Stair Calculator

Enter your total rise — the vertical distance from the bottom finished floor to the top finished floor — and your preferred tread depth. The calculator outputs the number of risers, the actual riser height, total run, stringer length, stair angle, and a green / yellow / red IRC compliance indicator. All inputs accept inches or centimeters.

What to enter

The calculator takes five inputs: total rise (required), tread depth (default 10 in), target riser height (default 7 in), tread thickness (default 1.5 in for 2x lumber), and stair width (default 36 in). The unit toggle switches every field between US imperial and metric.

What you get

Twelve outputs: number of risers, actual riser height, number of treads, total run, stringer length, stair angle, 2R+T comfort score, IRC compliance indicator per field, stringer count, stringer board length with 12-in waste, the recommended 2×12 PT stock board length, and a 2026 installed cost range.

Real project example — a 9-foot basement stair

Source: brief Section F-5 H2-1 worked example. The calculator above produces these exact numbers when the default inputs are submitted.

To calculate stair rise and run, divide your total rise by the number of risers you want — typically 7 to 7.75 inches per riser for residential stairs. The run (tread depth) should be at least 10 inches per IRC code. Multiply the number of treads (risers minus one) by tread depth to get total run.

The rise-run formula

Why riser height must be consistent

IRC R311.7.5 limits the difference between the tallest and shortest riser in a single flight to 3/8 inch. Inconsistent risers cause trips because the brain registers the first step rhythm and applies it to the rest of the flight. Dividing total rise by a whole number of risers — what the formula above does — eliminates the variation entirely.

The 7-11 rule for comfortable stairs

The 7-11 rule is a comfort heuristic: riser plus tread should equal 17 to 18 inches. A 7-inch riser pairs with an 11-inch tread (sum 18 in.); a 7.75-inch riser pairs with a 10-inch tread (sum 17.75 in.). This rule predates the modern IRC and is still taught in carpentry trades. It gives the right answer for most flights, but the more rigorous test is the 2R+T formula below.

The 2R+T comfort formula

The 2R+T formula — twice the riser plus the tread — was published by Architect François Blondel in 1675 and adopted by NFPA and FHA as the modern ergonomic comfort standard. Comfortable stairs land between 24 and 25 inches; below 24 the stair feels shallow and long, and above 25 it feels steep. The calculator displays the 2R+T score with every result.

Source: NFPA / FHA 2R+T = 24–25 in ergonomic comfort standard.

The 2021 International Residential Code (IRC R311.7) sets the minimum safety standards for residential stairs: maximum riser height of 7.75 inches, minimum tread depth of 10 inches with a nosing or 11 inches without, minimum headroom of 6 feet 8 inches, minimum stair width of 36 inches, and handrails between 34 and 38 inches above the tread nosing.

IRC R311.7 code requirements table

Source: International Residential Code 2021 §R311.7. Per Alex Rivera PE's structural review, all values reflect the 2021 IRC edition currently adopted by most US jurisdictions.

Deck stair code (IRC R507.4)

Exterior deck stairs are governed by IRC R507.4 in addition to R311.7. The most consequential rule: cut stringers must retain a minimum 3.5 inches of solid board after notching. Combined with a 7.75-in riser and 10-in tread, that minimum forces 2×12 lumber (actual 11.25 in) for any cut stringer. A 2×10 (actual 9.25 in) leaves only 5.75 inches after notching — too little for most geometries.

Commercial vs. residential code (IBC 1011.5 vs. IRC R311.7)

Commercial stairs follow IBC 1011.5, which is more restrictive: maximum riser 7 inches and minimum tread 11 inches. Residential stairs follow IRC R311.7 (max 7.75 in / min 10 in). The CalcSummit calculator targets residential — confirm with your AHJ if your project is mixed-use or commercial.

