Finished surface to finished surface.
Top of decking to grade.
Concrete pad or paver depth.
Nosing to nosing. IRC minimum 10 in.
Drives the bottom stringer cut.
IRC minimum 36 in clear.
Advanced options
0 = calculate it.
Required 3/4–1¼ in when tread under 11 in.
0 to skip. IRC minimum 80 in.
0 to skip the fit check.
This stair calculator lays out risers, treads and the stringer to IRC R311.7 — including the bottom stringer cut that causes more stair inspection failures than any other single mistake.
The short answer
Divide total rise by 7 for a comfortable step, round to a whole number, then divide back. A 36 in rise gives 5 risers of 7³⁄₁₆ in and 4 treads — because the top “tread” is the floor you’re arriving at.
Then cut one tread thickness off the bottom of the stringer. Skip that and your bottom step is an inch taller than the rest, which fails the 3/8 in uniformity rule outright.
On this page
How to use this stair calculator
Everything follows from total rise, so get that right first. Three ways in:
- Total rise known — you’ve measured finished surface to finished surface
- Deck to ground — deck height less the landing pad thickness
- Floor to floor — subfloor to subfloor, adjusted for finishes at each end
The header toggle chooses between comfort, which targets a 7 in riser, and fewest steps, which uses the legal maximum of 7¾ in. On a 60 in rise that’s the difference between 9 risers and 8.
Results show every dimension to the nearest 1/16 in, a pass or fail against each IRC limit, and the framing-square settings for marking the stringer.
The bottom riser is where stairs fail
This is pure geometry, it catches people every time, and most stair calculators don’t mention it.
You mark a stringer with a framing square: five identical triangles, each 7³⁄₁₆ in high. You cut it, stand it up, and it looks right. Then you screw the treads down — and every tread sits on top of the horizontal cut it was marked to.
The result: every riser measures 7³⁄₁₆ in except the bottom one, which now measures 8³⁄₁₆ in, because the stringer is resting on the ground while the first tread has been lifted by its own thickness.
IRC R311.7.5.1 allows the tallest and shortest riser in a flight to differ by no more than 3/8 in. Here’s what an uncorrected stringer actually produces:
| Tread material | Thickness | Bottom riser | Over the limit by |
|---|---|---|---|
| ¾ in plywood | 0.75 in | 7¹⁵⁄₁₆ in | 2.0× |
| 5/4 decking | 1 in | 8³⁄₁₆ in | 2.7× |
| 2× lumber | 1½ in | 8¹¹⁄₁₆ in | 4.0× |
The fix: cut the bottom of the stringer shorter by exactly one tread thickness. The first triangle becomes 6³⁄₁₆ in instead of 7³⁄₁₆, and once the tread is on, every riser measures the same. Mark it before you cut — a stringer can’t be lengthened.
The same logic applies at the top, in reverse. If the stringer hangs from a rim joist and the decking is fitted afterwards, the top riser shrinks by the decking thickness unless you allow for it. Measuring to finished surfaces at both ends is what keeps it honest.
The three equations
1. Risers = Total rise ÷ target riser, rounded
2. Riser height = Total rise ÷ Risers
3. Treads = Risers − 1
4. Stringer = √(Rise² + Run²)
Treads are always one fewer than risers — the upper floor is the last tread
Worked example — 36 in deck, 11 in treads, 1 in decking
What the IRC requires
| Requirement | Limit | Section |
|---|---|---|
| Maximum riser height | 7¾ in | R311.7.5.1 |
| Minimum tread depth | 10 in | R311.7.5.2 |
| Riser uniformity | 3/8 in | R311.7.5.1 |
| Tread uniformity | 3/8 in | R311.7.5.2 |
| Minimum clear width | 36 in | R311.7.1 |
| Minimum headroom | 80 in | R311.7.2 |
| Nosing, when tread under 11 in | ¾–1¼ in | R311.7.5.3 |
| Maximum rise between landings | 12 ft 7 in | R311.7.3 |
| Handrail required at | 4+ risers | R311.7.8 |
| Handrail height above nosing | 34–38 in | R311.7.8.1 |
The IRC sets no minimum riser height — only a maximum and the uniformity rule. The IBC, which governs commercial work, sets a 4 in minimum and requires 11 in treads rather than 10.
The 2024 IRC renumbered this section to R318.7. The dimensions are unchanged.
