Staircase Calculator — Risers, Treads & Stringer Length
Calculate the number of risers and treads, individual riser height, tread depth, stringer length, and total staircase run for any total rise. Includes IBC and IRC code compliance checks.
How to Use This Calculator
- Measure the total rise — the floor-to-floor or floor-to-landing height in inches
- Enter the desired staircase width
- Select your target riser height (7" is ideal; IRC max is 7-3/4")
- Choose tread depth (11" is comfortable; IRC minimum is 10")
- Click "Calculate Staircase" to see all dimensions and a code compliance check
How the Calculation Works
num_risers = ceil(total_rise_in / ideal_riser_height)
riser_height = total_rise_in / num_risers // adjusted to fit evenly
num_treads = num_risers - 1 // one fewer tread than risers
total_run_in = num_treads × tread_depth_in
stringer_length = sqrt(total_rise_in² + total_run_in²) // hypotenuse
// Code check: 4" ≤ riser ≤ 7.75", tread ≥ 10", (riser + tread) = 17"–18" The number of risers is found by dividing total rise by the target riser height and rounding up. The actual riser height is then the total rise divided by that riser count — ensuring all risers are equal. The number of treads is always one less than the number of risers (the top landing counts as the last tread). Total run is treads × tread depth. Stringer length is the hypotenuse of the right triangle formed by total rise and total run.
Variables:
- num_risers: Total number of steps/risers in the staircase
- riser_height: Height of each individual riser (must be equal for all)
- num_treads: Number of tread boards (always one less than risers)
- total_run_in: Horizontal distance the staircase travels (footprint length)
- stringer_length: Length of diagonal stringer boards — use for lumber purchase
Pro Tips
- The classic comfort rule: riser + tread = 17"–18"; e.g., 7" riser + 11" tread = 18" (ideal)
- IRC code for residential stairs: max riser 7-3/4", min tread 10", max variation between risers 3/8"
- Cut stringers require at least 3.5" of net cross-section remaining (the "effective depth") after notching
- Stringer lumber should be at minimum 2×12 pressure treated; 3-ply LVL for heavy loads
- Handrails are required on any staircase with 4 or more risers — plan for post locations when laying out stringers
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Frequently Asked Questions
What is the standard riser height and tread depth?
IRC residential code allows risers up to 7-3/4" and treads minimum 10" (measured from nosing to nosing). The most comfortable stair geometry follows the 17"–18" rule: riser + tread should equal 17 to 18 inches. A 7" riser with 11" tread (total 18") is considered ideal for most people.
How do I calculate the number of stairs?
Divide your total rise (floor-to-floor height in inches) by your target riser height (typically 7"). Round up to the nearest whole number. Example: 9-foot floor height = 108 inches ÷ 7" = 15.4, rounded up to 16 risers. Actual riser height = 108 ÷ 16 = 6.75 inches each.
How long does a stringer need to be?
Stringer length is the hypotenuse of the triangle formed by total rise and total run. Example: 108" rise, 15 treads at 11" each = 165" run. Stringer = √(108² + 165²) = √(11664 + 27225) = √38889 ≈ 197 inches = 16.4 feet. Purchase 2×12×18ft stringers to allow for cuts.
How many stringers do I need?
Residential stairs typically use 3 stringers: one on each side and one in the center. For staircase widths over 36", add a stringer every 16 inches of width. Wide stairs (48"+) may need 4 stringers. Stringers must be supported at the top and bottom — header joist at top, concrete pad or ledger at bottom.
What lumber do I need for a staircase?
Stringers: 2×12 (3 pieces at stringer length + overage). Treads: 5/4×12 or 2×12 decking at staircase width per tread. Risers (if closed): 1×8 at staircase width per riser. Hardware: joist hangers or angle brackets for top connection, anchor bolts for bottom. Handrail posts every 48" on center.