Stair Calculator
How to Use the Stair Calculator Enter the total rise — the floor-to-floor height in inches, measured from finished floor to finished floor. Select your preferred riser height and tread depth, stair width, and tread material. Click Calculate for the number of stairs, exact riser height, total horizontal run, stringer length, and a complete IRC […]
How to Use the Stair Calculator
Enter the total rise — the floor-to-floor height in inches, measured from finished floor to finished floor. Select your preferred riser height and tread depth, stair width, and tread material. Click Calculate for the number of stairs, exact riser height, total horizontal run, stringer length, and a complete IRC code check.
Always consult the current IRC (International Residential Code) and your local building department before building stairs. Stair dimensions vary by jurisdiction and occupancy type. The calculator provides a guide — not a substitute for local code review.
The Riser-Tread Relationship
Comfortable stairs follow the Blondel formula, developed by 17th-century architect François Blondel: two risers plus one tread should equal 24–25 inches. This formula predicts the natural stride length for stair climbing. A 7-inch riser and 11-inch tread gives 7+7+11 = 25 — perfectly comfortable. Stairs that violate this relationship feel wrong underfoot even when code-compliant: very tall risers with shallow treads feel steep and unsafe; very short risers with deep treads feel plodding and tiring to climb.
IRC Code Minimums (R311.7)
The International Residential Code requires: maximum riser height of 7-3/4 inches; minimum riser height of 4 inches; minimum tread depth (run) of 10 inches; minimum stair width of 36 inches clear; and minimum headroom clearance of 6 feet 8 inches measured vertically from the tread nosing to the ceiling above. All risers must be consistent within 3/8 inch of each other — an inconsistent riser is a major tripping hazard and a common injury cause. The calculator will flag if your dimensions fall outside these limits.
Stringer Length and Lumber Selection
Stringers are the structural diagonal boards that support the treads and risers. The stringer length is calculated using the Pythagorean theorem from the total rise and total run — always order stringer stock at least 1 foot longer than the calculated length to allow for cutting. For standard residential stairs, 2×12 lumber is the minimum stringer size. The effective depth remaining after cutting the rise and run notches must be at least 3.5 inches at the narrowest point — verify this before cutting.
FAQs
How do I measure total rise for stairs?
Measure from the finished floor on the lower level to the finished floor on the upper level. If subfloor is installed but flooring is not yet laid, add the flooring thickness (typically 3/4 inch for hardwood, 1/2 inch for tile or LVP). Measuring accurately is critical — a difference of even 1/4 inch in total rise affects riser consistency. An inconsistent riser height is both a code violation and a serious tripping hazard.
How many stairs does a standard 9-foot ceiling need?
A standard 9-foot ceiling with 3/4-inch subfloor top and bottom gives a total rise of approximately 109.5 inches. Dividing by 7 (target riser height) gives 15.6, which rounds to 16 risers. Actual riser height = 109.5 ÷ 16 = 6.84 inches — well within code and comfortable. 16 risers = 15 treads, and at 11-inch tread depth the total horizontal run is 15 × 11 = 165 inches = 13.75 feet. This is why standard staircases require about 13–14 feet of floor plan space.