Stair Calculator

Calculate stair rise, run, stringer length, stair angle, number of steps, and building code compliance for any staircase design.

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Basic Stair Calculator

Enter the total rise (vertical height) and either the total run or a single step run to calculate all stair dimensions and check building codes.

Floor-to-floor vertical height (e.g. 108" = 9 ft)
Horizontal depth of one tread (min 10" per IRC)
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Advanced Stair Calculator

Full calculation with tread thickness, nosing, headroom check, floor thickness, and stringer details for professional stair design.

Thickness of tread board (typically 1"–2")
Overhang beyond riser (IRC: 0.75"–1.25")
Min 80" (6'8") per IRC building code
Thickness of upper floor/landing structure
Min 36" per IRC code
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Material Estimator

Estimate lumber and material quantities needed to build your staircase including stringers, treads, risers, and handrails.

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Fraction to Decimal Reference

Common fractions used in stair measurement conversions to decimals.

16ths8ths4thsHalvesDecimal (inches)
How to Read Measurements
Stair measurements are typically given in inches and fractions. For example, 7-3/16" means 7 inches plus 3/16 of an inch = 7.1875". When using a calculator, convert: 3/16 = 0.1875, so enter 7.1875.
Common Tread Thicknesses
5/4 deck board: 1.0" actual (1-1/8" nominal)
2x lumber: 1.5" actual
2x12 stringer: 1.5" thick, 11.25" wide
Standard tread: 1" thick × 10" wide
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Stair Building Guide & Code Reference
Key Stair Terminology
Rise: Vertical height of one step (top of tread to top of next tread)
Run/Tread: Horizontal depth of one step (where your foot lands)
Total Rise: Full vertical height from floor to floor
Total Run: Horizontal distance covered by all steps
Stringer: Diagonal structural board that supports treads and risers
Nosing: Overhang of tread beyond the riser face
Headroom: Vertical clearance above the stair from tread to ceiling
IRC Building Code Requirements (USA)
Max riser height: 7-3/4" (197mm)
Min riser height: 4" (102mm)
Min tread depth: 10" (254mm)
Min headroom: 6'-8" / 80" (2032mm)
Min stair width: 36" (914mm)
Nosing: 3/4" min, 1-1/4" max when riser < 7-3/4"
Max rise variation: 3/8" between any two risers
Handrail height: 34"–38" above tread nosing
Guard height: 36" min for stairs with >30" total rise
Max total rise between landings: 151" (12'7")
The 17–25 Rule (Comfort Formula)
For comfortable stairs: Rise + Run should be between 17" and 25"
Ideal: Rise + Run = 17"–18" for interior stairs
Example: 7" rise + 11" run = 18" ✓ (comfortable)

Alternative: 2 × Rise + Run = 24"–25"
Example: 2(7") + 11" = 25" ✓ (ideal)
Standard vs Flush Stringer Mount
Standard Mount: Stringer sits on top of the floor. The bottom of the first riser starts at floor level. First step height = step height − tread thickness.

Flush Mount: Top of stringer is flush with the landing. The stringer is notched and attached to a ledger or header joist. Often used for decks.
Stringer Sizing Guide
Standard stair stringers are cut from 2×12 lumber (actual: 1.5" × 11.25").
The uncut portion of the stringer must be at least 3.5" (IRC) to maintain structural integrity.
For stairs wider than 36", a third center stringer is required.
Stringer length = √(Total Rise² + Total Run²) + safety margin of ~6"