Choosing the riser count
Riser count is a whole number, so it’s the one decision that fixes everything else. Divide by 7 and you get a comfortable stair; divide by 7.75 and you get the fewest legal steps and the shortest run.
| Total rise | Fewest legal | Comfort | Riser at comfort | Run at 11 in |
|---|---|---|---|---|
| 24 in | 4 | 4 | 6 in | 33 in |
| 30 in | 4 | 4 | 7½ in | 33 in |
| 36 in | 5 | 5 | 7³⁄₁₆ in | 44 in |
| 42 in | 6 | 6 | 7 in | 55 in |
| 48 in | 7 | 7 | 6⅞ in | 66 in |
| 60 in | 8 | 9 | 6¹¹⁄₁₆ in | 88 in |
| 72 in | 10 | 10 | 7³⁄₁₆ in | 99 in |
| 105 in | 14 | 15 | 7 in | 154 in |
| 120 in | 16 | 17 | 7¹⁄₁₆ in | 176 in |
Most of the time both approaches agree. Where they don’t — 60 in, 105 in, 120 in — the extra riser buys a noticeably easier climb at the cost of about a foot of run. A 105 in floor-to-floor, which is a common 8 ft ceiling with joists, lands on exactly 7 in risers at 15.
The 7-11 rule and why it works
A 7 in riser with an 11 in tread is the classic target. It comes from a formula the French architect François Blondel published in 1675, and it has survived because it describes how people actually walk:
2 × Riser + Tread = 24 to 25 inches
7-11 gives exactly 25 · A comfortable stride is about 25 in on the flat
| Riser | Tread | 2R + T | Angle | Feel |
|---|---|---|---|---|
| 6½ in | 12 in | 25.0 | 28.4° | Gentle, generous |
| 7 in | 11 in | 25.0 | 32.5° | The benchmark |
| 7¼ in | 10½ in | 25.0 | 34.6° | Comfortable, compact |
| 7½ in | 10 in | 25.0 | 36.9° | At the steep end of ideal |
| 7¾ in | 10 in | 25.5 | 37.8° | Legal maximum steepness |
Anything under 24 feels cramped and hurried; over 25 feels like you’re breaking stride. Both stay legal — the code sets safety limits, not comfort ones — but a flight that sits inside the range is noticeably easier to use, and you’ll climb it thousands of times.
Nosing
Nosing is the tread overhanging the riser below. It matters more than it looks, because it adds effective foot room without adding run.
IRC R311.7.5.3 requires a projection of ¾ to 1¼ in whenever the tread depth is less than 11 in. So a stair built to the 10 in minimum must have nosing — it isn’t a style choice.
At 11 in or more it becomes optional. Note that nosing does not count toward tread depth: the 10 in is measured nosing to nosing, not from the riser face.
Cutting the stringer
- Set the framing square to your riser on the tongue and your tread on the blade. For the worked example, 7.2 in and 11 in.
- Step it along the 2×12, marking each triangle. Use a stair gauge so the setting can’t drift.
- Cut the bottom short by one tread thickness. This is the step everyone forgets.
- Keep 5 in of throat — the remaining material behind the cuts. Less than that and the stringer is structurally a row of teeth.
- Cut one, test-fit it in place, then use it as the template for the rest.
Stringers go at 16 in on centre for most decking, tightening to 12 in for composite treads, and never fewer than three on a 36 in stair. Buy stock about a foot longer than the calculated stringer length — the diagonal is longer than people expect, and the end cuts eat into it.
Total run — check it fits before you cut
Rise is fixed by the building. Run is not, and it’s where stairs collide with reality.
A 36 in deck needs 44 in of run at 11 in treads. A 105 in floor-to-floor needs 154 in — nearly 13 feet of floor space, before you add a landing at either end. Interior stairs get squeezed for exactly this reason, and the temptation is to shorten treads to the 10 in minimum, which buys you 14 in and costs comfort at every step for the life of the house.
The calculator has a space-available field. Enter it and you’ll be told before you cut anything, rather than after.
Ten ways stairs fail inspection
- Bottom riser too tall — the stringer wasn’t shortened by a tread thickness
- Top riser wrong — finished flooring added after the layout
- Risers over 7¾ in
- Treads under 10 in
- Variation over 3/8 in anywhere in the flight
- No nosing on treads under 11 in
- Width under 36 in, or handrails eating into it
- Headroom under 80 in
- No handrail on a flight of 4 or more risers
- Guard openings passing a 4 in sphere
Numbers one and two are the same error in different places, and between them they account for most first-attempt failures. Both come from measuring to something other than the finished surface.
Stair mistakes to avoid
- Not shortening the bottom stringer cut. The most common failure there is.
- Measuring to subfloor instead of finished floor. Every riser inherits the error.
- Counting treads equal to risers. Treads are always one fewer.
- Rounding the riser height. Divide the exact rise; 1/16 in per step compounds.
- Shortening treads to fit the space. You’ll feel it every day for decades.
- Forgetting the landing pad. It raises the bottom and shortens the first riser.
- Cutting all stringers before test-fitting one. Cut one, offer it up, then batch the rest.
- Leaving under 5 in of throat. The stringer becomes a comb.
Frequently asked questions
How do I calculate stairs?
