Free online stair 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.