Stair calculator: how many steps do I need, and how tall?
A free stair calculator: type the floor-to-floor height and get every riser, tread and mark for one straight stair. No sign-up, and nothing leaves your browser.
Your stair
Type the total rise, floor to floor, and your stair appears here.
One fewer or one more?
Nothing you type leaves your browser, and nothing is saved.
Marks from the lower floor
| Step | Height above the lower floor | Level distance from the first tread's edge |
|---|
Your stair against the IRC figures
industryoversight.com/tools/stair-calculator/
How the stair calculator works
You know one number: the height from the finished floor at the bottom to the finished floor at the top. The rest of the stair follows from it.
Divide that height by the tallest step you'll allow and round up. That's the fewest risers that fit. Take 8 ft 10 3/4 in, which is 106.75 in. Over a 7 3/4 in maximum that's 13.77, so 14 risers, and 106.75 divided by 14 is exactly 7 5/8 in. A straight stair has one tread fewer than it has risers, because the upper floor is the last step: 14 risers, 13 treads.
Multiply the treads by the run per step, measured from the front edge of one tread to the front edge of the next, and you have the floor the stair takes: 13 at 10 in is 10 ft 10 in. The diagonal from the floor to the top edge is the square root of the rise squared plus the run squared, 14 ft 0 3/16 in here. That's not the board length to buy. A stringer needs extra for the cuts at both ends.
The part people get wrong is the marking. 108 in over 14 risers is 7.714 in, which a tape reads as 7 11/16. Step off 7 11/16 thirteen times and you reach 99 15/16 in, so the last riser comes out 8 1/16 in: taller than the IRC's 7 3/4 in and 3/8 in off its neighbors. The tape isn't wrong. Repeating its rounding is. We round each mark from the floor on its own, so the last one lands on 108 in and no two risers differ by more than 1/16 in.
Two more numbers keep the ends honest. Type your tread thickness and you get the bottom riser cut: under 1 1/2 in treads, 7 5/8 in risers need the stringer's first step cut to 6 1/8 in. Skip it and the first finished step grows by the tread's thickness while the top one shrinks by the same amount. And if the flooring isn't down yet at either end, add it under More details. Three quarters of an inch of hardwood still to go on the upper floor can turn 14 risers into 15.
Every code line comes from the IRC's published figures and carries its edition, its section number and the date we read it. The lines tell you where your stair sits against those figures. Whether it passes is your building department's call.
Who this is for
- Homeowners finishing a basement, replacing worn stairs or adding steps down from a deck or porch, who know the floor-to-floor height and want the step count before buying lumber.
- Anyone holding a carpenter's plan that says "14 risers at 7 5/8" who wants to check it against their own floors.
- Carpenters who want the marks, the bottom cut and the code figures in one place on a phone.
Homeowner questions about stair layout
How many stairs do I need for a 9 foot ceiling?
Count from the total rise, not the ceiling. The rise is the ceiling height plus the whole floor above it: joists, subfloor and finished flooring. Say that comes to 9 ft 11 in, or 119 in. Under the IRC's 7 3/4 in maximum you need 16 risers of 7 7/16 in, so 15 treads, and at 10 in per tread the stair runs 12 ft 6 in. Measure your own. Floors vary too much to guess.
How tall should stair risers be?
No taller than 7 3/4 in under the IRC, measured from the top of one tread to the top of the next (2021 R311.7.5.1, 2024 R318.7.5.1). The code sets no minimum for homes. It cares just as much that the risers match: in one flight, the tallest may beat the shortest by 3/8 in at most. That's why we round each mark from the floor instead of repeating one rounded step.
How many risers before a handrail is required?
Four. The IRC wants a handrail on at least one side of any flight with four or more risers (2021 R311.7.8, 2024 R318.7.8). With three risers or fewer, that line doesn't apply. Guards along an open side are a separate rule, and we don't cover them.
How do I measure the total rise for stairs?
Measure from the top of the finished floor below to the top of the finished floor above, at the spot where the stair will land. For deck steps, run a straight board level off the deck edge and measure down to the ground where the bottom step will sit. Flooring not down yet? Measure what's there and add the flooring under More details. Leave carpet out: the IRC measures steps without carpet, rugs or runners.
How do you calculate stair stringer length?
