Stair Calculator

Project name

No IRC edition selected. Choose one to get results · Confirm your required IRC edition with your local building department before using these results for a permit application.

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The stair calculator is fully usable — every dimension and the code check stay on screen. Your inputs are kept in this browser automatically.

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Floor to floor heightFinished floor to finished floor. Accepts fractions — 109 1/8".Run availableHorizontal space the stair can occupy.Stair widthHeadroom availableVertical, from the nosing line to whatever is above it.Tread depthNosing to nosing — not the width of the board.Tread nosingOverhang past the riser below.Stringer stock

Handrail height

Top of the rail above the nosing line.

Landing

Waste allowance

WITHIN EVERY LIMIT — NOT YET VERIFIED

Your stair is inside every limit we hold. Those limits have not been checked against the code text or your state's amendments yet, so this is not a compliance statement. Confirm the dimensions against R311.7 in your adopted edition.

1 further item could not be checked — see the code compliance list below for what each one still needs.

Risers

Riser height

6 13/16"

exact 6.8203"

Treads

Tread depth

10 1/2"

Total rise

9' 1 1/8"

Total run

13' 1 1/2"

Stringer length

15' 11 5/8"

Stair angle

34.7°

Cut to 6 13/16", 16 risers come to 0 1/8" under the true floor-to-floor height. Absorb it in the bottom riser when you cut the stringer — that is what the bottom-riser adjustment for finished floor thickness is for.

16 risers at 6 13/16" · 15 treads at 10 1/2" · stringer 15' 11 5/8" · 34.7° from horizontal. Dashed line is the underside of the stringer.

Code compliance

NEEDS MORE INFO

More information needed to check this.

InputsVERIFICATION REQUIRED

Every dimension needed for this calculation has been given and is usable.

IRC R311.7 stair limitsNEEDS MORE INFO

The stair limits for No IRC edition selected are not loaded (missing: riser.max, tread.min, width.min, headroom.min, riser.max-variation, tread.max-variation, handrail.required-at-risers, handrail.height.min, handrail.height.max), so nothing can be checked. The dimensions below are arithmetic only.

Checked against No IRC edition selected · IRC R311.7. These checks are an aid, not an approval — your building department has the final say.

Material takeoff

Stringers 2x123 @ 18'

· stringer marked out at15' 11 5/8" + 12" cutting allowance

Treads15 @ 3'

Risers16 @ 3'

Stringer hangers3

Structural screws180

Handrail brackets—

Lumber, as stock147.00 lf

Waste @ 10%+ 14.70 lf

Total to buy161.70 lf

No landing. A flight is limited in total rise before one is required (R311.7.3); that limit is NOT loaded in this app, so whether this flight needs a landing has not been checked. Nothing was assumed in its place.

Stair Calculator and Stringer Layout

Work backward from the one dimension the building fixes for you: the floor-to-floor height. Enter it — fractions welcome, 109 1/8 inches is fine — and the calculator picks the smallest riser count that keeps every riser inside the code limit, divides the rise exactly, and gives you the riser height to the nearest 1/16 inch.

It also reports the cumulative rounding error, which is the number that actually bites. Sixteen risers cut to 1/16 inch drift about an eighth of an inch off the true rise, and that has to be absorbed in the bottom riser when you cut the stringer. That is what the bottom-riser adjustment for finished floor thickness is for.

Every riser in a flight must come out the same. A stair whose treads are not identical is the most reliable way there is to put someone on the floor, which is why the code limits the variation across a flight to 3/8 of an inch and why this calculator will not produce an uneven layout.

Which code tables this works from

IRC R311.7.5.1 — riser height

Maximum riser height, and the maximum variation between the tallest and shortest riser in a single flight.

IRC R311.7.5.2 — tread depth

Minimum tread depth nosing to nosing, and the maximum tread variation.

IRC R311.7.1 and R311.7.2 — width and headroom

Minimum stairway width, and minimum headroom measured vertically from the nosing line.

IRC R311.7.8 — handrails

The riser count at which a handrail becomes required, and the height range it must sit in above the nosing line.

The inputs that matter

Floor-to-floor height

Finished floor to finished floor. Everything else follows from it, so measure it properly — and remember it changes if you add flooring later.

Tread depth

Measured nosing to nosing, not the width of the board. Deeper treads mean a longer total run, which is usually what runs you out of space.

Run available

The horizontal space the stair can occupy. If the stair does not fit, a landing with a turn is the usual answer.

Headroom

Measured vertically from the nosing line to whatever is above it. This is the one that catches people converting attics and basements.

Common questions

How do I calculate the number of stair risers?

Divide the total floor-to-floor rise by a comfortable riser height — around 7 inches — and round to a whole number, then check that the result keeps every riser inside the code maximum. If it does not, add a riser. Adding risers is the only safe direction: more risers means each one is shorter. A 109-1/8 inch rise comes out at 16 risers of 6-13/16 inches.

How many treads for a given number of risers?

One fewer than the risers, when the flight lands on the upper floor — the top tread is the floor itself. Sixteen risers gives fifteen treads. The total run is the tread count times the tread depth, so fifteen treads at 10-1/2 inches is 157-1/2 inches, just over 13 feet.

What is the maximum riser height allowed?

The IRC sets a maximum riser height and a minimum tread depth, and also limits how much the risers in one flight may vary from each other. This calculator pulls those limits from its code-rules data by edition and jurisdiction rather than hard-coding them, and tells you plainly whether your stair is inside or outside each one.

How long does the stringer need to be?

The stringer is the hypotenuse of the total rise and the total run. A 109-1/8 inch rise with a 157-1/2 inch run gives a stringer just over 191 inches, about 16 feet — so you are buying 2x12 stock longer than a standard 16 foot stick once you allow for the cuts.

GoFramePro provides preliminary planning and estimating information. Confirm structural requirements, local amendments and permitting requirements with the authority having jurisdiction.