🪜 Stair Stringer Calculator
Professional stair rise, run & stringer calculator with IBC code compliance
Stair Calculator
"The biggest mistake I see is builders forgetting to subtract the thickness of the first tread from the bottom riser. Your bottom riser should be one tread thickness shorter than the rest, or your first step will be higher than the others—instant trip hazard and code violation. Also, always use a framing square marked with stair gauges for consistent cuts."
📐 How to Calculate Stair Stringers
Building code-compliant stairs requires precise calculations. Here's the complete process:
Step 1: Measure Total Rise
The total rise is the vertical distance from the bottom floor surface to the top floor surface. This must be exact—measure from finished floor to finished floor, accounting for any flooring materials like carpet or tile that will be installed.
Step 2: Calculate Number of Steps
Divide your total rise by your target riser height (7 to 7.5 inches is most comfortable). Round to the nearest whole number. Then divide your actual total rise by this number to get your exact riser height.
Example: Total rise = 108 inches
- 108 ÷ 7.5 = 14.4 → Round to 14 steps
- Actual riser = 108 ÷ 14 = 7.714 inches per step
Step 3: Determine Total Run
The total run is the horizontal distance the staircase will cover. With 14 steps and 10-inch treads, you'll have 13 runs (one fewer than the number of steps because the top step is the floor itself).
- Total Run = (Number of Steps - 1) × Tread Depth
- Example: 13 × 10" = 130 inches
Step 4: Calculate Stringer Length
Use the Pythagorean theorem to find the diagonal stringer length:
- Stringer Length = √(Total Rise² + Total Run²)
- Example: √(108² + 130²) = √28,564 = 169 inches (14 feet 1 inch)
- You'll need 2×12 lumber at least this long for each stringer
Jake Morrison
Licensed Carpenter, ICC Residential Building Inspector
25+ years building custom staircases
⚠️ Common Stair Building Mistakes to Avoid
1. Inconsistent Riser Heights
The Mistake: Each step has a slightly different height, creating a tripping hazard.
How to Avoid: Calculate the exact riser height by dividing total rise by number of steps. Mark all cuts on your stringer carefully using a framing square with stair gauges set to your exact measurements. The maximum variation allowed is 3/8 inch—but aim for zero variation.
2. Forgetting the First Tread Thickness
The Mistake: The bottom riser ends up one tread thickness taller than all others.
How to Avoid: Subtract the thickness of your tread material (typically 1 to 1.5 inches) from the first riser cut on your stringer. If your tread is 1.5" thick and your riser is 7.5", cut the bottom riser at 6 inches.
3. Ignoring Code Requirements
The Mistake: Building stairs that won't pass inspection due to exceeding maximum riser height (7.75") or insufficient tread depth (minimum 10").
How to Avoid: Use this calculator to verify IBC compliance before cutting. Remember that local codes may be stricter—always check with your building department. Commercial stairs have different requirements (7" max riser, 11" min tread).
4. Inadequate Stringer Support
The Mistake: Using only two stringers on stairs wider than 36 inches, causing bounce and eventual failure.
How to Avoid: Use 3 stringers for stairs 36" to 48" wide, and add an additional stringer for every 16" of additional width. Always use 2×12 lumber for stringers—2×10 lacks sufficient material after cutting and will fail inspection.
📋 IBC Residential Stair Code Requirements (2021)
Critical Dimensions
- Maximum Riser Height: 7.75 inches (7-3/4")
- Minimum Tread Depth: 10 inches (measured from nosing to nosing)
- Maximum Variation: 3/8 inch (9.5mm) between largest and smallest riser or tread in a flight
- Minimum Width: 36 inches (measured above handrail if present)
- Minimum Headroom: 80 inches (6' 8") vertical clearance
- Nosing Overhang: 0.75 to 1.25 inches
- Maximum Nosing Radius: 9/16 inch
Handrail Requirements
- Height: 34 to 38 inches above nosing
- Graspability: 1.25 to 2 inches diameter for round rails
- Required: On at least one side for stairs with 4 or more risers
- Returns: Must return to wall at top and bottom
Landing Requirements
- Width: Not less than the stair width
- Depth: Minimum 36 inches in direction of travel
- Maximum Rise Between Landings: 151 inches vertical
🪵 Stringer Materials & Cutting Guide
Recommended Lumber
- Stringers: 2×12 pressure-treated or SPF (Spruce-Pine-Fir) #2 or better
- Treads: 2× (5/4×6 decking, 2×10, or 2×12 depending on design)
- Risers: 1× or 3/4" plywood (if enclosed stairs)
Number of Stringers Needed
- Up to 36" wide: 2 stringers minimum
- 36" to 48" wide: 3 stringers
- Over 48" wide: Add 1 stringer per 16" of additional width
Cutting Tips
- Use a framing square with stair gauges clamped at your rise and run measurements
- Mark all cuts before making any—measure twice, cut once
- Minimum remaining width after cuts should be 5 inches (this is why 2×12 is required)
- Make a template stringer first and test-fit before cutting all stringers
- Cut with a circular saw, finish corners with a handsaw to avoid over-cutting
❓ Frequently Asked Questions
What is the ideal stair riser and tread size?
For residential stairs, the ideal riser (vertical height) is 7 to 7.5 inches, and the ideal tread (horizontal depth) is 10 to 11 inches. The IBC requires risers to be no more than 7.75 inches and treads to be at least 10 inches. A comfortable rule of thumb: 2 × Rise + Run = 24 to 25 inches (known as the Blondel formula).
How do I calculate the number of steps for my stairs?
Divide your total rise (floor-to-floor height) by your desired riser height (typically 7-7.5 inches). Round this number to the nearest whole number to get the number of steps. Then divide your total rise by this number of steps to get the exact riser height for each step.
What is stringer length and how is it calculated?
The stringer length is the diagonal measurement of the stair's structural support board. It's calculated using the Pythagorean theorem: √(Total Rise² + Total Run²). This tells you the minimum length of lumber (usually 2×12) you need to cut your stringers from.
Can stair risers vary in height on the same staircase?
Building codes require that riser heights be uniform within ±3/8 inch (9.5mm maximum variation). Inconsistent riser heights are a major tripping hazard and will fail inspection. Always divide your total rise evenly among all steps to ensure uniformity.
What is the minimum headroom required for stairs?
The IBC requires a minimum vertical headroom clearance of 80 inches (6 feet 8 inches), measured vertically from the nosing of the treads. This ensures people of all heights can safely ascend and descend without hitting their heads.
Do outdoor deck stairs have different requirements?
Outdoor stairs follow the same basic IBC requirements (7.75" max riser, 10" min tread), but with additional considerations: use pressure-treated lumber rated for ground contact, ensure proper drainage (avoid water pooling on treads), and consider adding a slight pitch (1-2°) to treads for water runoff.