How to Calculate Stair Dimensions
I built this stair calculator to take the guesswork out of one of the most safety-critical parts of any building project. Stairs built to the wrong dimensions are not just a code violation — they're a fall hazard. The key relationship in stair design is between riser height (the vertical face of each step) and tread depth (the horizontal surface you step on). These two dimensions must stay within specific ranges and have a fixed geometric relationship to feel comfortable and safe to walk.
The most widely used formula in stair design is: 2 × riser height + tread depth = 24 to 25 inches. This ratio produces a natural walking stride. The International Residential Code (IRC) sets the limits at maximum 7-3/4" riser height, minimum 10" tread depth, and requires that all risers within a flight vary by no more than 3/8" — because inconsistent riser heights are the leading cause of stair-related falls.
Code Requirements and Common Mistakes
Meeting code requirements isn't just about passing inspections — these numbers exist because decades of injury data show what stair geometry is safe. Key requirements to remember:
- Maximum riser height (IRC): 7-3/4 inches; comfortable range is 7 to 7-1/2 inches.
- Minimum tread depth (IRC): 10 inches measured from nosing to nosing; comfortable is 10 to 11 inches.
- Minimum headroom clearance: 6 feet 8 inches measured vertically from the stair nosing.
- Handrails required on any stair with 4 or more risers; graspable rail height 34–38 inches above nosing.
Calculating the Stringer Length
The stringer is the diagonal structural member that supports each step. Its length is calculated using the Pythagorean theorem: the run (total horizontal distance) squared plus the rise (total vertical height) squared equals the stringer length squared. Add 10–15% for the actual length of material you need to buy, to account for the notching at the top and bottom and the thickness losses from cutting.
For a standard 3-stringer staircase (the minimum), stringers should be spaced no more than 36 inches apart. Wider stairs (over 36 inches wide) need a centre stringer. Stringers are typically cut from 2 × 12 lumber and should retain at least 3.5 inches of solid wood below the deepest notch point — less than that and the stringer can crack under load.
Frequently Asked Questions
How do I find the right number of risers?
Measure the total rise (floor-to-floor height in inches), then divide by your target riser height (try 7.5 inches first). Round to the nearest whole number — that's your riser count. Then divide the total rise by that number to get the exact riser height for each step. For example: 108 inches total rise ÷ 7.5 = 14.4, so use 14 risers. Exact riser height = 108 ÷ 14 = 7.71 inches — within code.
What is stair nosing and is it required?
A nosing is the portion of the tread that overhangs the riser below it, typically 3/4 to 1-1/4 inches. When treads have a nosing overhang, the IRC allows a minimum tread depth of 10 inches measured from nosing to nosing. Nosings aren't strictly required if the tread depth is at least 11 inches (measured without overhang). Nosings improve safety by giving your foot a clear landing edge and are standard practice on most residential stairs.
Can I build stairs without a permit?
Interior stairs that are part of a permitted remodel or new construction are covered by that permit. Exterior stairs (deck stairs, entry stairs) frequently require a separate permit if they're attached to the structure. Even if a permit isn't required in your jurisdiction, building to IRC stair dimensions is strongly recommended for safety and to avoid complications when selling the property. An inspector or real estate agent may flag non-code stairs as a defect.