Check your local code

Local jurisdictions can amend the IRC. Some adopt earlier code editions (still using the older 7.75-in or 8.25-in riser maximum), and a few have steeper enforcement on tread depth. Verify the exact requirements with your local building inspector before pouring a footing or cutting a stringer.

Calculate stair stringer length using the Pythagorean theorem: Stringer Length = √(Total Rise² + Total Run²). For a 9-foot-rise stair (108 in) with a 130-inch total run, the stringer is √(108² + 130²) = √(11,664 + 16,900) = √28,564 = 169 inches, or just over 14 feet. Add 12 inches of waste for cuts.

The stringer length formula

What size lumber for stair stringers?

Cut stringers require 2×12 lumber at minimum. The actual depth of a 2×12 is 11.25 inches; after notching for a 7.75-in riser and 10-in tread, 7.75 inches of solid board remains — comfortably above the IRC R507.4 minimum of 3.5 inches. A 2×10 (actual 9.25 in) leaves 5.75 inches after notching, which is insufficient for most stair geometries. For exterior stairs, specify pressure-treated 2×12 to resist rot.

How many stringers do you need?

A standard residential stair uses three stringers — one on each side and one in the middle — spaced no more than 24 inches on center. Wider stairs add a fourth or fifth stringer. The calculator returns the exact stringer count for your stair width input. For a 36-in. wide stair, three stringers are sufficient; at 48 in., four are required.

Cut vs. uncut stringers

A cut stringer is notched in a sawtooth pattern so each tread sits on a horizontal cut (the seat cut) and each riser butts against a vertical cut (the plumb cut). An uncut (or solid) stringer keeps the full board depth and uses metal tread brackets or let-in tread cleats — typically stronger and used for very long runs or heavy loads. Cut stringers are faster to build; uncut stringers carry more load. The IRC 3.5-in remaining-board rule applies only to cut stringers.

For framing lumber takeoff beyond the stair itself — joists, beams, posts — use the framing calculator. For the concrete footing at the base of the stringer landing, use the concrete calculator.

Building stairs costs between $100 and $300 per step installed for interior stairs, and $25 to $50 per step in materials for exterior deck stairs using pressure-treated lumber. A standard 10-step flight costs $1,000–$3,000 installed, including stringers, treads, and labor at $50–$100 per hour (RSMeans / HomeAdvisor 2026).

Stair cost by type (2026)

Source: RSMeans Construction Cost Data 2026 and HomeAdvisor 2026 Cost Guide. Reviewed by Sarah Kim, CPE.

Factors that affect stair cost

Five drivers move stair cost: material choice (pine, oak, or hardwood), stringer type (cut or uncut), tread width, finish (paint, stain, or natural), and permit fees. Tight stair openings, landings, and winder turns add 30–50% to labor. Custom millwork — wraparound nosings, decorative stringers, ornate balusters — can double total project cost.

DIY vs. hiring a contractor

Exterior deck stairs are a reasonable DIY project for a homeowner with framing experience and the right saw. Interior stairs — especially main-stair runs that an inspector will check — are worth hiring out: code violations on a primary stair are expensive to remediate. Multi-flight stairs with landings, structural opening modifications, or any stair on a load-bearing wall always require a licensed contractor and may require a PE-stamped drawing.

For a full project budget including labor, permits, and site work, use the construction cost calculator. For hardwood tread surface coverage and cost, see the flooring calculator.

Riser and tread limits come directly from IRC 2021 §R311.7. Stringer length uses the Pythagorean theorem applied to total rise and total run. Comfort scoring uses the NFPA / FHA 2R+T = 24–25 in ergonomic standard. Cost ranges come from RSMeans Construction Cost Data 2026 cross-checked against HomeAdvisor 2026.

Fraction-to-decimal conversion for stair measurements

The five questions below cover the most-searched stair PAA queries: standard flight length, stair angle, handrail requirements, 2×10 vs. 2×12 stringers, and IRC riser variation. Each answer is self-contained and starts with a direct response.