Stair Calculator
Enter your total rise and target riser height to get the number of risers, treads and stringer length for a set of stairs.
- Total run
- 55 in
- Stringers needed
- 4
- Risers and treads for a single straight flight
- 7 in equal riser height
- Stringer count from the stair width
- 11 in tread depth (default)
- The IRC caps risers at 7.75 in — confirm with your local building department
- Keep riser height variation under 3/8 in across the flight
- Buy one extra stringer as a spare in case of a miscut
Most codes cap risers at 7.75 in and require 10 in+ treads. Confirm with your local building department.
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How to calculate it
Enter your total rise and target riser height to get the number of risers, treads and stringer length for a set of stairs.
Source: Risers = total rise ÷ desired riser height (rounded up). Treads = risers − 1. Stringer length = √(total run² + total rise²). Code typically limits risers to 7.75 in max.
A 42 in total rise at 7 in risers: 42 ÷ 7 = 6 risers, 5 treads. With 11 in treads the run is 55 in; stringer = √(55² + 42²) = 69.2 in.
How to use this calculator
- Enter the total rise (vertical height from landing to landing) in inches.
- Set your target riser height — the calculator rounds to equal, code-safe risers (≤ 7.75 in).
- Adjust the tread depth (11 in default) to see the total run and stringer length.
- Enter the price per stringer for an optional material cost estimate.
Purchasing & waste guidance
- Most US building codes (IRC) cap residential risers at 7.75 in, require treads at least 10 in deep, and keep riser height variation under 3/8 in across a flight.
- Aim for risers between 6.5 and 7.5 in for comfortable stairs. The comfort rule of thumb 2× riser + tread ≈ 25 in lands near an 11 in tread at a 7 in riser.
- Use three stringers for stairs up to 36 in wide; add one stringer per extra 16 in of width (four at 48 in, five at 60 in).
- Buy pressure-treated 2×12 stringer stock for deck stairs; interior stairs use kiln-dried 2×12 lumber. Add one extra stringer as a spare in case of a miscut.
- Maintain at least 6 ft 8 in of headroom over the stair and a 36 in minimum width for a residential egress stair.
Waste factor guidance
- Buy one extra stringer as a spare in case of a miscut — stairs can’t be short a stringer.
- 15% waste on tread and riser boards covers cuts and selective grain matching.
- For painted pine stairs, 10% waste is usually enough.
Common mistakes to avoid
- Exceeding the 7.75 in IRC riser cap or varying riser height by more than 3/8 in within a flight.
- Spacing stringers too far apart (over 16 in) so treads flex and crack.
- Forgetting the 6 ft 8 in minimum headroom clearance over the stair.
Assumptions & what’s included
- Total rise is the vertical height from landing to landing.
- Riser height is rounded up so all risers are equal and code-safe (≤ 7.75 in per the IRC).
- Tread depth defaults to 11 in; stringer length assumes a straight, single-flight run.
- Stringer count assumes 16 in maximum spacing. Price is per stringer and optional.
Frequently asked questions
How many steps do I need for a 36 inch rise?
A 36 in total rise at 7.5 in risers needs 5 risers (steps) and 4 treads. The calculator rounds to a code-safe, equal riser height for any total rise.
What is the maximum riser height allowed by code?
The IRC caps residential risers at 7.75 in, requires treads at least 10 in deep, and limits riser-height variation to 3/8 in within a flight. Always confirm with your local building department, since local amendments can be stricter.
How long should a stair stringer be?
The stringer length is the diagonal: √(total run² + total rise²). The calculator gives this so you can buy the right length of 2×12 stringer stock — round up to the next stocked length and account for the cut at the top and bottom.
How many stringers do I need for a 36 inch wide stair?
Three stringers for stairs up to 36 in wide. Add one stringer for every additional 16 in of width — four at 48 in, five at 60 in. Spacing stringers no more than 16 in apart (24 in for composite treads) keeps treads from flexing.