?
The IRC sets the maximum riser height at 7.75 inches and the minimum at 4 inches for residential stairs. Risers taller than 7.75 inches force an unnaturally high step that increases fall risk, particularly on descent. Most comfortable residential stairs use risers between 7 and 7.5 inches. This calculator targets 7.375 inches as the ideal starting point and then adjusts to divide the total rise into equal risers.
How do I calculate stringer length?
Stringer length is the hypotenuse of a right triangle where total rise is one leg and total run is the other. The formula is: stringer = sqrt(totalRise² + totalRun²). For example, a stair with 108 inches of rise and 147 inches of run has a stringer of sqrt(11664 + 21609) = sqrt(33273) = about 182 inches (15.2 feet). Add at least 6 inches to this for the cuts at the top and bottom, and buy lumber in the next standard length up.
Why does the number of treads equal risers minus one?
Each riser lifts you to the next tread. The top riser lifts you onto the upper floor or landing, which is already a flat surface and does not count as a tread. So a staircase with 14 risers has 13 treads. This is a consistent rule in stair construction: treads = risers minus 1. The total horizontal distance the staircase covers (total run) is the number of treads multiplied by the tread depth.
What is the riser-plus-tread rule?
The riser-plus-tread rule states that the sum of one riser height and one tread depth should fall between 17 and 18 inches for a comfortable stair. This is an ergonomic guideline based on a natural walking stride, not an IRC code requirement. A 7.25-inch riser with a 10.5-inch tread sums to 17.75 inches, which is well within the comfortable range. Stairs outside this range feel either too steep (sum under 17) or too shallow (sum over 18).