Stair Stringer Calculator
Stair design + material takeoff — IRC-compliant riser height and tread depth, stringer count, balusters, handrail, and labor by riser.
Finished floor to next finished floor
IRC min 36 in
IRC max 7-3/4 in
IRC min 10 in; 11 in standard
2×12 PT 16 ft: ~$65
Oak: ~$48. PT: ~$22. Composite: ~$55.
3.3 standard (4 in OC)
1.25 hr standard. Outdoor PT: 1.0. Custom newel: 2.0.
Result
- RisersIRC-compliant
- 16 × 6.75 in
- Treads
- 15 × 11 in deep
- StringersFor 36 in wide stair
- 3
- Total run
- 165.0 in (13.8 ft)
- Balusters
- 46
- Stringer cost
- $195
- Tread cost
- $720
- Riser boards
- $288
- Balusters
- $552
- Handrail
- $216
- Labor
- 20.0 hr · $1,300
- Total (material + labor)
- $3,271
This estimate is based on national average costs and may vary by region, project specifics, and market conditions. Use as a starting point for your bids.
How to calculate stair dimensions
Two numbers do most of the work: total floor-to-floor rise and your target riser height. Divide total rise by target riser and round UP — that gives you the riser count. The actual riser becomes total rise ÷ riser count. The IRC max is 7-3/4 in, and you want at least 4 in.
Tread count is always one less than riser count (you don't tread on the floor at the bottom). Total run is tread count times tread depth — typical 10 in IRC minimum, 11 in or 12 in for comfort.
IRC residential stair requirements
| Spec | Requirement |
|---|---|
| Riser height | Max 7-3/4 in, min 4 in |
| Tread depth | Min 10 in (nose-to-nose) |
| Width (clear) | Min 36 in |
| Riser variation | Max 1/4 in between any two |
| Headroom | Min 6 ft 8 in |
| Baluster spacing | No 4-in sphere passes through |
| Handrail height | 34–38 in above nosing |
Always confirm with your local building department — older homes sometimes get grandfathered into legacy specs but new work has to meet current code.
Stringer count by stair width
- ≤ 36 in wide: 3 stringers (residential minimum)
- 36–60 in wide: 4 stringers
- ≥ 60 in wide: 5 stringers
Production rates
An experienced carpenter builds a typical 16-riser interior stair in about 20 hours (labor only, materials sized and cut beforehand). That's ~1.25 hours per riser including layout, stringer cuts, install, tread + riser fasten, and finish. Outdoor stairs (PT, simpler trim) run faster — about 1 hour per riser. Custom newel posts and curved railings push the labor side dramatically.
Frequently asked questions
What are the IRC stair requirements?
The IRC residential code (R311.7) sets: riser height max 7-3/4 in (and min 4 in), tread depth min 10 in (measured nose to nose), 36 in clear width minimum, max 1/4 in variation between any two risers, and 6 ft 8 in headroom. Always check your local jurisdiction — many states amend these for stairs in older homes.
How do I calculate riser count?
Total floor-to-floor rise divided by your target riser height (typically 7 in), then rounded up. A 108 in (9 ft floor-to-floor) stair targeting 7 in risers needs 16 risers (108 ÷ 7 = 15.4 → 16). The actual riser becomes 108 ÷ 16 = 6.75 in. Always round UP — rounding down would push the riser past the 7-3/4 max.
How many stringers do I need?
Three is the residential minimum for stairs up to 36 in wide. Four stringers for 36–60 in wide (typical front-porch stairs). Five for stairs 60 in and wider. Modern engineered treads can sometimes go 4 ft between stringers, but 16-in OC is the conservative spec that any inspector will approve without questions.
Open riser vs closed riser — what's the cost difference?
Closed risers (with a riser board between each tread) add the cost of the riser boards themselves but are required for stairs serving primary egress in most jurisdictions. Open risers (no boards) save material cost but the gap between treads must be less than 4 in to comply with the same IRC baluster rule. Open risers look modern but lose some structural cross-bracing.
How many balusters do I need?
IRC requires no opening to allow a 4-in sphere through. For balusters perpendicular to the rail, that translates to spacing them so the on-center spacing is ≤ 4 in minus baluster width. For a 1.5-in baluster: 4-1.5 = 2.5 in clear gap = 4 in OC. For 100 LF of railing at 4 in OC, that's 300 balusters. The calculator above takes a balusters-per-foot input — 3 to 3.3 covers most cases.
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