Stair Calculator
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Risers
- Actual riser height
- 7.2 in
- Treads
- 14 treads at 10 in
- Total run
- 11 ft 8 in (140 in)
- Stringer length
- 14 ft 8.82 in minimum
- 2R + T comfort rule
- 24.4 in — within the 24–25 in band
- R × T rule
- 72 — within the 71–75 band
- IRC compliance
- Meets IRC riser and tread limits
- The preferred 7.5 in riser gives 14.4 steps, which is not a whole number — so the count is 15 and each riser is actually 7.2 in. Every riser in a flight must be within 0.375 in of every other, so the total rise has to divide exactly. There is no rounding available here.
- 14 risers would also satisfy the IRC maximum, but at 7.714 in it falls outside the 2R+T comfort band. 15 risers was chosen instead.
- Riser 7.2 in is within the 7.75 in IRC maximum and tread 10 in meets the 10 in minimum.
- 2R + T = 24.4 in, inside the 24–25 in band that makes a stair comfortable at a natural walking stride.
- The flight needs 11 ft 8 in of floor space and at least 6 ft 8 in of headroom measured vertically from the tread nosing line — the constraint that most often forces a redesign in a retrofit.
Riser counts for this rise
Every option that divides 108 in into equal risers. Only some satisfy both the code limit and the comfort rule.
| Risers | Riser height | Total run | 2R + T | Verdict |
|---|---|---|---|---|
| 12 | 9 in | 9 ft 2 in | 28 | Riser over IRC max |
| 13 | 8.308 in | 10 ft 0 in | 26.62 | Riser over IRC max |
| 14 | 7.714 in | 10 ft 10 in | 25.43 | Code only |
| 15 (chosen) | 7.2 in | 11 ft 8 in | 24.4 | Code and comfort |
| 16 | 6.75 in | 12 ft 6 in | 23.5 | Code only |
| 17 | 6.353 in | 13 ft 4 in | 22.71 | Code only |
| 18 | 6 in | 14 ft 2 in | 22 | Code only |
How do you calculate stair risers and treads?
Divide the total floor-to-floor rise by your preferred riser height, round to a whole number, then divide the rise by that count for the actual riser. A 108 inch rise with 15 risers gives 7.2 inch steps, 14 treads and a 140 inch total run.
risers = round(total rise ÷ preferred riser) actual riser = total rise ÷ risers treads = risers − 1 run = treads × tread depth- total rise
- Finished floor to finished floor, vertically
- actual riser
- The real step height once the count is fixed
- treads
- Always one fewer than risers — the top floor is the last step
- 2R + T
- The comfort rule; 24 to 25 inches matches a walking stride
A stair is a division problem with a hard constraint: the riser count must be a whole number, and every riser in the flight must be within three-eighths of an inch of every other, so the total rise has to divide exactly. That means your preferred riser height is only ever a starting point. A 108 inch rise at a preferred 7.5 inches gives 14.4 steps, which does not exist — so you pick 14 or 15 risers and accept the height that results, 7.714 or 7.2 inches. Treads are always one fewer than risers, because the upper floor itself forms the last stepping surface; forgetting this is the most common error and leaves a flight one tread long. Two limits then decide which count is right. The IRC caps the riser at 7.75 inches and requires at least a 10 inch tread. Beyond code, the classical rule 2R + T between 24 and 25 inches matches the geometry of a natural stride — at 14 risers this stair gives 25.4, slightly outside it, while 15 risers gives 24.4, comfortably inside. A stair can pass inspection and still be unpleasant to climb.
- Measure the total rise from finished floor to finished floor.
- Divide by your preferred riser height and round to a whole number.
- Divide the total rise by that count for the actual riser height.
- Check the riser is at or below the 7.75 inch IRC maximum.
- Count treads as one fewer than risers, then multiply by tread depth.
- Verify 2R plus T falls between 24 and 25 inches.
