Stair Calculator

Every stair starts with the total rise, the vertical distance from the finished lower floor to the finished upper floor. The basic version divides it into the fewest equal risers that stay within the 7¾-inch limit of the International Residential Code (IRC): a 10-foot rise needs 16 risers of 7½ inches. With a standard stringer, the top step is the upper floor itself, so there is one fewer tread than risers. Enter the run of one tread, or the total horizontal space available, and the calculator returns the tread depth, overall run, stair angle and the length of stringer stock you need.

Comfortable stairs follow a simple rule: twice the riser height plus the tread depth should come to about 24 to 25 inches, matching a natural stride. Risers of 7 to 7½ inches with treads of 10 to 11 inches feel right to most people.

Stair terminology and common building codes

The terms below are used in building codes and on plans. The code limits quoted are from the IRC, the model code adopted, often with amendments, by most US states for houses; commercial buildings follow the stricter International Building Code. Always confirm with your local building department before you build.

• Total rise: the height from finished floor to finished floor. Every riser in a flight must be equal; the IRC allows no more than 3/8 inch difference between the tallest and shortest riser.
• Riser: the vertical face of a step, and the height from one tread to the next. Maximum 7¾ inches in the IRC. Open risers are allowed if the gap would not let a 4-inch sphere pass through (where the stair is more than 30 inches high).
• Tread: the horizontal surface you step on. The tread depth, measured from the edge of one nosing to the next, must be at least 10 inches.
• Run: the horizontal depth of one step (unit run), or the horizontal length of the whole stair (total run).
• Nosing: the front edge of a tread that projects past the riser below. On stairs with solid risers and treads under 11 inches deep, the IRC requires a nosing of ¾ to 1¼ inches.
• Stringer: the inclined board that supports the treads and risers, either notched (cut stringer) or with treads housed in grooves. Stringers are commonly cut from 2 × 12 lumber.
• Headroom: the vertical clearance measured from a sloped line along the tread nosings to the ceiling or floor above. At least 6 feet 8 inches is required.
• Width: stairways must be at least 36 inches wide above the handrail height, with the handrail allowed to project into that width.
• Handrail: required on at least one side of a flight with four or more risers, mounted 34 to 38 inches above the nosings and graspable.
• Guard: a barrier on the open side of stairs and landings more than 30 inches above the floor below, at least 34 inches high along the stair.
• Landing: a level area at the top and bottom of each flight, at least as wide as the stair and 36 inches deep in the direction of travel. Codes also cap how high one flight can climb between landings, a little over 12 feet in the IRC.
• Winder: a tread that is wider at one end, used to turn a corner. Winders need at least 10 inches of depth at a point 12 inches from the narrow end.

Fraction to Decimal Conversion

Tape measures in the US are marked in fractions of an inch, while calculators show decimals. Use this table to convert between them when you mark out a stringer.

Frequently asked questions