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Stair Calculator

Free stair calculator for contractors and builders. Calculate risers, treads, stringer length, stairwell opening, headroom clearance, and material estimates. IRC code compliance checker included.

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Floor-to-floor height. Standard: 97"-110" (8'-9' ceilings + framing)

Code max: 7.75" · Ideal: 7"-7.5"

Code min: 10" nosing-to-nosing

Code min: 36"

Code: 3/4"-1-1/4" if tread < 11"

Stringer Mount

Stair Dimensions

Number of Risers

14

Number of Treads

13

Actual Riser Height

7.71"

Tread Depth

10.50"

Riser adjusted from 7.50" to 7.71" to evenly divide the total rise.

Calculated Measurements

Total Rise108.00" (9.0')
Total Run136.50" (11.4')
Stringer Length174.06" (14.5')
Recommended Lumber (Stringer + 12" for cuts)186.06" (15.5')
First Step Height (Riser height minus tread thickness - stringer drop)6.71"
Stringer Placement (Horizontal distance from header to stringer base)136.50" (11.4')
Stairwell Opening (Based on 80" headroom clearance)101.11" (8.4')
Stair Angle38.4°
Riser + Tread Sum (ideal: 17"-18")18.21"
Effective Tread (w/ nosing)11.50"
Available Headroom108.00" (9.0')

IRC Code Compliance

Riser Height: 7.71" (max 7.75")
Tread Depth: 10.50" (min 10")
Nosing: 1.00" (3/4"-1-1/4" required when tread < 11")
Riser + Tread: 18.21" (17"-18" comfort rule)
Width: 36.00" (min 36")
Headroom: 108.00" (9.0') (min 80" / 6'8")

Quick Answer

For a 108.00" (9.0') total rise (9-foot ceiling), you need 14 risers at 7.71" each with 13 treads. Total run: 136.50" (11.4'). Stringer length: 174.06" (14.5').

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Stair Profile

Stringer: 174.06" (14.5')Total Rise108.00" (9.0')Total Run: 136.50" (11.4')Stringer Placement: 136.50" (11.4')Step Ht.7.71"Tread: 10.50"Thickness: 1.00"1st Step6.71"14 Risers38.4°Showing 6 of 14 risers (proportions maintained)EstimationPro.ai

Stair Design & Code Guide

Building code requirements, calculation methods, and construction costs for residential stairs.

Standard Stair Dimensions & Building Code

IRC residential code requires a maximum 7-3/4" rise and minimum 10" run per step, with minimum 36" width for interior stairs.

  • Maximum riser height: 7-3/4" (7.75 inches)
  • Minimum tread depth: 10 inches (nosing not counted)
  • Minimum width: 36 inches clear
  • Maximum variation: 3/8" between tallest and shortest riser
  • Headroom: Minimum 6'-8" clearance above the stair nosing line
  • Handrail height: 34"–38" measured from stair nosing

The comfort rule: 2 × Riser + Tread = 24"–25". A 7" rise with an 11" tread (25") is considered ideal.

Key Takeaways

  • Max riser: 7-3/4", min tread: 10"
  • Min width: 36" clear (residential)
  • Comfort rule: 2 × Rise + Tread = 24"–25"

How to Calculate Number of Stairs

Divide total rise (floor-to-floor height) by desired riser height to determine the number of steps needed.

  • Standard floor-to-floor height: 8'–9' (96"–108")
  • Example: 108" total rise ÷ 7.5" per riser = 14.4 → round to 14 risers
  • Actual riser: 108" ÷ 14 = 7.71" per step (within code)
  • Number of treads: Always one fewer than risers (14 risers = 13 treads)
  • Total run: 13 treads × 10" = 130" (10'-10") horizontal distance needed

Always measure actual floor-to-floor height, not assumed height. Even 1/2" difference across 14 risers creates noticeable unevenness.

Key Takeaways

  • Number of risers = total rise ÷ riser height
  • Treads = risers minus 1
  • Always measure actual floor-to-floor (not assumed)

Stair Construction Costs in 2026

A standard straight staircase costs $2,000–$5,000 to build (framing + basic treads), while custom stairs run $5,000–$25,000+.

  • Basic straight stair (carpeted): $2,000–$4,000
  • Hardwood treads (oak, maple): $3,000–$8,000
  • L-shaped or U-shaped: $4,000–$10,000 (landing adds cost)
  • Spiral staircase: $3,000–$15,000+ (kit vs. custom)
  • Railing/balustrade: $1,500–$5,000+ (wood, iron, cable, glass)

Key Takeaways

  • Basic straight stairs: $2,000–$4,000
  • Hardwood treads: $3,000–$8,000
  • Railing: $1,500–$5,000+ depending on style

How to Use This Calculator

Measure the total rise

Measure the vertical distance from the finished floor at the bottom to the finished floor at the top. This is your total rise in inches. For a standard 8-foot ceiling with framing, expect 97" to 100".

Set riser and tread dimensions

Adjust the desired riser height (default 7.5") and tread depth (default 10.5"). The IRC requires a maximum 7.75" riser and minimum 10" tread. Set the nosing overhang — code requires 3/4" to 1-1/4" when tread depth is under 11".

Choose mount type and width

Select Standard or Flush stringer mount (flush reduces total run by one tread). Set stair width — minimum 36" for residential per IRC. Use Advanced Options to enter ceiling height for accurate headroom calculation.

Review code compliance and materials

Check green/red/yellow compliance badges for IRC requirements. Expand the Material Estimates panel for stringer, tread, and riser lumber quantities. Toggle Imperial/Metric or Fractions, and print results for the job site.

