Stair Calculator

Stairs must meet strict dimensional requirements for safety. Failure to comply can result in failed inspections, insurance issues, and serious injuries:

IRC (Residential) Requirements - 2021:

IBC (Commercial) Requirements - 2021:

Critical Code Points:

• All risers must be within 3/8" of each other in height
• All treads must be within 3/8" of each other in depth
• Open risers may have maximum 4" gap (child safety)
• Winding treads have specific measurement requirements
• Guard rails required when drop exceeds 30"

The 2R + T formula ensures stairs match natural human gait for comfortable, safe ascent and descent:

The Rule of 24-25:

2R + T = 24 to 25

Where:
R = Riser height in inches
T = Tread depth in inches

Why 24-25? This range matches the average adult stride when climbing stairs. Too steep (high 2R+T) creates tiring, dangerous stairs. Too shallow (low 2R+T) results in awkward gait and trip hazards.

Optimal Combinations Meeting Code:

What Happens Outside the Range:

Rule of 17-18 (Alternative): Some builders use R + T = 17 to 18

• Same principle, different expression
• 7" + 11" = 18 (ideal)
• 7.5" + 10" = 17.5 (acceptable)

Accurate calculations ensure code compliance and consistent, safe stairs:

Step 1: Measure Total Rise Measure floor-to-floor including ALL finish materials:

Total Rise = Subfloor to Subfloor + Upper Finish Floor + Lower Finish Floor Gap
Example: 97" + 0.75" (hardwood) + 0.25" (vinyl) = 98"

Step 2: Calculate Number of Risers

Target Risers = Total Rise ÷ 7" (ideal height)
Example: 108" ÷ 7" = 15.43
Round to: 15 or 16 risers

Step 3: Calculate Actual Riser Height

Riser Height = Total Rise ÷ Number of Risers
With 15 risers: 108" ÷ 15 = 7.2" ✓ (within code)
With 16 risers: 108" ÷ 16 = 6.75" ✓ (within code)

Step 4: Calculate Tread Depth (using 2R+T = 25)

Tread Depth = 25 - (2 × Riser Height)
With 7.2" riser: 25 - 14.4 = 10.6" tread
With 6.75" riser: 25 - 13.5 = 11.5" tread

Step 5: Calculate Total Run

Number of Treads = Number of Risers - 1
Total Run = Number of Treads × Tread Depth
Example: (15 - 1) × 10.6" = 148.4" = 12.4 feet

Step 6: Verify Code Compliance

• Riser: 7.2" < 7.75" ✓
• Tread: 10.6" ≥ 10" ✓
• Variance: All risers equal (calculated) ✓

Quick Reference Table:

Stringers are the structural backbone of stairs—proper layout and cutting is critical:

Stringer Sizing Requirements:

Minimum Remaining Stringer Depth:

Stringer Length Calculation:

Stringer Length = √(Total Rise² + Total Run²)

Example for 108" rise, 148" run:
Length = √(108² + 148²) = √(11,664 + 21,904) = √33,568 = 183.2"
Add 12" for bottom cut = 195" (order 16' or 18' lumber)

Framing Square Layout:

1. Set stair gauges at riser height (e.g., 7.2") and tread depth (e.g., 10.6")
2. Position square on stringer stock
3. Mark each rise and run, progressing along the board
4. Mark bottom cut (drop by one tread thickness)
5. Mark top cut (attach to header or landing)

Stringer Cutting Checklist:

2026 Stringer Lumber Costs:

Tread and riser materials affect both appearance and durability—choose based on location and use:

Tread Material Comparison (2026):

Riser Material Options:

Tread Sizing:

Nosing Requirements:

• Projection: 3/4" to 1-1/4" (IRC)
• Radius: 9/16" maximum
• Uniform: All treads must have same nosing
• Open risers: Still require nosing

Materials List Example (15-Step Stair, 36" wide):

Headroom violations are a common cause of failed inspections—plan ceiling openings carefully:

Minimum Headroom:

Calculating Ceiling Opening:

Opening Length = (Number of treads beyond wall × Tread Depth) + Stringer Thickness

Headroom Calculation Method:

1. Draw stairs to scale on graph paper
2. Mark 80" vertical line from each nosing
3. Connect tops of lines to find headroom line
4. Ceiling must be above this line throughout

Opening Sizing Example: For 108" rise, 10.6" treads, with 80" headroom requirement:

• Determine how many treads fit below existing ceiling
• Calculate remaining treads requiring opening
• Add 2" clearance for framing

Common Headroom Issues:

Landing Requirements:

Door Swing at Landing:

• Doors cannot swing over stairs
• Landing must accommodate full door swing
• Exception: Screen doors, storm doors

Handrails and guards are critical safety features with specific code requirements:

Handrail Requirements (IRC R311.7.8):

Guard (Baluster) Requirements:

Handrail Graspability:

Material Costs (2026):

Baluster Quantity Calculation:

Balusters per Tread = (Tread Depth - 1") ÷ 4" (spacing)
Example: (11" - 1") ÷ 4 = 2.5, round up to 3 balusters per tread
Total Balusters = Treads × Balusters per Tread

Different configurations solve space constraints while maintaining code compliance:

Straight Run Stairs:

• Simplest design, easiest to build
• Requires most floor space (12-15' run typical)
• Best for basements, exterior applications
• Lowest cost per step

L-Shaped Stairs with Landing:

• 90° turn at landing
• Reduces required straight run
• Landing must be minimum 36" × 36"
• Add one tread equivalent for landing

U-Shaped Stairs with Landing:

• 180° turn at landing
• Most compact footprint
• Landing typically equals stair width × 2
• Common in two-story residential

Winding Stairs (Winders):

Spiral Stairs (IRC R311.7.10.1):

Space Requirements Comparison:

Different framing methods suit different applications and skill levels:

Cut Stringer (Traditional):

• Stringers cut with rise/run notches
• Treads rest on notches
• Most common residential method
• Requires 2×12 minimum stock

Closed Stringer with Cleats:

• Stringers not notched
• Metal cleats support treads
• Cleaner appearance
• Stronger stringer section

Closed Stringer with Routed Housing:

• Stringers routed with tread/riser pockets
• Treads wedged from behind
• Highest quality appearance
• Most labor-intensive

Prefab Steel Stringer:

• Pre-manufactured adjustable stringers
• Bolt together assembly
• Fastest installation
• Limited to standard rises

Framing Method Comparison:

Attachment Methods:

2026 Hardware Costs: