Stair Calculator
Work out how many risers and treads a stair flight needs, and how much floor space it takes, from the floor-to-floor rise.
Important: this tool is a layout estimate using model-IRC limits. Stairs are life-safety construction — the final design must be verified against your local building code (which may be stricter), and a permit is typically required. Like all BuildyWork tools, it supports your planning; it is not a substitute for a professional or your local building official.
Stair dimensions
Riser count uses the IRC maximum riser height of 7.75" and the minimum tread depth of 10" — model-code values that your local jurisdiction may amend. This is a layout estimate, not a stair design.
Results
For a total rise of 106 in with 10 in treads, you'll need 14 risers of 7.57 in each and 13 treads, for a total run of 130 in (10.83 ft).
Risers needed
14 risers of 7.57 in
That's a full flight, and the riser height comes out within a quarter inch of the 7.75" code maximum — the steepest layout the model IRC allows.
Construction tolerance can push a near-limit riser over the line at inspection. Consider adding one more riser for margin (and recheck the longer total run fits), and confirm your jurisdiction hasn't adopted a stricter maximum before cutting stringers.
Treads (risers − 1): 13
Actual riser height: 7.57 in
Total run: 130 in (10.83 ft)
How this works
The core move is a division with a constraint: your total rise divided by the code's maximum riser height (7-3/4" per IRC R311.7.5.1), rounded up to a whole number, is the minimum number of risers for which every riser can stay at or under the limit. Rounding up is not a convention here but a safety property — rounding down would put every riser over the legal maximum, as the second worked example shows. The rise is then divided by that riser count to give the actual riser height, and dividing equally (rather than stacking maximum-height risers with a short one at the end) is itself a code requirement: the IRC allows no more than 3/8" of variation within a flight, because an odd step is a documented trip hazard. Tread count is risers minus one, since the upper finished floor serves as the final walking surface, and total run is tread count times the tread depth you choose — with the IRC's 10" minimum tread as the floor of the input range. What this calculator deliberately does not do: check headroom (that depends on your stairwell opening), lay out landings, size stringers, or account for local code amendments, several of which are stricter than the model IRC values used here. It is a layout estimator in the same spirit as our other calculators — it gets the arithmetic and the code constants right so you can think about the carpentry — but stairs are life-safety construction: the resulting design must be verified against your local code, and a building permit is typically required. As our About page says, these tools support your planning; they are not a substitute for a professional or your local building official. The unit toggle switches between imperial and metric; internally every calculation runs in canonical SI units, so switching never introduces rounding drift. Riser and tread limits (max riser 7-3/4", min tread 10", 3/8" flight variation): IRC R311.7.5, via the ICC's published code.
Worked example
A basement stair with a measured total rise of 106 inches, using 10" treads: 106 ÷ 7.75 = 13.7, rounded up to 14 risers. Actual riser height: 106 ÷ 14 ≈ 7.57" — comfortably under the 7-3/4" IRC maximum, with about 3/16" of margin for construction tolerance. Treads: 14 − 1 = 13. Total run: 13 × 10" = 130", or 10 ft 10 in of horizontal space, before the landing required at the bottom. Riser + tread = 17.57", right in the traditional comfort band.
A second example: a taller rise where one riser changes everything
A main-floor stair with a 118" total rise and 11" treads: 118 ÷ 7.75 = 15.2, rounded up to 16 risers of 118 ÷ 16 = 7.375" each, 15 treads, and a total run of 15 × 11" = 165" (13 ft 9 in). Now see what rounding down would have done: 15 risers of 7.87" each — every single riser over the legal maximum, an inspection failure spread across the whole flight. One riser is the difference between a compliant stair and a rebuild, which is why the calculator only ever rounds the riser count up.
Common mistakes
Measuring the rise at the wrong spot or to the wrong surface
The rise must be plumb, at the stair location, finished floor to finished floor. Measuring along a sloped floor, at a different corner of the room, or subfloor-to-subfloor when one floor will get 3/4" hardwood and the other tile, bakes an error into every riser. The equal-riser division then spreads that error across the flight — until the flooring goes in and the top or bottom step is suddenly a code-violating odd one.
Forgetting the tread thickness when cutting the first and last riser
A classic stringer-cutting error: once treads are installed, every step gains a tread thickness — which cancels out everywhere except at the ends. The bottom of the stringer must be trimmed by one tread thickness ("dropping the stringer") or the first step ends up a tread-thickness too tall and the top step too short. The calculator gives you the theoretical equal risers; dropping the stringer is how the cut version preserves them.
Using the maximum riser as the design riser
Dividing the rise by 7.75" and rounding up guarantees compliance, but treating 7-3/4" as a target rather than a ceiling produces the steepest legal stair — and if construction tolerances push one riser even slightly over, it fails inspection. The actual riser this calculator reports is already below the max by construction; if it's close to the limit, seriously consider one more riser for a stair that both passes with margin and walks better.
Planning the run without checking what the stairs land on and under
A 13-tread flight at 10" per tread needs 130" of horizontal run — plus a code-required landing at least as deep as the stair is wide at the bottom, and headroom clearance under the floor opening above along the entire descent. It's common to compute a compliant flight that physically doesn't fit the house. Sketch the flight in section, with the stairwell opening, before cutting anything.
Confusing tread depth with the physical board width
Tread depth is the horizontal riser-face-to-riser-face distance — the run per step — measured nosing to nosing. An 11-1/4"-wide board (a 2×12) making a 10" run with a 1-1/4" nosing overhang is a 10" tread, not an 11-1/4" one. Entering the board width as the tread depth inflates the total run and, worse, misplaces every riser cut on the stringer.
Ignoring local amendments and permit requirements
The IRC numbers this calculator uses (7-3/4" max riser, 10" min tread) are the model code, but stairs are among the most commonly amended provisions — some jurisdictions require 7" maximum risers and 11" treads. Most new or replacement interior stairs also need a permit. A call to your building department before cutting stringers costs minutes; rebuilding a flight that fails inspection costs the whole job.
Building to the plan's rise instead of the as-built rise
Framing drifts: a floor system speced at 109" of rise can come out at 108-1/4" as built. Stringers laid out from the plan number then land wrong by nearly an inch, concentrated at one end. Always measure the actual rise after the upper floor is framed (and re-run this calculator with the real number) rather than trusting the drawing.
When this calculator's model stops applying
- Spiral and circular stairs: the IRC has an entirely separate rule set for spirals (minimum tread dimensions at the walk line, different rise limits) — this straight-flight model doesn't transfer.
- Winder treads turning a corner: legal under the IRC with specific walk-line and minimum-dimension requirements, but their variable tread geometry is outside this calculator's uniform-tread model.
- Flights serving more than a dwelling unit (commercial, multi-family common stairs): the IBC applies instead, with a 7" maximum riser and 11" minimum tread — stricter than the IRC values used here.
- Very tall rises without landings: the IRC limits a single flight's vertical rise (12 ft 7 in between landings); a taller total rise needs an intermediate landing, which this single-flight model doesn't lay out.
- Attic ladders, ship's ladders, and alternating-tread devices: permitted only for specific limited-access uses, under their own rules — their geometry deliberately violates normal stair limits.
- Existing-stair replacement in older homes: some jurisdictions allow existing non-conforming dimensions to remain in a like-for-like replacement where the geometry can't be fixed without altering the structure — a legal question for your local official, not a calculation.
- Stairs with finish buildup applied after layout: thick tile or a double-layer tread added later changes the first and last riser heights; the model assumes the rise you enter is to final finished surfaces.
- Guards and handrails: required (handrails on flights of 4 or more risers, guards where drops exceed 30") but entirely outside this calculator — a dimensionally perfect flight with no handrail still fails inspection.
FAQ
What exactly is the total rise, and how do I measure it?
Total rise is the vertical distance from the finished floor at the bottom of the stairs to the finished floor at the top — not subfloor to subfloor, unless both finish floors will be the same thickness. Measure it plumb with a level and tape (or a laser) at the actual stair location, because floors are rarely perfectly level and an inch of error at measurement becomes a code violation at the top riser.
Why does the calculator use 7-3/4" as the maximum riser height?
That figure comes from IRC R311.7.5.1, the model residential code most US jurisdictions adopt: riser height shall not be more than 7-3/4 inches. Dividing the total rise by 7.75" and rounding up gives the fewest risers that all stay at or under the limit. Note that some states and cities amend this — several use a 7" maximum — which is one reason the local-verification disclaimer on this page is not boilerplate.
Why must all risers be the same height?
IRC R311.7.5.1 also limits the variation between the tallest and shortest riser in a flight to 3/8". The deeper reason is biomechanics: after two or three steps, your body calibrates to the rhythm, and a single odd-height step is a genuine trip hazard — it's consistently cited among the most common causes of stair falls. That's why the calculator divides the rise into perfectly equal risers rather than using a stack of maximum-height ones plus a short leftover.
Why is there one fewer tread than risers?
In a standard flight that lands on the upper floor, the top riser is 'capped' by the upper floor itself — the floor is the last walking surface, so it isn't a tread you build. Fourteen risers therefore means thirteen treads. The exception is a flight that ends at a landing platform you're also building; if you frame the landing as the top tread, count treads = risers there.
What tread depth should I use?
The IRC minimum is 10 inches (R311.7.5.2), measured from riser face to riser face — nosing to nosing, not the physical board width. 10" is a code floor, not a comfort target: many stair builders prefer 10-1/2" to 11" treads for main stairs, and commercial code (IBC) requires 11". Deeper treads cost total run — each extra inch of tread depth adds one inch times the number of treads to the horizontal space the stairs consume.
Does the calculator check headroom?
No — headroom depends on the floor opening dimensions above the stairs, which this calculator doesn't model. The IRC requires at least 6 ft 8 in of clear headroom measured vertically from the tread nosing line. When you have the riser count and total run from this tool, check where the flight passes under the floor above: total run is frequently constrained not by floor space but by where your head clears the stairwell trimmer.
What is the comfort rule for riser and tread combinations?
The traditional carpenter's rule of thumb: riser + tread should land around 17–18 inches (a related version: 2 × riser + tread ≈ 24–25"). A 7-3/8" riser with a 10-1/2" tread sums to 17-7/8" — comfortable. The code limits define the legal envelope; the comfort rules find the pleasant spot inside it. If this calculator gives you a riser near the maximum, consider adding one more riser and rechecking: slightly shorter risers with the same treads often walk better.
How does nosing affect the numbers?
Tread depth is measured nosing-to-nosing, so a projecting nosing doesn't change the total run — the run is set by the riser-face spacing you enter as tread depth. The IRC does regulate the nosing itself (typically 3/4" to 1-1/4" projection where treads are less than 11" deep). Practically: cut your stringers to the tread-depth run, and the nosing overhangs beyond the cut.
Can I use this for a deck or exterior stairs?
Yes — the same IRC riser and tread limits apply to deck stairs, and the math is identical, with the total rise measured from the landing pad (or grade) to the deck surface. Two exterior wrinkles: the landing pad's final height matters (pour or set it before final stringer layout, or your bottom riser ends up odd), and if the grade slopes, measure the rise at the point where the stairs actually land.
How much lumber do stairs need once I have the layout?
The layout drives it directly: stringers are typically cut from 2×12s long enough to span the diagonal (roughly √(rise² + run²) plus waste — order the next standard length up), with three stringers standard for stairs up to 36" wide. Treads are one board per tread; risers one per riser if the design is closed. This calculator's riser/tread counts and total run give you every quantity except the board widths you choose.
What if my result's actual riser height comes out well under the maximum?
That's normal and good. The calculator uses the fewest risers that satisfy the limit, so the actual riser is whatever the rise divides into — for example, a 106" rise needs 14 risers of about 7.57" each. If the result lands near the maximum and feels steep, add a riser manually (divide your rise by riser count + 1) and see if the shallower flight fits your available run: more risers means shorter risers but a longer total run and one more tread.
Do landings or winders change the calculation?
A landing splits one flight into two, but the riser math stays global: the total rise still divides into equal risers across both flights, and the landing simply replaces one tread with a platform (the landing surface counts as one step in the sequence, and IRC requires it at least 36" deep in the direction of travel). Winders — wedge-shaped treads that turn a corner — have their own IRC dimensional rules at the walk line and are beyond this straight-flight model.