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Stair Calculator

Plan stair risers, treads, total run, and stringer length. IRC code compliance check + side-view diagram.

Construction

Stair Calculator

Generated on April 25, 2026

Floor to floor

Max code: 7.75 in

Min code: 10 in

Optional, typical 0-1.25 in

Side view (drawn to scale)

rise = 108.0 inrun = 154.0 in15 risers · 14 treads
# of risers
15
# of treads
14
Actual riser
7.20 in
Tread depth
11 in
Total run
154.00 in
Angle
35.0°
Stringer length
188.10 in
2R + T (rule)
25.40 in

IRC code check

Riser height 7.20 in ≤ 7.75 in (max)
Tread depth 11 in ≥ 10 in (min)
Comfort rule: 2R + T = 25.40 in outside 24-25 in

Step-by-step calculation

Formula

# risers = round(total rise / preferred riser). Actual riser = total rise / # risers. # treads = # risers − 1. Total run = # treads × tread depth. Stringer = √(rise² + run²).

  1. 1Total rise: 108 in (floor to floor).
  2. 2# risers = round(108 / 7) = 15.
  3. 3Actual riser height = 108 / 15 = 7.200 in.
  4. 4# treads = 15 − 1 = 14 (the top step is the floor).
  5. 5Total run = 14 × 11 = 154.00 in.
  6. 6Stringer length = √(108² + 154.00²) = 188.10 in.
  7. 7Stair angle = arctan(108 / 154.00) = 35.04°.
  8. 8Code: max riser 7.75 in, min tread 10 in. Comfort rule: 2R + T should be 24-25 in.

?What is the Stair Calculator?

The Stair Calculator computes the number of risers, riser height, total run, stair angle, and stringer length from a known total rise (floor-to-floor measurement) and your preferred riser height. It then checks compliance against the International Residential Code (IRC) limits — max riser 7.75 inches, min tread 10 inches, plus the comfort rule '2R + T should equal 24-25 inches'. Includes a drawn-to-scale side view of the staircase. Used by carpenters, DIYers, and architects to plan internal stairs, deck stairs, and porch stairs that feel comfortable and meet code.

The Formula

# risers = round(total_rise / preferred_riser). Actual riser = total_rise / # risers. # treads = # risers − 1. Total run = # treads × tread_depth. Stringer = √(rise² + run²).

The number of treads is always one less than risers because the topmost riser ends at the upper floor (the floor itself counts as the final 'tread'). Stringer length is the diagonal cut of the support board — calculated by the Pythagorean theorem from total rise and total run. The 2R + T rule of thumb (sum should be 24-25 inches) was derived from studies of comfortable stride length: humans naturally take a step that combines rise and tread depth into this approximate distance, so violating the rule produces stairs that feel awkward.

Practical Examples

1

Floor-to-floor 9 ft (108 in) with 7-inch preferred riser: 16 risers × 6.75 in actual + 15 treads × 11 in = 165 in run, 35.6° angle.

2

Deck stair 36 in down to grade with 7-in preferred: 5 risers, 4 treads — short porch staircase.

3

Code minimum: 7.75 in max riser, 10 in min tread (IRC 2021). Some local codes allow up to 8.25 in for retrofits.

4

Service stairs (basements, attics) often go up to 8 in riser — uncomfortable but sometimes necessary in tight spaces.

5

Comfortable stairs: 7 in riser + 11 in tread = 25 in (2R+T) — feels natural going up and down.

Frequently Asked Questions

The top of the staircase IS the upper floor, so there's no separate top tread. Going down: you step off the landing onto tread 1, then 2, then 3... and the bottom riser drops you onto the lower floor. Risers always exceed treads by one.

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