Divide the total rise by about 7 in and round to a whole number for the riser count, then divide the rise back by that count for the exact riser height. Treads are one fewer than risers. A 36 in rise gives 5 risers of 7³⁄₁₆ in and 4 treads, with 44 in of run at 11 in treads.
Why is my bottom step taller than the others?
Because the stringer wasn’t cut short at the bottom. Treads sit on top of the horizontal cuts, so the first tread is raised by its own thickness while the stringer sits on the ground. With 1 in decking that makes the bottom riser 1 in taller — nearly three times the 3/8 in tolerance. Cut the bottom of the stringer shorter by exactly one tread thickness.
What is the maximum riser height?
7¾ inches under IRC R311.7.5.1, measured vertically between the leading edges of adjacent treads. The IRC sets no minimum, but the tallest and shortest riser in a flight cannot differ by more than 3/8 inch. The IBC, for commercial work, sets a 4 inch minimum.
What is the minimum tread depth?
10 inches under the IRC, measured nosing to nosing rather than from the riser face. The IBC requires 11 inches for commercial stairs. When a tread is under 11 inches the code also requires a nosing projection of ¾ to 1¼ inches.
How many treads for a given number of risers?
Always one fewer. The floor you’re arriving at serves as the last tread, so 5 risers need 4 treads. Counting them equal is a common error that adds a step’s worth of run and leaves you with a spare board.
What is the 7-11 rule?
A 7 inch riser with an 11 inch tread, the classic comfort target. It satisfies Blondel’s formula — twice the riser plus the tread should equal 24 to 25 inches — at exactly 25, giving a 32.5° flight. The IRC permits steeper, up to 7¾ and 10, but the 7-11 stair is noticeably easier to climb.
How long does a stringer need to be?
The hypotenuse: √(rise² + run²). A 36 in rise with 44 in of run gives a 56⅞ in stringer. Buy stock about a foot longer than that, because the end cuts consume material and the diagonal is longer than most people estimate.
How much space do stairs need?
More than most people plan for. A 36 in deck needs 44 in of run, but a 105 in floor-to-floor needs about 154 inches — nearly 13 feet — plus landings. Check the run against your available space before cutting, not after.
What is the 3/8 inch rule?
Within one flight, the tallest and shortest riser cannot differ by more than 3/8 inch, and the same applies separately to tread depth and nosing projection. It exists because people climb stairs by rhythm rather than by looking, and an odd step is a trip hazard. It’s the most commonly failed stair requirement.
Do I need a handrail?
On any flight of four or more risers, per IRC R311.7.8. It must be 34 to 38 inches above the tread nosings, continuous for the full flight, and graspable. Most deck stairs from a 30 inch deck have at least four risers, so assume you need one.
How far apart should stair stringers be?
16 inches on centre for most decking, tightening to 12 inches for composite treads, which sag between wider supports. A 36 inch stair needs at least three stringers. Keep 5 inches of uncut material — the throat — behind the notches or the stringer loses strength.
Can I use this stair calculator in metric?
Dimensions are in inches, since the IRC is written that way. Multiply inches by 25.4 for millimetres — so the 7¾ in maximum riser is 196 mm and the 10 in minimum tread is 254 mm, which are the figures the code itself gives in brackets.
Standards and sources
- IRC R311.7.5.1 — riser height maximum and the 3/8 in uniformity tolerance
- IRC R311.7.5.2 — tread depth minimum and its own uniformity tolerance
- IRC R311.7.5.3 — nosing projection, required where tread depth is under 11 in
- IRC R311.7.1, R311.7.2, R311.7.3 — width, headroom and maximum rise between landings
- IRC R311.7.8 — handrail requirement and height. The 2024 IRC renumbers this section to R318.7 without changing the dimensions
- Blondel’s rule (1675) for the 2R + T comfort range, still the basis of the 7-11 guideline
Related calculators
How this stair calculator works out its numbers
Riser count comes from dividing total rise by a target height and rounding — 7 in for the comfort setting, 7¾ in for fewest steps — then taking whichever is larger so the result is always legal. Riser height is the exact rise divided by that count, carried at full precision and displayed to the nearest 1/16 in, which is how it gets marked out.
Treads are risers minus one, because the upper surface serves as the final tread. Run is treads times tread depth, and the stringer is the hypotenuse of rise and run. The angle reported is per step — arctan of riser over tread — which is the pitch you feel, rather than the overall flight diagonal.
Every IRC limit is checked individually and reported as a pass or a specific failure, including the ones that interact: nosing becomes mandatory below an 11 in tread, and total rise above 12 ft 7 in requires an intermediate landing regardless of everything else. The bottom stringer cut is calculated from your tread thickness rather than assumed.
This is a layout tool, not a structural design. Stringer sizing and span, guard and handrail construction, and landing framing are separate structural questions. Local amendments to the IRC are common, and stairs are an inspected item almost everywhere. Cut one stringer, test-fit it in place, and confirm every riser measures the same before you cut the rest.