onlinecalculator.me

Free online stair calculator

A stair calculator turns total rise into step count, actual riser height, and stringer length. It also checks whether your design meets IRC residential building code limits.

About this calculator

How to use

  1. Measure total rise in inches (floor to floor, vertically).
  2. Enter the stair width in inches.
  3. Set the tread run — the horizontal depth of each step (IRC minimum 10 in).
  4. Set your target riser height (7 to 7.5 in is comfortable).
  5. Read step count, actual riser, stringer length, and code compliance.

Formulas

numberOfSteps    = round(totalRise / targetRiser)
                   [increase until actualRiser ≤ 7.75 in]

actualRiser      = totalRise / numberOfSteps

totalRun         = numberOfSteps × runLength   (inches)

stringerLength   = sqrt((totalRise / 12)² + (totalRun / 12)²)   (ft)

stringersNeeded  = max(2, ceil(stairWidth / 16))

treadBoards      = numberOfSteps × ceil(stairWidth / 144)

Worked example

108 in total rise (9 ft floor), 36 in wide, 10 in run, 7.5 in target riser:

numberOfSteps    = round(108 / 7.5) = round(14.4) = 14
actualRiser      = 108 / 14 ≈ 7.71 in  ✓ (≤ 7.75 in)

totalRun         = 14 × 10 = 140 in = 11.67 ft
stringerLength   = sqrt(9² + 11.67²) = sqrt(81 + 136.19) ≈ 14.74 ft

stringersNeeded  = max(2, ceil(36 / 16)) = max(2, 3) = 3
treadBoards      = 14 × 1 = 14 boards (36 in width fits on a 12-ft board)

riser + run      = 7.71 + 10 = 17.71 in  ✓ (17–25 in range)
→ IRC compliant

Notes

  • All stair dimensions should be verified with your local building official before construction. Some jurisdictions adopt amendments to the IRC.
  • Stringer lumber is typically 2×12 pressure-treated for exterior stairs. A 14-step stringer is about 15 ft long — allow for some length overage when purchasing.
  • Tread boards are commonly 5/4×6 (actual 1 in × 5.5 in) or 2×6 lumber; composite tread systems are also available.

Frequently asked

How many steps do I need for a given rise?
Divide the total rise (in inches) by your target riser height and round to the nearest whole number. Then verify the actual riser (total rise / step count) does not exceed 7.75 in per IRC R311.7. The calculator does this automatically.
What is the IRC code limit for risers and treads?
IRC 2021 R311.7 requires: riser height between 4 in and 7.75 in, tread run at least 10 in, and the sum of riser + run between 17 in and 25 in. The calculator flags any violation.
What is a stringer?
A stringer is the diagonal structural member that supports the treads and risers. Most staircases need at least two stringers — one on each side. Wider stairs (over 36 in) require a center stringer. The calculator estimates one stringer per 16 inches of stair width.
How do I measure total rise?
Total rise is the vertical distance from the finished floor at the bottom to the finished floor at the top of the staircase, measured in inches. For a 9-ft floor-to-floor height that would be 108 in.
What is nosing and does it affect step count?
Nosing is the lip of the tread that overhangs the riser below. IRC requires at least 0.75 in of nosing. Nosing does not change the step count — it affects the tread board length and total horizontal footprint of the stair.
How do I share my stair calculation?
Click Share to copy the page link, or Share with my numbers to encode your inputs in the URL.