Square the total rise, square the total run, add them and take the square root. For 106 3/4 in of rise and 130 in of run, that's 168.21 in, or 14 ft 0 3/16 in. That's the diagonal from the floor at the foot to the top edge, not the board to buy. A stringer needs extra length for its end cuts and for how it hangs, so buy longer and lay it out from the marks.
What is the minimum tread depth for stairs?
10 in under the IRC, measured level from the front edge of one tread to the front edge of the next (2021 R311.7.5.2, 2024 R318.7.5.2). That's the run per step, not the board. Under 11 in, the code also asks for a nosing of 3/4 in to 1 1/4 in, so a 10 in run means a tread board 10 3/4 in to 11 1/4 in deep.
How many steps before you need a landing?
The IRC doesn't count steps. It caps one flight at 12 ft 7 in of rise between floors or landings (2021 R311.7.3, 2024 R318.7.3). At the full 7 3/4 in per riser that's 19 risers, but the cap is on height: a 13 ft 4 in rise needs a landing however short you make the steps. The code also wants a floor or landing at the top and bottom, 36 in deep on a straight stair, with a few exceptions at doors.
How do I lay out stair stringers from these numbers?
Lay the stringer out from the marks table: each tread cut sits one tread thickness below its mark, measured from the floor line, so rounding never piles up. The bottom riser cut is the first of those. With 7 5/8 in risers and 1 1/2 in treads it's 6 1/8 in, and once the treads go on, every step rises 7 5/8 in, the first one included.
Stair riser height code: the IRC figures, edition by edition
The IRC is a model code. States and towns adopt it on their own schedule, sometimes with changes, which is why every figure here carries its edition. For ordinary straight stairs, the two editions share these general numbers. Stairs to nonhabitable attics or crawl spaces sit outside these rules, and qualifying basement bulkhead stairs are exempt. IRC 2024 also has exceptions for open risers and some existing stairs, explained after the figures:
| What | IRC figure | 2021 | 2024 |
|---|---|---|---|
| Riser height | at most 7 3/4 in (196 mm) | R311.7.5.1 | R318.7.5.1 |
| Run per step (the code's "tread depth") | at least 10 in (254 mm), nosing to nosing | R311.7.5.2 | R318.7.5.2 |
| Difference between risers, or between treads, in one flight | at most 3/8 in (9.5 mm) | R311.7.5.1, R311.7.5.2 | R318.7.5.1, R318.7.5.2 |
| Nosing | 3/4 in to 1 1/4 in (19 to 32 mm); none asked for at a run of 11 in (279 mm) or more | R311.7.5.3 | R318.7.5.3 |
| Rise of one flight | at most 12 ft 7 in (3835 mm) between floors or landings | R311.7.3 | R318.7.3 |
| Handrail | on at least one side, from 4 risers | R311.7.8 | R318.7.8 |
| Headroom | at least 6 ft 8 in (2032 mm) | R311.7.2 | R318.7.2 |
| Width | at least 36 in (914 mm) above handrail height | R311.7.1 | R318.7.1 |
| Landing depth, straight stair | at least 36 in (914 mm) | R311.7.6 | R318.7.6 |
The 2024 edition changed a few things around those numbers. The section moved from R311.7 to R318.7, open risers may now carry a nosing past 1 1/4 in, and a new R318.7.9 says alterations to an existing stair don't have to meet these figures where the space and construction don't allow a lower pitch. That last one matters if you're replacing a steep old basement stair in a town on the 2024 edition. Garden steps that don't serve a building, porch or deck sit outside the section in both editions.
Sources and last checked
- Stair figures: International Residential Code 2021, section R311.7, and 2024, section R318.7, Chapter 3, read on ICC Digital Codes on October 4, 2026. We cite figures and section numbers; the code text belongs to the International Code Council.
- Inches to millimeters: 1 in = 25.4 mm exactly, NIST Handbook 44 (2026), Appendix C.
- Data: stair-code-limits, version of October 7, 2026.
- We haven't read the 2018 edition. If your town still builds to it, ask your building department for its stair figures.
- Printings: we read the 2021 edition in its first printing and the 2024 edition in version 1.0. When we tried the live ICC Digital Codes pages again on October 8, 2026, they wouldn't open for us, so we can't confirm the newest printing or any errata. Archived copies of later printings of both editions on the Wayback Machine show the same figures.
More tools for your project
Want it built or checked? Find a carpenter near you, or a deck builder for outdoor steps.
Bookmark this calculator for the job. Nothing you type is saved, so copy or print your stair layout before you close the tab.