This stair calculator converts a total rise — the vertical distance between two finished floors — into a complete stair layout: number of risers, exact riser height, tread count, total run, and the stringer length to cut. Pick a target riser height between 7 and 7.75 inches, and it divides the rise into equal steps that comply with the IRC residential code.
Use it as a stair stringer calculator before you mark up a 2×12: the stringer length comes from the Pythagorean theorem on the total rise and run, and the tool warns you if a riser or tread falls outside code limits.
How to Calculate Stairs: Rise and Run Step by Step
Every staircase starts with one measurement — total rise, taken from finished floor to finished floor.
- Step 1: divide the total rise by your target riser height (7.5" is typical) and round up to get the number of risers. Rounding up keeps each step at or below the target.
- Step 2: divide the total rise by that riser count to get the exact riser height — every riser must be equal, with no more than 3/8" variation across the flight.
- Step 3: treads = risers − 1, because the upper floor acts as the top step.
- Step 4: total run = treads × tread depth (10.5" is a comfortable standard; 10" is the code minimum).
- Step 5: stringer length = √(rise² + run²).
If headroom or floor space is tight, adjust the riser target and rerun the numbers rather than making one odd step.
Stair Code Limits and the 7-11 Rule
The International Residential Code (IRC R311.7) sets the limits most US inspectors enforce:
- Maximum riser height: 7.75 inches (some jurisdictions allow 8.25")
- Minimum tread depth: 10 inches (11" with no nosing in some codes)
- Maximum variation between risers or treads: 3/8 inch
- Minimum stair width: 36 inches; minimum headroom: 6 ft 8 in
- Handrail required for 4 or more risers, 34–38 inches above nosings
Carpenters also use the classic 7-11 rule — a 7" riser with an 11" tread — and the comfort formula 2 × riser + tread = 24–25 inches. A 7.5" riser with a 10.5" tread scores 25.5", right at the comfortable edge; steeper combinations quickly feel like a ladder.
Worked Example: Stairs for a 105-Inch Floor-to-Floor Rise
Say your basement stairs must climb a total rise of 105 inches and you target a 7.5" riser.
Step 1 — risers: 105 ÷ 7.5 = 14 exactly, so 14 risers. Step 2 — actual riser height: 105 ÷ 14 = 7.5 inches. Perfectly on target. Step 3 — treads: 14 − 1 = 13 treads at 10.5 inches deep. Step 4 — total run: 13 × 10.5 = 136.5 inches, about 11 ft 4-1/2 in of floor space. Step 5 — stringer: √(105² + 136.5²) = 172.2 inches, so each stringer needs a 2×12 at least 14 ft 4 in long.
If the rise had been 106 inches instead, 106 ÷ 7.5 = 14.13 rounds up to 15 risers of 7.07 inches each — slightly gentler stairs that need one more tread and 10.5 more inches of run.
Frequently Asked Questions
How do you calculate the number of steps for stairs?
Divide the total rise (floor-to-floor height in inches) by your desired riser height, usually 7 to 7.75 inches, and round up. A 105-inch rise with 7.5-inch risers needs 14 risers, which means 13 treads since the upper floor serves as the top step.
What is the maximum riser height for residential stairs?
The IRC allows a maximum riser height of 7.75 inches for residential stairs, and all risers in a flight must be within 3/8 inch of each other. Some state and local codes permit up to 8.25 inches, so always check the code your inspector enforces.
What is the 7-11 rule for stairs?
The 7-11 rule says a comfortable stair has roughly a 7-inch riser and an 11-inch tread. A related comfort formula is 2 × riser + tread = 24–25 inches. Stairs that are much steeper feel like ladders, while much shallower stairs break normal walking stride.
How long should a stair stringer be?
Stringer length equals the square root of total rise squared plus total run squared. For a 105-inch rise and 136.5-inch run, that is √(11,025 + 18,632) ≈ 172 inches, so you need a 14 ft 4 in board — in practice a 16 ft 2×12 per stringer, with three stringers for a 36-inch-wide stair.
How much room do I need for a staircase?
A straight staircase for a standard 8-foot floor-to-floor rise (about 105 inches) needs roughly 11.5 feet of horizontal run plus a 36-inch landing at each end — about 17.5 feet total. L-shaped or switchback stairs with landings can cut the footprint length nearly in half.