Measuring your stairs before you calculate
The calculator is only as accurate as the one number you feed it. Total rise decides everything else — get that measurement right first.
Find your total rise
Hold a tape measure or a straight board plumb from the finished floor you're leaving to the finished floor you're arriving at. On an existing opening, drop a plumb bob from the top nosing line to the finished floor below.
Note your available run
Measure the horizontal floor space you actually have, from the base of the rise to the nearest wall, doorway or obstruction. This tells the calculator whether your ideal stair will physically fit.
Decide riser height, or let it auto-set
Leave it on Auto and the calculator targets a comfortable 7.5" riser, then rounds to a whole step count. If your project has a fixed riser requirement, switch to Custom and enter it directly.
Read the compliance chips
Green means the result sits inside IRC residential limits for riser height and tread depth. Red means adjust total rise, riser target or tread depth before you cut anything.
The four formulas behind every result
Nothing on this page is a black box. Here's exactly what runs when you change a number.
Step Count
Total rise divided by your target riser height, rounded to the nearest whole step — you can't build a fractional stair.
Actual Riser Height
The rounding above means your real riser is rarely the exact target. This is the number you cut every riser to, uniformly.
Tread Depth (2R+T rule)
A standard comfort formula: twice the riser plus the tread should land near 24–25 inches. Floored at the 10" IRC minimum.
Stair Angle & Stringer
Angle from the riser-to-tread ratio; stringer length from the Pythagorean triangle formed by total rise and total run.
Walking through a real 108" rise
Same four formulas, filled in step by step, for a basement stair climbing 9 feet (108") floor to floor — the default this calculator loads with.
Step count
108" total rise, targeting a 7.5" riser: 108 ÷ 7.5 = 14.4, which rounds to 14 steps. You can't cut a fractional step, so the calculator always rounds to the nearest whole number here.
Actual riser height
Re-divide the total rise by that whole step count: 108 ÷ 14 = 7.71" per riser. That's the number every riser gets cut to — not the 7.5" target, which was only a starting point.
Tread depth
Using the 2R+T comfort rule: 25 − (2 × 7.71) = 9.57", which is below the 10" IRC minimum, so the calculator floors it at exactly 10". This is a common outcome once riser height climbs past roughly 7.5" — the comfort formula and the code minimum start to conflict, and code always wins.
Run, angle, and stringer
13 treads (one fewer than the step count) at 10" each gives a total run of 130". The angle works out to atan(7.71 ÷ 10) ≈ 37.6° — just past the comfortable range, which is a direct consequence of the tread being floored in step 3. Stringer length is √(108² + 130²) ≈ 169.0", the board length a stringer has to be cut from before notching.
This is exactly why the angle chip can fail even when the riser and tread chips both pass individually — the three checks interact, and a tight rise is the usual cause. Increasing available run, or accepting a 15-step flight instead of 14, brings the angle back into range.
Comfort formulas compared
Code sets the outer limits. These three rules of thumb, all satisfied by this calculator's defaults, decide whether a code-legal stair also feels good to climb.
| Rule | Formula | Target | What it optimizes for |
|---|---|---|---|
| 7-11 Rule | R = 7", T = 11" | R+T ≈ 18" | Simple carpenter's rule of thumb, slightly conservative on tread depth. |
| 2R + T Rule | 2R + T = 24–25" | ≈ 24.5" | The industry-standard comfort formula; used as this page's Auto default. |
| Blondel's Formula | 2R + T = 63–65 cm | ≈ 24–25.6" | The 19th-century architectural origin of the 2R+T rule, in metric. |
Want all three run against the same inputs side by side? Use the Stair Comfort Formula Calculator.
IRC residential stair code, quick reference
This calculator checks your result against the two limits below. Full residential code covers headroom, width and handrails too — see the linked calculators for those.
| Element | Residential (IRC) | Commercial (IBC) |
|---|---|---|
| Max riser height | 7.75" | 7" |
| Min tread depth | 10" | 11" |
| Riser variance across a flight | 3/8" | 3/8" |
| Min headroom | 6'8" | 6'8"–7'6" (varies) |
| Min clear width | 36" | 44"+ (varies) |
Reference only — always confirm against your local jurisdiction's adopted code before building.
Landings, headroom & stringer planning
A stair can pass every riser and tread check and still fail inspection on three things this section covers — flight length, vertical clearance, and how many stringers actually carry the load.
When you need a landing
IRC caps a single flight at 147" (12'-3") of vertical rise. This calculator's Landings Needed result and the compliance chip flag it automatically — a 108" basement stair never triggers it, but a full two-story run often does. Each landing must be at least as deep as the stair is wide.
Checking headroom
Measured perpendicular from the nosing line — not straight up-and-down — to any ceiling, header, or duct above. Rise and run math says nothing about this, which is why it's the single most common item that trips up an otherwise-compliant basement stair. Enter your measured clearance in the Advanced panel to check it here.
How many stringers
Cut stringers spaced no more than roughly 16" on center keep a stair from feeling bouncy underfoot. A standard 36" wide stair lands on 3; anything wider, or a run getting a heavy finish like tile or stone, typically needs a 4th. Set your stair width in the Advanced panel to see the count.
Board length per stringer
The straight-line stringer length this calculator returns is the hypotenuse only. Add roughly 14" for the birdsmouth cut at the top and notch waste along the length, then round up to a stocked lumber length — 8', 10', 12', 14', 16', 18' or 20'. The Stringer Board result already does this rounding for you.
These are planning-stage estimates, not a stamped structural design. A wide stair, an unusual load, or a long single stringer span may need an engineer's sign-off regardless of what this calculator shows.
Common mistakes when planning rise and run
Most stair complaints — and most failed inspections — trace back to one of these five planning errors, not to bad carpentry.
Measuring to sub-floor, not finished floor
Total rise has to account for finished flooring and underlayment on both levels — tile, hardwood or carpet pad on either end can shift the number by half an inch or more.
Rounding the target riser instead of recalculating it
Once you round to a whole step count, go back and divide total rise by that count. Building to the original, unrounded target riser produces an inconsistent final step.
Ignoring the last step down
The finished floor at the bottom counts as the last "riser" in the sequence. Forgetting it is the most common reason a site-built stair ends up one step short or with an oversized final riser.
Locking tread depth before checking angle
A tread that satisfies the 2R+T rule can still be too shallow once the code minimum overrides it, as in the worked example above — always check the resulting angle, not just the riser and tread individually.
Forgetting stringer material loss
Calculated stringer length is the straight-line hypotenuse. Add extra for the notched cuts, the birdsmouth at the top, and standard lumber lengths — a 169" calculated stringer is typically cut from a 14' or 16' board.
Skipping the headroom check
Rise and run tells you nothing about vertical clearance above the stair. A layout that passes every riser and tread check can still fail on headroom — check that separately before finalizing.
When to use a stair rise and run calculator
Rise and run planning shows up at the very start of almost every stair project, indoor or out.
Home Renovation
Replacing a worn or non-compliant staircase during a remodel.
New Construction
Setting stair layout before framing, so it matches the plan set.
Deck & Outdoor Builds
Laying out stringers before cutting deck stair lumber.
Code Compliance & Permits
Confirming riser and tread numbers before a plan check or inspection.
Basement Finishing
Fitting a compliant stair into a fixed, often tight, basement opening.
Accessibility Planning
Checking whether a gentler rise and run is achievable before design-freeze.