Treads
14
Stair Calculator
Risers, tread depth, total run and stringer length — from one measurement, the floor-to-floor height.
Finished floor below to finished floor above. Not the ceiling height.
7 in is comfortable. The calculator adjusts to divide evenly.
Inches, nosing included. 10 in is the usual minimum.
Nearly always the first one.
15 risers at 7.20 in
14 treads, total run 11 ft 8 in.
Total run
140.0 in
Stringer length
176.8 in
Angle
37.6°
Within the usual residential limits.
Check your own local code before you cut anything. The limits used
here are the common US residential ones — riser no more than 7¾ in, tread no less
than 10 in, and no more than 3/8 in difference between the tallest and shortest riser
in a flight. Your city may be stricter, and commercial work certainly is.
How this is worked out
Everything comes from the total rise. You cannot pick a riser height and make it fit — you pick one you'd like, then let the arithmetic round it.
total rise ÷ riser height you want = number of risers, rounded to a whole numbertotal rise ÷ number of risers = actual riser heighttreads = risers − 1, when the upper landing is the top steptotal run = treads × tread depthstringer = √(total rise² + total run²)
Step two is the one people miss. Every riser in a flight must be the same height, so the number you actually build to is almost never the round number you started with.
Worked example — 9 ft floor to floor, aiming at 7 in risers
- Total rise:
9 × 12 = 108 in - Risers:
108 ÷ 7 = 15.43, round to 15 - Actual riser:
108 ÷ 15 = 7.2 in - Treads:
15 − 1 = 14 - Total run:
14 × 10 = 140 in, or 11 ft 8 in - Stringer:
√(108² + 140²) = √31,264 = 176.8 in, so a 16 ft board
The limits, and why they exist
| Measure | Usual residential limit |
|---|---|
| Riser height | 7¾ in maximum |
| Tread depth | 10 in minimum |
| Variation between risers | 3/8 in maximum across the flight |
| Comfortable angle | 30° to 37° |
That 3/8 in variation rule is the one that catches people. A flight where the bottom step is half an inch different from the rest is the single most common cause of falls on stairs, because your foot learns the rhythm after two steps and stops looking.
Questions people ask
Why is the bottom step always wrong?
Because the stringer sits on the lower floor but the treads add thickness on top. You have to cut the bottom of the stringer down by one tread thickness, or the first step ends up taller than all the others. It is the classic stair-building mistake and it fails inspection every time.
How do I measure total rise properly?
Finished floor to finished floor. If the lower floor is getting tile or the upper one is getting carpet, measure to where the surface will end up, not where it is now. Getting this wrong by half an inch throws every riser out.
What if I don't have room for the run?
You have three choices: a steeper stair with taller risers, up to the code limit; a landing with a turn, which buys you space but takes a chunk of floor; or a winder. You cannot simply shorten the treads below the minimum — that is what makes a stair genuinely dangerous to come down.
What length stringer board do I buy?
Take the stringer figure and go up to the next standard length, then add a foot. In the example above, 176.8 in is 14 ft 9 in, so a 16 ft board. You need the extra for the cuts at each end, and stringers are not something to splice.
Does this handle a landing?
Not directly, but you can do it in two passes: work out the rise to the landing, then the rise from the landing to the top floor, as two separate flights. Each flight must have consistent risers within itself, and in most codes the two flights have to match each other as well.