Stair Calculator Building Code. Stair calculations follow strict building codes for safety. Formula: Number of Risers equals Total Rise divided by Maximum Riser Height (7.75 inches IRC). Run equals Risers minus 1 times Tread Depth (10 inches minimum). Example 108 inch total rise: 108 divided by 7.75 equals 13.94, round to 14 risers. Each riser: 108 divided by 14 equals 7.71 inches. Treads: 13 treads at 10 inches equals 130 inch run or 10 feet 10 inches. International Residential Code requirements: Maximum riser height 7.75 inches residential, 7 inches commercial. Minimum tread depth 10 inches measured horizontally. Maximum riser variation 3/8 inch between steps. Minimum headroom 80 inches above nose line. Minimum width 36 inches clear. Handrail required one side for width under 44 inches, both sides over 44 inches. Handrail height 34-38 inches above nose. Maximum space under rail 4 inches prevents child falling. Landing required every 12 feet vertical rise. Stringer calculation: Number of stringers depends on width. 36 inches needs 3 stringers (both sides plus center). 48 inches needs 4 stringers. Space stringers maximum 16 inches on center. Stringers use 2×12 lumber for strength, cut pattern for treads and risers. Costs: Basic wood stairs 1,000-3,000 for interior straight run 10 feet. Carpet runner adds 300-800. Curved stairs 5,000-15,000. Exterior deck stairs 500-2,000. Commercial steel stairs 3,000-10,000. Tread materials: Solid wood oak, maple 30-60 per tread. Carpet treads 15-30 each. Rubber treads 20-40 each. Composite 25-50 per tread. Metal diamond plate 40-80 per tread. Choose based on use and safety. Nosing requirements: Minimum 3/4 inch to maximum 1.25 inch projection. Radius 1/16 to 9/16 inch prevents trip hazard. Nosing must be uniform all treads. Contrasting color recommended for visibility. Open riser stairs: Maximum opening 4 inches between treads prevents child foot entrapment. Open risers not recommended for residential with children. Solid risers safer. Winder stairs: Triangular treads turn corners, save space. Minimum 10 inches at narrow end, 11 inches at 12 inches from narrow. More complex to build. Not ideal for heavy traffic or elderly. Landing requirements: Change of direction needs landing minimum 36 inches square. Doors opening onto landings need landing minimum 36 inches in travel direction. Exterior landings cannot slope more than 2 percent (1/4 inch per foot). Common mistakes: Miscalculating number of risers often off by one, affects top or bottom step height creating trip hazard. Not following code causes inspection failure. Unequal riser heights code violation and dangerous. Insufficient headroom bumping head. Handrail wrong height or missing. Inadequate width for furniture moving. Stringers spaced too far causes bounce. Pro tips: Use stair calculator app or online tool ensures code compliance and cuts layout. Cut one stringer first, use as template for others. Test fit before attaching treads. Sand all edges prevents splinters. Seal or stain before installation easier than after. Install temporary treads during construction protects finished treads. Example calculation detailed: Basement stairs floor to floor 108 inches. Divide by 7.75 max equals 13.94, round to 14 risers. Actual riser: 108 divided by 14 equals 7.714 inches each. Number of treads: 14 minus 1 equals 13 treads. Using 10 inch treads: 13 times 10 equals 130 inches run equals 10 feet 10 inches horizontal space required. Total stair angle: arctan(108/130) equals 39.7 degrees typical comfortable slope. Materials needed: Three 2×12 stringers at 12 feet each, 13 treads at 36 inches wide (oak 30 dollars equals 390), 14 risers if closed (optional 280), nosing molding 30 feet at 3 per foot 90, hardware 50, total materials 900-1,200. FAQ: What is comfortable rise and run? 7-7.5 inch rise with 10-11 inch tread most comfortable. How many stringers? Minimum 3 for 36 inch width, 4 for 48 inches. Do I need permit? Yes for new stairs or structural changes. Can I build my own? Yes if experienced, must follow code, inspection required. What about spiral stairs? Compact, save space, harder to move furniture, code restrictions apply minimum 26 inch diameter 7.5 inch center tread.