Free Stair Calculator — Rise, Run & Stringer Length

How to Use the Stair Calculator

How to Calculate Stair Rise and Run

To calculate stairs, divide the total rise (floor-to-floor height in inches) by your desired riser height to get the number of risers, then subtract one to get the number of treads. Multiply the number of treads by your tread depth to find the total run. This calculator handles all of these calculations instantly, including stringer length, code compliance, and headroom clearance.

Measuring Total Rise

Total rise is the vertical distance from the surface of the lower finished floor to the surface of the upper finished floor. This is the single most critical measurement in stair layout — every other dimension derives from it.

Measure from finished floor to finished floor, not from subfloor to subfloor. If the finished flooring has not been installed yet, add its thickness to your measurement. For example, if you measure 108" from subfloor to subfloor and both floors will receive 3/4" hardwood, your total rise is 108" + 3/4" (upper floor) − 3/4" (lower floor) = 108". If only the upper floor gets finish material, add only that thickness.

For a floor-to-floor height of 9' 1" (109 inches), dividing by a target riser height of 7-1/2" gives 14.53 — round up to 15 risers. The actual unit rise becomes 109" ÷ 15 = 7.267", or approximately 7-1/4". This keeps each riser comfortably within code limits.

The Rise-Run Relationship

The number of treads always equals the number of risers minus one. The top "tread" is the upper floor itself, so a flight with 15 risers has 14 treads. Multiply the tread count by the unit run (tread depth minus any nosing overhang) to calculate total run — the horizontal distance the stair covers.

The industry standard known as the 7-11 rule recommends a 7" rise paired with an 11" run. This combination produces a comfortable walking angle of roughly 32 degrees. Both the IRC and IBC reference this proportion, and most experienced framers use it as a starting target before adjusting to fit the available space.

A useful cross-check: the sum of one riser plus one tread should fall between 17" and 18". Values outside this range tend to feel either too steep or too shallow for normal foot traffic.

IRC vs IBC Code Requirements

Building codes set maximum and minimum dimensions to ensure safety. The two most widely adopted codes in the United States are the International Residential Code (IRC) for dwellings and the International Building Code (IBC) for commercial and public buildings.

• IRC R311.7.5.1 (2021 edition) — Residential: Maximum riser height 7-3/4", minimum tread depth 10", minimum headroom 6'8", minimum stair width 36", maximum nosing projection 1-1/4", maximum riser variation 3/8" within a flight.
• IBC 1011.5.2 (2021 edition) — Commercial: Maximum riser height 7", minimum tread depth 11", minimum headroom 6'8", minimum stair width 44" (or 36" for occupant loads under 50), maximum riser variation 3/8".

Important: Many states and municipalities amend these codes. Always check your local jurisdiction's adopted code edition and any local amendments before finalizing your stair design. This calculator flags violations against the standard IRC and IBC limits, but your local inspector has the final word.

Stringer Layout Basics

Stringers are the notched diagonal boards that support the treads and risers. Use a framing square with stair gauges clamped at the unit rise and unit run to mark each step along the stringer stock. The cut follows a sawtooth pattern, with each notch removing material from the board.

The stringer throat — the narrowest remaining width of the stringer after notching — must be at least 3-1/2". This typically means a 2×12 is required for risers above 7-1/2" or runs above 10". For smaller stairs (such as short deck stairs with a 6" rise and 10" run), a 2×10 may provide adequate throat depth, but always verify by drawing the layout to scale or using this calculator's stringer diagram.

Headroom Clearance (IRC R311.7.2)

Both the IRC and IBC require a minimum of 6'8" (80 inches) of headroom above every point on the stairway. Headroom is measured vertically from the stair nosing line — an imaginary diagonal line connecting the front edges of all nosings — up to the nearest overhead obstruction.

This measurement must be maintained along the full walking path of the stairs, not just at the bottom or top. Common headroom problems include floor joists that cross the stairwell opening, HVAC ductwork in basement stairways, and headers framed too low. Basement stair renovations are especially prone to headroom violations because the existing framing often was not designed for a finished ceiling below.

If you are tight on headroom, increasing the tread depth (and thus the total run) pushes the stairs further from the overhead obstruction, which effectively increases headroom. Alternatively, increasing the number of risers (which reduces unit rise) makes the stair angle shallower and can recover headroom.

Stair Width and Handrail Requirements

The IRC requires a minimum clear stair width of 36" measured between finished surfaces (drywall, not studs) at and below the handrail height. The IBC requires 44" for most commercial stairs, dropping to 36" only when the occupant load served is under 50 (Section 1011.2).