Worked example: A 108 inch total rise with a preferred 7.5 inch riser and 10 inch treads
A 108 inch total rise with a preferred 7.5 inch riser and 10 inch treads
Dividing the 108 inch rise by the preferred 7.5 inches gives 14.4 risers, which is not a whole number, so the count must be 14 or 15. Fourteen risers would make each step 108 ÷ 14 = 7.714 inches — under the 7.75 inch IRC maximum, so legal — but 2R + T comes to 2(7.714) + 10 = 25.43 inches, outside the 24 to 25 inch comfort band, and R × T is 77.1 against a target of 71 to 75. Fifteen risers gives 108 ÷ 15 = 7.2 inches exactly, with 2R + T = 24.4 and R × T = 72, both comfortably inside. So 15 is the right answer even though 14 passes code. That means 14 treads, one fewer than the risers because the upper floor is itself the last stepping surface, and a total run of 14 × 10 = 140 inches, or 11 ft 8 in of floor space. The stringer spans the hypotenuse of a 108 by 140 inch triangle, so it needs to be at least 176.8 inches — 14 ft 8.8 in — of stock.
What each stair input means
- Total rise
- Finished floor to finished floor, vertically. This is the one measurement that must be exact — every riser divides into it, and getting it wrong by an inch throws every step off.
- Preferred riser height
- What you would like each step to be. The riser count is derived from this and then the actual height is recalculated, because the total rise must divide exactly.
- Tread depth
- The horizontal walking surface, excluding any nosing overhang. IRC requires at least 10 inches.
- Nosing overhang
- How far the tread projects past the riser below. Required between 0.75 and 1.25 inches when the tread is less than 11 inches.
What this stair does not account for
- Treads are always one fewer than risers, because the upper floor forms the last stepping surface. Counting them equal leaves a flight one tread too long.
- Measure the total rise between finished floor surfaces. Measuring to a subfloor and then adding flooring on one level only will throw every riser out.
- All risers in a flight must be within 3/8 inch of each other under IRC. This is why the total rise must divide exactly and the preferred height cannot simply be used.
- Passing code is not the same as being comfortable. A stair can meet the 7.75 inch riser limit and still fail the 2R+T rule, which makes it tiring or cramped to climb.
- Headroom of 6 ft 8 in measured vertically from the tread nosing line is required, and it is the constraint that most often forces a stair redesign in a retrofit.
- Tread depth excludes the nosing overhang. A 10 inch tread with a 1 inch nosing gives an 11 inch total footprint but still counts as 10 inches for code.
- Local amendments vary and commercial stairs are governed by the IBC with different limits. Confirm with your building department before cutting stringers.
Frequently asked questions
What is the maximum stair riser height?
7.75 inches for residential stairs under the IRC, with all risers in a flight within 3/8 inch of each other. Commercial stairs under the IBC have a lower maximum, typically 7 inches.
What is the minimum tread depth?
10 inches under the IRC, measured excluding the nosing overhang. A nosing of 0.75 to 1.25 inches is required when the tread is less than 11 inches deep.
How many treads for 15 risers?
Fourteen. Treads are always one fewer than risers, because the upper floor itself is the final stepping surface. This is the single most common mistake in laying out a flight.
What is the 2R plus T rule?
Twice the riser height plus the tread depth should fall between 24 and 25 inches, which matches a natural walking stride. A stair outside that band feels either cramped or stretched even if it passes code.
How much space does a staircase need?
The total run is the tread count times the tread depth. A 108 inch rise with 15 risers and 10 inch treads needs 140 inches — 11 ft 8 in — of floor length, plus landings.
How long should stair stringers be?
At least the hypotenuse of the rise and run triangle. For a 108 inch rise and 140 inch run that is 176.8 inches, and stock should be longer to allow for cutting and attachment.
Sources and review
- 2021 International Residential Code, Section R311.7 — Stairways — International Code Council
Last reviewed .