Every stair starts with one measurement — the total rise from one finished floor to the next — and a set of rules about how tall and deep each step may be. This calculator divides the rise into equal risers that stay under your maximum, then works out the treads, total run, stringer length and angle, and checks the result against typical International Residential Code (IRC) limits and long-standing comfort rules.
How to use the stair calculator
- Measure the total rise from the finished floor at the bottom to the finished floor at the top. If flooring is not installed yet, add its thickness.
- Enter your maximum riser height. The IRC limit is 7¾ inches; many builders aim for 7 to 7½.
- Enter the tread depth you want — at least 10 inches for code-compliant residential stairs.
- Optionally add the stair width and nosing projection so they are included in the checks.
- Read the step count, riser height, run and stringer length, then review the checks and the alternative riser counts.
Stair formulas
Dividing the rise evenly matters: the IRC allows riser heights within one flight to differ by no more than 3/8 inch, because people subconsciously expect every step to be the same.
Worked example
Total rise 105 in, maximum riser 7¾ in, tread 10 in:
105 ÷ 7.75 = 13.55 → 14 risers of 105 ÷ 14 = 7½ in
13 treads × 10 in = 130 in, a total run of 10′ 10″
Stringer = √(105² + 130²) = 167.1 in ≈ 13′ 11 1/8″; angle = arctan(7.5 ÷ 10) ≈ 36.9°
Comfort check: 2 × 7.5 + 10 = 25 in — right at the top of the 24–25 in range.
Choosing 15 risers instead lowers each step to exactly 7 inches but adds a tread, stretching the run to 11′ 8″. The comparison table under the result shows these trade-offs for any rise.
Typical IRC stair requirements
| Element | IRC guideline (R311.7) |
|---|---|
| Riser height | 7¾ in maximum |
| Tread depth | 10 in minimum, nosing to nosing |
| Variation between risers or treads | 3/8 in maximum within a flight |
| Nosing | ¾ – 1¼ in projection where treads are under 11 in |
| Stair width | 36 in minimum clear width |
| Headroom | 6 ft 8 in minimum |
| Handrail | 34 – 38 in high on flights with four or more risers |
These figures reflect recent IRC editions and are offered as guidance. States and cities adopt and amend the code on their own schedules, so confirm requirements with your local building department before you cut.
Comfort rules of thumb
Stair builders have long balanced riser and tread with simple formulas. The most widely quoted, attributed to the 17th-century French architect François Blondel, says that twice the riser plus the tread should equal 24 to 25 inches, matching an average adult stride. Two other checks are riser + tread ≈ 17 to 18 inches and riser × tread ≈ 70 to 75. A stair that satisfies these feels natural to climb even when it is near the code maximums.
Laying out stringers
Stringers are usually cut from 2×12 lumber with a framing square and stair gauges set to the riser height and tread depth. Because the bottom tread’s thickness is added on top of the first cut, the bottom of the stringer is typically trimmed by one tread thickness so every finished riser comes out equal. Leave enough solid wood below the cuts for strength, follow your code’s span limits, and for decks check how the stringer attaches to the rim joist. For converting tape-measure readings, the feet and inches calculator handles fractions, and the concrete calculator estimates poured concrete steps.
Frequently asked questions
How many steps do I need for a 108-inch rise?
Divide the rise by the maximum riser height and round up. With the IRC's 7¾-inch limit, 108 ÷ 7.75 = 13.9, so you need 14 risers of 108 ÷ 14 ≈ 7 11/16 inches each, and 13 treads.
What is the maximum riser height allowed by code?
The International Residential Code, used for most US homes, allows risers up to 7¾ inches with treads at least 10 inches deep. Commercial buildings under the International Building Code are stricter: risers of 4 to 7 inches and treads of at least 11 inches.
Why is there one fewer tread than risers?
The top riser brings you up to the upper floor, which serves as the final step, so it does not need a separate tread. A staircase with 14 risers therefore has 13 treads.
How long should stair stringers be?
The stringer runs along the hypotenuse of the triangle formed by the total rise and total run, so its length is √(rise² + run²). Buy lumber at least a foot longer to allow for the plumb and level cuts at each end.
What is a comfortable stair angle?
Most residential stairs fall between about 30 and 37 degrees. A 7½-inch riser with a 10-inch tread gives about 36.9°, close to the steep end of the range; a 7-inch riser with an 11-inch tread gives about 32.5° and feels noticeably easier.