Stair Calculator — Risers, Treads and Stringer Length
Give it one measurement — floor to floor — and it works out the risers, the exact riser height, the treads, the total run and the stringer length, and tells you whether it passes code.
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How to work it out
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Measure the total rise accurately
Finished floor below to finished floor above — including the flooring that is going down later. Everything else follows from this number, so measure it twice. If the new floor is not laid yet, add its thickness now.
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Divide into equal risers
Divide the total rise by about 7 in. and round to a whole number. Then divide the total rise back by that number to get the exact riser height. Every riser must be identical — code allows only 3/8 in. variation across the whole flight, because the foot learns the first step and expects the rest to match.
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There is always one fewer tread than risers
The top floor is the last tread, so a flight with 15 risers has 14 treads. Getting this wrong is the classic stair-building mistake.
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Check the total run fits
Treads × tread depth gives the horizontal distance the stair needs. Measure that space before you cut anything — a comfortable stair that does not fit the room is no use, and the fix is usually a landing rather than steeper steps.
Risers by floor-to-floor height
Aiming for 7 in. risers, with 10.5 in. treads.
| Total rise | Risers | Each riser | Treads | Total run |
|---|---|---|---|---|
| 7.5 ft (90 in.) | 13 | 6.92 in. | 12 | 10.5 ft |
| 8 ft (96 in.) | 14 | 6.86 in. | 13 | 11.4 ft |
| 8.5 ft (102 in.) | 15 | 6.80 in. | 14 | 12.2 ft |
| 9 ft (108 in.) | 15 | 7.20 in. | 14 | 12.2 ft |
| 9.5 ft (114 in.) | 16 | 7.12 in. | 15 | 13.1 ft |
| 10 ft (120 in.) | 17 | 7.06 in. | 16 | 14.0 ft |
| 10.5 ft (126 in.) | 18 | 7.00 in. | 17 | 14.9 ft |
| 11 ft (132 in.) | 19 | 6.95 in. | 18 | 15.8 ft |
Nine feet floor-to-floor is common in newer US homes; older houses are often closer to eight.
US stair code limits (IRC)
| Dimension | Limit | Note |
|---|---|---|
| Maximum riser height | 7.75 in. | Measured vertically, tread to tread. |
| Minimum tread depth | 10 in. | Nosing overhang does not count. |
| Minimum stair width | 36 in. | Measured above the handrail. |
| Minimum headroom | 6 ft 8 in. | Measured vertically from the nosing line. |
| Variation between risers | 3/8 in. | Across the entire flight, not step to step. |
Your local code may be stricter. Always confirm with your building department before cutting stringers.
The 2R + T comfort rule
Carpenters have used this for a century: twice the riser height plus the tread depth should land between 24 and 25 inches. A 7 in. riser with a 10.5 in. tread gives 24.5 — right in the middle, which is why that combination feels natural.
Stairs that fail this rule are legal but uncomfortable. Too low a number and you shuffle; too high and you feel you are climbing. The calculator checks it for you.
Why every riser must be identical
People climb stairs without looking. After two steps the foot has learned the rhythm and expects it to continue. A single riser even half an inch different is a genuine trip hazard, and it is the most common cause of stair falls in homes.
This is why you divide the total rise by a whole number rather than picking a round riser height and letting the last step take up the difference.
Cutting the stringer
The stringer length above is the diagonal — the raw board length before cuts. Buy longer: you need material beyond both ends for the plumb cut at the top and the seat cut at the bottom. Add at least a foot.
Remember to drop the bottom of the stringer by one tread thickness, or the first step ends up taller than all the others by exactly that amount. It is the second classic stair mistake.
Deck stairs and outdoor steps
The same arithmetic applies, with two differences. Measure the total rise from the deck surface to the finished ground, not to bare soil — the landing pad raises it. And outdoor stairs often use a gentler riser, around 6.5 in. with a 12 in. tread, which reads as more relaxed in an open space.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
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risers = total rise ÷ target riser height, roundedThen increased until the riser height falls under the code maximum. -
riser height = total rise ÷ risersDividing back gives identical risers — code allows only 3/8 in. variation across a flight. -
treads = risers − 1The upper floor is the final tread. -
total run = treads × tread depthThe horizontal space the stair needs. -
stringer = √(run² + rise²)Pythagoras on the stair triangle. Buy a foot longer for the end cuts. -
check: 24 ≤ (2 × riser) + tread ≤ 25The old carpenter's comfort rule. 7 in. riser with a 10.5 in. tread gives 24.5.
Common questions
What is the standard stair riser height?
About 7 inches in US homes, with an absolute code maximum of 7.75 in. Commercial stairs tend to be gentler at 6.5 to 7 in. The exact figure is always your total rise divided by a whole number of risers, never a round number chosen in advance.
How many stairs for a 9 foot ceiling?
A 9 ft floor-to-floor height is 108 in. Divided by 7 gives 15.4, so use 15 risers at 7.2 in. each, with 14 treads. At 10.5 in. per tread the stair needs about 12 ft 3 in. of floor space.
Why is there one fewer tread than risers?
Because the upper floor itself is the final tread. A flight with 15 risers steps up 15 times but only has 14 boards to walk on, the last step landing on the floor above.
What is the minimum tread depth?
Ten inches under the IRC, measured from the face of one riser to the face of the next — the nosing overhang does not count towards it. Most builders use 10.5 or 11 in. because 10 feels tight underfoot.
How long a board do I need for the stringer?
The diagonal shown above, plus at least a foot for the top and bottom cuts. A 2×12 is standard; after the notches are cut you must leave at least 5 in. of solid wood along the back edge or the stringer is too weak.
Can all the risers be slightly different?
No. Code allows a maximum of 3/8 in. variation across the entire flight, and even that is generous. Uneven risers are the leading cause of stair falls because the climber's foot is expecting the previous height.
Do I measure to the subfloor or the finished floor?
Finished floor, at both ends. If the flooring is not down yet, add its thickness — forgetting a 3/4 in. hardwood at the top makes the final riser 3/4 in. short of all the others.
What about the handrail?
Required on at least one side for four or more risers, set between 34 and 38 in. above the nosing line, and it must be graspable — a 1.25 to 2 in. round rail rather than a flat board. It is the part inspectors check most often.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- International Residential Code R311.7 — maximum 7.75 in. riser, minimum 10 in. tread, 36 in. width, 3/8 in. variation
- IRC R311.7.8 — handrail height between 34 and 38 in. and graspability
- Architectural Graphic Standards — the 2R + T comfort relationship
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 11, 2026.