Stair Calculator
Calculate rise, run, stringer length and headroom for a code-compliant staircase.
Your details
Measure from finished floor to finished floor, not to the subfloor.
The walking surface depth, excluding any nosing overhang.
Materials
Joist depth plus subfloor and finished floor. Used for the headroom check.
This calculator runs entirely on your device. Nothing you enter is uploaded, stored, or sold. How this works
Riser height
7.267"
15 risers · 14 treads · 147.00" total run
Includes your inputs, the full breakdown, and every row of the table.
Number of risers
15
Number of treads
14
Total run
147.00"
Stringer length
183.00"
- Total rise
- 109.00"
- Risers
- 15
- Riser height
- 7.267" (116/16")
- Treads
- 14
- Tread depth
- 10.50"
- Total run
- 147.00" (12.25 ft)
- Stringer length
- 183.00" (15.25 ft)
- Stair angle
- 36.6°
- Minimum floor opening for headroom
- 131.5"
- Stringer stock needed
- 17 ft per stringer
- Tread material
- 40.3 sq ft
| Measure | Your design | Requirement | Result |
|---|---|---|---|
| Riser height | 7.267" | max 7.75" | Pass |
| Tread depth | 10.50" | min 10" | Pass |
| 2 × riser + tread | 25.03 | 24 to 25 | Outside ideal |
| Stair angle | 36.6° | 30 to 37 comfortable | Pass |
| Stair width | 36" | min 36" | Pass |
| Riser variation | 0.000" | max 0.375" between risers | Pass |
What this means
- A riser of 7.267" and a tread of 10.50" both satisfy the IRC limits, and the stair sits at 36.6 degrees.
- Every riser must be within three-eighths of an inch of every other. The classic mistake is forgetting that the bottom riser must be cut down by the thickness of the tread material, and the top one adjusted for the finished floor — get that wrong and the first or last step is a trip hazard.
- Building codes require 80 inches of headroom measured vertically from the tread nosing. With a 11.0" floor assembly you need a floor opening of roughly 131" to clear it.
How the stair calculation works
Stairs are the part of a build where small errors are most obvious and most dangerous. The human gait is remarkably sensitive to inconsistency — a single riser differing by half an inch from its neighbours is enough to catch a foot, which is why building codes limit the variation between risers to three-eighths of an inch across the entire flight. That tolerance is on the total set, not between adjacent steps.
The arithmetic starts from the total rise, which must be measured from finished floor to finished floor. Divide by a comfortable target riser height, round to a whole number, then divide the total rise by that number to get the exact riser. It almost never lands on a neat fraction, and that is fine — what matters is that every riser is identical. Note that there is always one fewer tread than riser, because the upper floor itself serves as the final tread.
Two rules govern comfort. The IRC caps risers at seven and three-quarter inches and requires treads of at least ten. Beyond code, the old carpenter's rule that twice the riser plus the tread should fall between twenty-four and twenty-five inches produces stairs that feel natural to climb, because it approximates the relationship between step height and stride length.
The detail that catches most first-time builders is the stringer cut at top and bottom. The bottom of the stringer must be trimmed by the thickness of the tread material, or the first step ends up taller than the rest. The top attachment likewise has to account for the finished floor thickness. Getting either wrong produces exactly the uneven riser that codes exist to prevent.
Frequently asked questions
What is the maximum riser height allowed by code?
Seven and three-quarter inches under the International Residential Code, with a minimum tread depth of ten inches. Commercial codes are stricter, typically capping risers at seven inches and requiring eleven-inch treads.
Why is there one fewer tread than riser?
Because the upper floor acts as the last tread. A flight with fourteen risers has thirteen treads, since you step onto the landing itself rather than a fourteenth board.
What is the 2R + T rule?
Twice the riser height plus the tread depth should total between twenty-four and twenty-five inches. It approximates the relationship between step height and natural stride, and stairs built to it feel comfortable rather than cramped or stretched.
How much headroom do stairs need?
Eighty inches measured vertically from the nosing of each tread to the ceiling or floor structure above. This is what usually determines how long the floor opening must be, and it is the constraint most often discovered too late.
Why must I cut down the bottom of the stringer?
Because the tread material sits on top of every cut except the bottom one, which rests on the floor. Trimming the bottom by exactly the tread thickness keeps the first riser the same height as the rest — skip it and you create precisely the uneven step codes are designed to prevent.
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Disclaimer. This calculator is provided for general information and educational purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Results are estimates based on the inputs you provide and the assumptions described above. Confirm any figure with a qualified professional before acting on it.