Stair Calculator
Calculate stair risers, treads, total run and stringer length from total rise. Includes code checks and comfort rule.
Risers required
15
- Riser height
- 7.2 in
- Treads
- 14
- Total run
- 11.67 ft
- Stringer length
- 14.73 ft
- Stair width
- 3 ft
Riser height is the total rise divided by the number of risers. The IRC limits risers to 7.75 in and requires treads of at least 10 in; always confirm local code and handrail requirements.
A safe stair is built from a consistent riser height and tread depth. This calculator takes the total rise, divides it into equal risers no taller than your limit, and returns the tread count, total run and stringer length.
The numbers follow the common residential rule, but stairs are code-regulated. Check the maximum riser, minimum tread, width, handrail and landing requirements with your local building department before cutting stringers.
How the stair calculator works
Risers = ceiling(total rise ÷ maximum riser). Riser height = total rise ÷ risers. Treads = risers − 1, because the top landing is not a tread. Total run = treads × tread depth. Stringer length = √(total rise² + total run²), converted to feet.
Worked example
A 108-inch rise with a 7.5-inch maximum gives 15 risers at 7.2 inches each. That is 14 treads, a 140-inch run (11.7 ft), and a stringer about 14.7 ft long.
Stair code basics
- The IRC limits residential risers to 7.75 inches and requires treads of at least 10 inches.
- Stair width is typically at least 36 inches above the handrail; check local amendments.
- Handrails are required on one side of most stairs and should be 34–38 inches above the nosing.
- Headroom must be at least 6 ft 8 in, and a landing is required at the top and bottom.
- Riser height and tread depth must be consistent within the flight; variations cause trips.
The rise/run comfort rule
A common comfort check is 2R + T = 24 to 25 inches, where R is the riser height and T is the tread depth. A 7.2-inch riser with a 10-inch tread gives 24.4, which is comfortable for most adults.
Steeper stairs save space but feel insecure, especially going down. Shallower stairs feel generous but consume more floor area. Keep the ratio consistent and provide a landing on long runs.
Stringer layout
- Use a framing square with stair gauges set to the riser and tread dimensions.
- The stringer's throat must leave enough wood at the notch; many builders keep at least 3.5 inches.
- Account for the tread thickness and nosing when marking the top and bottom cuts.
- Attach the stringer with a ledger or hanger and check the connection with an engineer for long spans.
Frequently asked questions
How many stairs do I need for a 9-foot ceiling?
A 9 ft floor-to-floor rise is 108 inches. At a 7.5-inch maximum riser, that is 15 risers. The actual riser height is 7.2 inches.
What is the maximum riser height for stairs?
The IRC allows a maximum riser of 7.75 inches for residential stairs. Commercial and local codes can be stricter, so confirm before building.
How do I calculate stair stringer length?
Use the Pythagorean theorem: stringer = √(total rise² + total run²). For a 108-inch rise and 140-inch run, the stringer is about 177 inches, or 14.7 ft.
What is the 2R + T rule?
Twice the riser height plus the tread depth should be about 24–25 inches for comfortable stairs. For a 7.2-inch riser, a 10-inch tread gives 24.4.
How wide should residential stairs be?
At least 36 inches is common, measured above the handrail. Check local code and consider how furniture will move up the stairs.
Do stairs need a landing?
Yes, at the top and bottom, and a landing is required between flights. Long continuous runs also need intermediate landings under most codes.
Assumptions and accuracy
- Dimensions are converted to feet before the calculation; 1 ft = 12 in = 0.3048 m.
- Waste allowances are estimates. Add more for irregular shapes, many cuts and uneven subgrade.
- Bag yields, coverage rates and material densities vary by product. Check the label or ask your supplier.
- Local building codes and site conditions always take precedence over a general-purpose calculator.
- Read our full methodology