Handrails must be between 34" and 38" above the stair nosing line (IRC R311.7.8.1). The handrail must be "graspable" — meaning a circular cross-section between 1-1/4" and 2" in diameter, or a non-circular profile with a perimeter between 4" and 6-1/4" and a maximum cross-section of 2-1/4". A flat 2×4 cap rail does not meet graspability requirements and will fail inspection.

Handrails must extend horizontally at least the depth of one tread beyond the top riser, and at least the depth of one tread beyond the bottom nosing at the bottom of the flight (IRC R311.7.8.2). These extensions give users something to grab as they transition on and off the stairs.

Common Code Violations That Fail Inspections

After decades of stair inspections, certain violations appear again and again. Knowing these before you build saves time, money, and the frustration of tearing out work:

• Inconsistent riser heights (3/8" rule): Per IRC R311.7.5.2.1, the tallest riser and shortest riser in a flight cannot differ by more than 3/8". This is the single most common stair code violation. It usually happens when the total rise is not measured accurately, or when finished flooring thickness is not accounted for. Our calculator automatically divides the total rise evenly to prevent this.
• Risers exceeding the maximum height: IRC allows a maximum of 7-3/4". This typically happens when a builder tries to reduce the number of risers to shorten the total run, pushing each riser above the limit. One extra riser is cheaper than a failed inspection.
• Inadequate headroom: The 6'8" minimum must be maintained at every point, not just at the top or bottom landing. Inspectors run a 6'8" pole along the nosing line. If it hits anything, you fail.
• Non-graspable handrails: A flat 2×4 or 2×6 cap rail is not a code-compliant handrail. You need a profiled rail or round stock that meets the graspability dimensions (1-1/4" to 2" diameter for round rails).
• Missing nosing on closed risers: If the tread depth is less than 11", a nosing projection of 3/4" to 1-1/4" is required (IRC R311.7.5.3). Treads 11" or deeper do not need nosing. Open risers have separate rules.
• Stringer throat less than 3-1/2": After notching, the remaining width at the narrowest point of the stringer must be at least 3-1/2". Using a 2×10 for stairs with 7-3/4" risers often violates this.
• Guardrail openings exceeding 4": The 4-inch sphere rule (IRC R312.1.3) applies to stair guardrails. A 4-inch sphere must not be able to pass through any opening in the guardrail assembly.

Worked Example: Standard Basement Stair

Here is a complete example for a typical basement stair with a 9'1" (109") floor-to-floor height:

1. Total rise: 109" (measured finished floor to finished floor)
2. Target riser height: 7-1/2" → 109 ÷ 7.5 = 14.53 → round to 15 risers
3. Actual unit rise: 109 ÷ 15 = 7.267"(approximately 7-1/4") — within IRC max of 7-3/4" ✓
4. Number of treads: 15 − 1 = 14 treads
5. Tread depth (unit run): 10-1/4" (meets IRC min of 10") ✓
6. Total run: 14 × 10.25 = 143.5"(11'11-1/2")
7. Rise + Run check: 7.267 + 10.25 = 17.517 — within the 17–18 comfort range ✓
8. Stringer length: √(109² + 143.5²) = √(11881 + 20592.25) = √32473.25 = 180.2" (15'0-3/16")
9. Stair angle: arctan(109/143.5) = 37.2° — comfortable walking angle ✓

For this stair, you would need 2×12 stringer stock at least 16 feet long. With a 7.267" rise and 10.25" run, the stringer throat after notching would be approximately 4.2" — safely above the 3.5" minimum.

Pro Tips

• Measure total rise in three spots across the stair opening — left edge, center, and right edge — and use the longest measurement. Floors are rarely perfectly level, and using the longest dimension prevents a short riser at the top.
• Check that both the upper and lower landing surfaces are level before measuring. A landing that slopes toward the stair opening will throw off your total rise and produce uneven risers.
• Add 1/16" to your unit rise when marking stringers to prevent cumulative error. Over a 15-riser flight, pencil-line thickness and saw kerf can shave nearly 1/4" from the total rise. The extra 1/16" per step compensates.
• Always check local code amendments before ordering materials — some jurisdictions modify the standard IRC or IBC riser and tread limits, and a few still enforce legacy codes with different requirements.
• Cut one stringer first and test-fit it in place before cutting the rest. Clamp it to the upper floor frame and set a level on the tread notches. Adjust if needed — it is far cheaper to recut one board than to waste an entire set of stringer stock.
• For basement stairs, verify headroom before finalizing your layout. Measure from the nosing line at the critical point (usually where the stair passes under the floor framing) to the bottom of the joists or ductwork. If you are under 80", consider adjusting the stair angle or relocating obstructions.
• When ordering materials, get at least one extra stringer board. If you make a cutting error on the first stringer, having a spare avoids a trip back to the lumber yard. For a standard 15-riser flight, plan for 3 stringers (2 outer + 1 center) plus 1 spare.