Stair Calculator
Calculate stair dimensions from total rise: number of risers, tread depth, stringer length, and total run. Includes IRC code requirements for riser height, headroom, and handrail dimensions.
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Enter your values above to see the results.
Tips & Notes
- ✓Calculate the number of risers first by dividing total rise by target riser height and rounding to the nearest whole number, then recalculate the actual riser height — never cut risers to the target height without this recalculation or the top riser will be a different height.
- ✓All risers must be within ⅜ inch of each other per IRC code — a single riser that is ½ inch different from the others is a tripping hazard and will fail inspection.
- ✓Add 1 inch of nosing overhang to each tread when calculating total horizontal run — nosing is included in the tread depth measurement but hangs over the riser, which affects the stringer length calculation.
- ✓Check headroom at multiple points along the stair path, not just at the bottom — the critical headroom constraint is usually at the point where the stair passes under a floor joist or header, not at the landing.
- ✓For exterior stairs, slope each tread ⅛ inch per foot toward the open edge for drainage — flat treads collect water that accelerates rot and creates ice hazards in freeze-thaw climates.
Common Mistakes
- ✗Measuring total rise from finished floor surface to finished floor surface rather than from subfloor to subfloor — the finished flooring thickness changes the actual rise if not accounted for in the calculation.
- ✗Cutting all stringers before verifying the layout with a story pole (a straight board marked with each riser increment) — a small error in rise calculation multiplied by 12 risers can produce a significant error at the top.
- ✗Installing stairs without a landing at the top — IRC requires a level landing at least as deep as the stair width at both the top and bottom of any stairway. Stepping directly from the last tread onto a slope is a code violation.
- ✗Using green or wet lumber for stair stringers — green lumber warps and twists as it dries, causing risers to become uneven over time. Use kiln-dried lumber or allow green lumber to fully dry before installation.
- ✗Forgetting to account for tread thickness when laying out stringers — if treads are 1.5 inches thick, the bottom riser must be 1.5 inches shorter than all others, or the bottom step will be higher than all other steps.
Stair Calculator Overview
Stair design calculation determines rise, run, number of steps, and total stringer length from the total vertical height being spanned. Building codes govern these dimensions precisely — stairs that are too steep or too shallow are both safety hazards, and stairs that do not meet code will fail inspection and require reconstruction. Getting the geometry right before cutting lumber or pouring concrete prevents the most expensive mistake in stair construction.
Core stair formulas:
Number of Risers = Total Rise ÷ Riser Height (round to nearest whole number)
Actual Riser Height = Total Rise ÷ Number of Risers
EX: Total rise 84 inches, target riser height 7 inches → 84 ÷ 7 = 12 risers → Actual riser = 84 ÷ 12 = 7.0 in exactly. Run per tread = 11 inches → Total horizontal run = 11 × 11 treads = 121 inches = 10.08 ftBuilding code requirements for residential stairs (IRC):
| Dimension | IRC Minimum | IRC Maximum | Recommended | Notes |
|---|---|---|---|---|
| Riser height | 4 inches | 7¾ inches | 6½–7 inches | All risers must be within ⅜ in of each other |
| Tread depth (run) | 10 inches | No maximum | 10–11 inches | Measured from nosing to nosing |
| Stair width | 36 inches | No maximum | 36–48 inches | Measured handrail to handrail |
| Headroom | 6 ft 8 in | No maximum | 7 ft minimum | Measured from tread nosing vertically |
| Handrail height | 34 inches | 38 inches | 36 inches | Measured from tread nosing |
| Landing depth | 36 inches | No maximum | Equal to stair width | Required at top and bottom of stairs |
| Stair Type | Riser Height | Tread Depth | Rise + Run Sum | Comfort Rating |
|---|---|---|---|---|
| Gentle / accessible | 5–6 in | 12–14 in | 17–19 in | Excellent — easy for all users |
| Standard residential | 6½–7 in | 10–11 in | 17–18 in | Very good — typical home interior |
| Standard exterior deck | 6–7 in | 11–12 in | 17–19 in | Very good — comfortable outdoors |
| Steep / compact | 7½–7¾ in | 10 in | 17½–18 in | Acceptable — meets code minimum |
| Code maximum | 7¾ in | 10 in | 17¾ in | Meets code — not recommended |
Frequently Asked Questions
Divide the total vertical rise (in inches) by your target riser height. Round to the nearest whole number — this is your number of risers. The number of treads equals the number of risers minus one (the top landing is not counted as a tread). Example: 102 inches total rise ÷ 7 inch target = 14.57 → round to 15 risers → 14 treads. Recalculate actual riser height: 102 ÷ 15 = 6.8 inches per riser.
The International Residential Code (IRC) sets the maximum riser height at 7¾ inches for residential stairs. However, all risers in a staircase must be within ⅜ inch of the tallest riser — you cannot have most risers at 7 inches and one at 7¾ inches. Many local jurisdictions adopt the IRC with modifications, and some have stricter limits. Commercial stairs under IBC have a 7-inch maximum riser height. Always verify requirements with your local building department.
Stringer length = √(Total Rise² + Total Run²). This is the Pythagorean theorem applied to the stair triangle. Example: 10-step stair with 7-inch risers and 11-inch treads → Total Rise = 70 inches, Total Run = 99 inches → Stringer = √(70² + 99²) = √(4,900 + 9,801) = √14,701 = 121.2 inches = 10.1 feet. Add 12 inches for the bottom foot and top bearing point: order 11.5-foot stringers.
Interior stairs in an existing home: typically yes, though some jurisdictions exempt replacement stairs to the original dimensions. Exterior deck stairs: yes in most jurisdictions — they are part of the deck permit. New construction stairs: always require a permit as part of the overall building permit. Stair inspections verify riser uniformity, tread depth, handrail height and graspability, and headroom — all of which directly affect safety. The permit process exists specifically because improperly built stairs cause thousands of falls annually.
For exterior stairs in contact with moisture: pressure-treated lumber rated UC3B or higher for above-ground applications. Common choices are pressure-treated Southern Yellow Pine (economical), cedar (naturally rot-resistant, more attractive), and redwood (premium, naturally rot-resistant). Composite decking materials work well for treads but require solid framing support. Avoid standard untreated pine, fir, or spruce for exterior stairs — they deteriorate quickly in outdoor conditions and become a maintenance liability within 3–5 years.
The comfort formula is: 2 × Riser Height + Tread Depth = 24 to 25 inches. An alternative version: Riser + Tread = 17 to 18 inches. These are not code requirements but universally-used design guidelines. A stair with a 7-inch riser and 11-inch tread: 7 + 11 = 18 (within range, comfortable). A stair with a 6-inch riser and 12-inch tread: 6 + 12 = 18 (comfortable). A stair with a 5-inch riser and 14-inch tread: 5 + 14 = 19 (outside range — feels awkward and shuffling, even though it meets code minimum).