Stair Calculation Formulas

Number of Risers = Total Rise ÷ Desired Riser Height (rounded to nearest whole)
Actual Riser Height = Total Rise ÷ Number of Risers
Number of Treads = Number of Risers − 1
Total Run = Number of Treads × Tread Depth
Stringer Length = √(Total Rise² + Total Run²)
Recommended Lumber = Stringer Length + 12" (for seat and plumb cuts)
Stairwell Opening = Total Run − ((Ceiling Height + Floor Thickness − 80") ÷ tan(Stair Angle))
Riser + Tread Comfort Rule: Riser Height + Tread Depth = 17" to 18"

Where:

Total Rise
= Vertical distance from finished floor to finished floor (inches)
Total Run
= Horizontal distance from first riser to last riser (inches)
Riser + Tread
= Should equal 17" to 18" for comfortable stairs (comfort rule)
Min Headroom
= 80 inches (6 feet 8 inches) per IRC R311.7.2
Max Riser Height
= 7.75 inches per IRC R311.7.5.1
Min Tread Depth
= 10 inches measured nosing-to-nosing per IRC R311.7.5.2
Nosing
= 3/4" to 1-1/4" required when tread depth < 11" per IRC R311.7.5.3
Min Width
= 36 inches clear per IRC R311.7.1

Stair Building Code Quick Reference (IRC 2021)

Requirement IRC Code Value
Maximum Riser HeightR311.7.5.17.75"
Minimum Tread DepthR311.7.5.210"
Nosing (when tread < 11")R311.7.5.33/4" to 1-1/4"
Maximum Riser VariationR311.7.5.13/8"
Minimum WidthR311.7.136"
Minimum HeadroomR311.7.26'8" (80")
Handrail HeightR311.7.8.134" to 38"
Guard Required (drop > 30")R312.1.1Min 36" height

Stair Terminology Glossary

Riser —
The vertical board between treads. Riser height is the vertical distance from one tread surface to the next.
Tread —
The horizontal surface you step on. Depth is measured from the front edge (nosing) to the back where the next riser meets.
Stringer —
The diagonal structural member (typically 2×12 lumber) that supports the treads and risers. Most stairs use 2 or 3 stringers.
Nosing —
The front edge of a tread that projects past the face of the riser below. Required by IRC when tread depth is less than 11 inches.
Total Rise —
The total vertical distance from the finished floor at the bottom to the finished floor at the top of the stairway.
Total Run —
The total horizontal distance of the stairway, calculated as the number of treads multiplied by the tread depth.
Throat —
The narrowest point of a notched stringer, measured perpendicular to the stringer edge. Must be at least 3.5" for structural integrity.
Stairwell Opening —
The rectangular hole in the upper floor that the stairway passes through. Must be large enough to maintain 80" headroom clearance.

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Frequently Asked Questions

What is the maximum riser height allowed by code?
The International Residential Code (IRC R311.7.5.1) limits riser height to 7.75 inches maximum. Most builders target 7" to 7.5" for comfortable stairs. The maximum variation between any two risers in a flight is 3/8 inch. There is no code-mandated minimum riser height, though risers below 6" are uncommon and can feel awkward.
What is the minimum tread depth for stairs?
The IRC (R311.7.5.2) requires a minimum tread depth of 10 inches, measured from the front edge (nosing) of one tread to the front edge of the next. A depth of 10.5" to 11" is considered ideal for comfort and safety. When the tread depth is less than 11", nosing of 3/4" to 1-1/4" is required (IRC R311.7.5.3).
How do I calculate stringer length?
Stringer length is calculated using the Pythagorean theorem: Stringer = square root of (Total Rise² + Total Run²). When purchasing lumber, add 12 to 18 inches to the calculated length to allow for the seat cut at the bottom and the connection at the top. Most residential stairs use 2×12 lumber for stringers.
What is the riser plus tread rule?
The riser-plus-tread comfort rule states that the riser height plus the tread depth should equal 17 to 18 inches. For example, a 7.5" riser with a 10.5" tread equals 18", which is within the ideal comfort range. Another formula (Blondel's) uses 2×Riser + Tread = 24 to 25 inches.
How much headroom do stairs need?
The IRC (R311.7.2) requires a minimum headroom of 6 feet 8 inches (80 inches) measured vertically from the stair nosing to the ceiling or any obstruction above. This applies to the entire length of the stairway, including at the top where the stair passes through the floor opening.
How many stairs do I need for a 9-foot ceiling?
A 9-foot (108") ceiling with standard framing gives approximately 118" of total rise (108" ceiling + ~10" floor structure). Dividing by a 7.5" target riser: 118 / 7.5 ≈ 16 risers (with 15 treads). At 10.5" tread depth, total run would be about 157.5" (just over 13 feet). Use this calculator with your exact measurements for precise results.
What is the difference between standard and flush mount stringers?
In a standard mount, the top tread is level with the upper landing and the top riser butts against the header. In a flush mount, the stringer notch sits directly on the header, which reduces the total run by one tread depth. Flush mount is common in deck stairs and some interior applications.
How wide do residential stairs need to be?
The IRC (R311.7.1) requires a minimum width of 36 inches for residential stairs, measured clear of handrails. When handrails are installed, each handrail can project up to 4.5" into the required width. Stairs wider than 36" typically need a center (third) stringer for structural support.
How do I calculate stairwell opening size?
Stairwell opening depends on the stair angle, total run, and the required 80" headroom clearance. The formula is: Stairwell Opening = Total Run − ((Ceiling Height + Floor Thickness − 80") / tan(Stair Angle)). The opening must be large enough that a person standing on any tread has at least 80" of clearance to the floor structure above.
What size lumber do I need for stair stringers?
Most stair stringers use 2×12 lumber. After notching for treads and risers, at least 3.5" of the stringer board must remain at the narrowest point (the throat) to maintain structural integrity. For stairs wider than 36", use three stringers — one on each side and one in the center.

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