Free Contractor Tool
Stair Calculator
Enter your floor-to-floor height and get the riser count, exact riser height, tread count, total run, and stringer length — checked against IRC limits and the comfort formulas as you type.
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Calculate Stair Rise and Run
Measure the finished floor to finished floor height and enter it below. Everything else — riser count, exact riser height, run, stringer, angle, and code compliance — is calculated for you.
Finished floor to finished floor
10 in minimum (IRC)
36 in minimum (IRC)
Required if tread < 11 in
Stair layout
Risers
14
Riser height
7.714"
Treads
13
Total run
136.5"
Stringer length
174.1" (14.5 ft)
Stair angle
38.4°
Total run (ft)
11.38 ft
Effective tread
11.5"
IRC code check
Riser 7.714" within the 7.75" maximum
IRC R311.7.5.1 satisfied
Tread 10.5" meets the 10" minimum
IRC R311.7.5.2 satisfied
Width 36" meets the 36" minimum
IRC R311.7.1 satisfied
Comfort formulas
Rise + Run = 18.21"
Comfortable range is 17 to 18 inches
Rise × Run = 81 in²
Comfortable range is 70 to 75 square inches
2 × Rise + Run = 25.93"
Blondel formula target is 24 to 25 inches
IRC Stair Code Requirements
Residential limits from the International Residential Code. Commercial stairs under the IBC are stricter — 7 in maximum rise and 11 in minimum tread.
Worked example: 9-foot ceiling
A 108-inch total rise divided by the 7.75-inch maximum gives 13.9, so you need 14 risers. Each riser is then 108 ÷ 14 = 7.714 inches — just inside code.
That gives 13 treads. At 10.5 inches each the total run is 136.5 inches, or 11 feet 4.5 inches. The stringer works out at about 174 inches — you will need 16-foot stock.
The mistake that fails inspection
Measuring subfloor to subfloor instead of finished floor to finished floor. If you add 3/4-inch hardwood upstairs and nothing downstairs, every riser shifts — and the top or bottom step ends up out of tolerance on its own.
The IRC allows only 3/8 inch of variation between the tallest and shortest riser in a flight. Decide the finished floor build-up on both levels before you cut a single stringer.
Stair Comfort Formulas
Code tells you what is legal. These three rules tell you whether the stair will actually feel right underfoot.
How much floor space do stairs need?
More than clients expect. A standard 8-foot floor-to-floor run needs about 10 feet 6 inches of horizontal space, before you add landings at either end.
Headroom is the other constraint: 6 feet 8 inches measured vertically from the nosing line. In a tight footprint that requirement is usually what forces a switchback with an intermediate landing.
Sizing the stringer
Stringer length is straight Pythagoras — the square root of total rise squared plus total run squared. A 108-inch rise over a 136.5-inch run gives roughly 174 inches, so you are buying 16-foot stock.
Cut from 2×12 so enough material remains below the notches; codes generally want at least 5 inches of solid stringer left. Add a few inches to the calculated length for the plumb cut at the top and the level cut at the bottom.
From Material Takeoffs to a Signed Proposal
Getting the layout right is the craft. Getting paid for it quickly is the business. SubcontractorHub turns your takeoff into a branded proposal your crew can present on a tablet before leaving the site.
Materials, labor, and markup roll in automatically. Show good-better-best options, offer financing, and capture a signature on the spot.
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Stair dimensions are calculated from the values entered and checked against International Residential Code limits. Local amendments are common and often stricter, and commercial work falls under the IBC. Always confirm requirements with your building department before cutting stringers.
Frequently Asked Questions
- How do you calculate stair rise and run?
- Measure the total rise — the finished floor-to-floor height. Divide it by the maximum riser height allowed, 7.75 inches under the IRC, and round up to get the number of risers. Divide the total rise by that number for the exact riser height. The number of treads is always one fewer than the number of risers, and total run equals treads multiplied by tread depth.
- What is the maximum stair riser height?
- The International Residential Code caps residential risers at 7 3/4 inches and requires a minimum tread depth of 10 inches. Commercial stairs under the IBC are stricter at 7 inches maximum rise and 11 inches minimum tread. Local codes can be tighter than either, so confirm before framing.
- How many stairs for 8 feet?
- A 96-inch total rise divided by the 7.75-inch maximum gives 12.4, so you need 13 risers. That makes each riser 96 ÷ 13 = 7.38 inches, with 12 treads. At a 10.5-inch tread depth the total run is 126 inches, or 10 feet 6 inches of floor space.
- Why must all stair risers be the same height?
- People climb stairs by muscle memory rather than by looking, so an inconsistent riser is a genuine trip hazard. The IRC allows no more than 3/8 inch of variation between the tallest and shortest riser in a flight. The most common failure is forgetting to account for finished flooring thickness at the top or bottom, which throws off the first or last step.
- What is the ideal stair rise and run ratio?
- Three rules of thumb converge on the same comfortable range. Rise plus run should total 17 to 18 inches, rise multiplied by run should land between 70 and 75 square inches, and the Blondel formula — twice the rise plus the run — should equal 24 to 25 inches. A 7-inch rise with an 11-inch tread satisfies all three.
- How much space do stairs need?
- More than most people plan for. A standard 8-foot floor-to-floor staircase needs about 10 feet 6 inches of horizontal run, plus landing space top and bottom. You also need 6 feet 8 inches of headroom measured from the tread nosing line, which is what usually forces a switchback or a landing in a tight footprint.