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Stair Calculator

Basis: IRC 2021 · R311.7 (Stairways) · R312 (Guards) · prescriptive tables only

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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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DOES NOT COMPLY

One requirement is not met.

  • Fits the space available

    The stair runs 157 1/2" but only 144" is available — 13 1/2" too long. Add a landing and turn the stair, or reduce the tread depth if the code minimum allows. NOTE: this app does NOT check landings — not their dimensions, and not the total rise at which R311.7.3 requires one. If you add a landing you are checking it yourself.

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

Risers

16

Riser height

6 13/16"

exact 6.8203"

Treads

15

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.

9' 1 1/8"13' 1 1/2" total runR 6 13/16"T 10 1/2"34.7°
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

DOES NOT COMPLY

Something here does not comply.

InputsVERIFICATION REQUIRED

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

Riser heightVERIFICATION REQUIRED
Your riser height: 6 13/16"Maximum allowed: 7 3/4"Margin: 0 15/16"

Riser height of 6 13/16", within the 7 3/4" maximum we compute. That figure has not been verified against the printed table yet, so check it against your code book before you build.

IRC 2021 R311.7.5.1 — Maximum riser height· this rule is marked unverified in our data, so it cannot return a PASS

Tread depthVERIFICATION REQUIRED
Minimum required: 10"Your tread depth: 10 1/2"Above the minimum by: 0 1/2"

Treads are 10 1/2", at or above the 10" minimum.

IRC 2021 R311.7.5.2 — Minimum tread depth· this rule is marked unverified in our data, so it cannot return a PASS

Stair widthVERIFICATION REQUIRED
Minimum required: 36"Your stair width: 36"Above the minimum by: 0"

36" wide, at or above the 36" minimum.

IRC 2021 R311.7.1 — Minimum stairway width above the handrail height· this rule is marked unverified in our data, so it cannot return a PASS

HeadroomVERIFICATION REQUIRED
Minimum required: 80"Your headroom: 82"Above the minimum by: 2"

82" of headroom, at or above the 80" minimum.

IRC 2021 R311.7.2 — Minimum headroom, measured vertically from the nosing line· this rule is marked unverified in our data, so it cannot return a PASS

Riser variation within the flightNEEDS MORE INFO

Not checked. Every riser this calculator lays out is identical, so variation only becomes real once the stair is BUILT — the bottom riser in particular changes with finished floor thickness. Measure all 16 risers and enter them to check the 0 3/8" limit.

IRC 2021 R311.7.5.1 — Maximum difference between the tallest and shortest riser in a flight· this rule is marked unverified in our data, so it cannot return a PASS

Tread variation within the flightNEEDS MORE INFO

Not checked. Every tread this calculator lays out is identical, so variation only becomes real once the stair is BUILT — the bottom riser in particular changes with finished floor thickness. Measure all 15 treads and enter them to check the 0 3/8" limit.

IRC 2021 R311.7.5.2 — Maximum difference between the deepest and shallowest tread in a flight· this rule is marked unverified in our data, so it cannot return a PASS

Handrail heightVERIFICATION REQUIRED
Permitted range: 34" – 38"Your handrail height: 36"

36" above the nosing line, inside the 34"–38" range.

IRC 2021 R311.7.8.1 — Minimum handrail height above the nosing line· this rule is marked unverified in our data, so it cannot return a PASS

GuardsNEEDS MORE INFO

A guard IS required — the 109 1/8" drop is above the 30" trigger in R312.1.1 — but no guard height has been given, so it cannot be checked. The minimum is 36".

IRC 2021 R312.1.2 — Minimum guard height· this rule is marked unverified in our data, so it cannot return a PASS

Handrail requiredVERIFICATION REQUIRED
Minimum required: 4 risersYour riser count: 16 risers

This flight has 16 risers, so R311.7.8 requires a handrail — and one is specified. Its height is checked separately.

IRC 2021 R311.7.8 — Number of risers in a flight at or above which a handrail is required· this rule is marked unverified in our data, so it cannot return a PASS

Fits the space availableDOES NOT COMPLY

The stair runs 157 1/2" but only 144" is available — 13 1/2" too long. Add a landing and turn the stair, or reduce the tread depth if the code minimum allows. NOTE: this app does NOT check landings — not their dimensions, and not the total rise at which R311.7.3 requires one. If you add a landing you are checking it yourself.

Checked against IRC 2021 · IRC R311.7. These checks are an aid, not an approval — your building department has the final say.

Not for purchase — design does not comply

One or more checks failed. The quantities below show what this design would take, so you can see how far off it is — they are not an order. Fix the failing check first.

Material takeoff

Stringers 2x123 @ 18'
· stringer marked out at15' 11 5/8" + 12" cutting allowance
Treads15 @ 3'
Risers16 @ 3'
Stringer hangers3
Structural screws180
Handrail brackets5
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.

⚠ The stair does not fit the run available. A landing with a turn is the usual answer — but this app does not check landings, so their dimensions and the maximum rise between them are yours to verify against R311.7.3.

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.

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