Stair Calculator
Turn a floor-to-floor height into a code-compliant stair layout — riser height, tread depth, total run, and stringer length — checked against IRC limits.
How the stair calculator works
This stair calculator takes a floor-to-floor height and lays out a code-compliant flight of stairs. You enter the total rise — the finished height from the lower floor to the upper floor — a preferred riser height, and your tread depth. It returns the number of risers, the exact riser height, the number of treads, the total run, and the stringer length.
It also works as a rise and run calculator and a stair stringer calculator in one. Beyond the raw numbers, it checks the layout against IRC R311.7 and flags whether your riser and tread fall inside code limits, so you know before you cut a stringer whether the flight will pass inspection.
How to calculate stair rise and run by hand
Begin with the number of risers. Divide the total rise by your preferred riser height and round to the nearest whole number. If the floor-to-floor height is 109 in and you prefer a 7.5 in riser, that is 109 / 7.5, or about 14.5, which rounds to 14 risers.
Divide the total rise back by that riser count to get the exact riser height — 109 / 14, or about 7.79 in. Doing it this way guarantees every step is identical, which matters because uneven risers are both a trip hazard and a code violation. The number of treads is always one less than the number of risers, because the upper floor itself serves as the final tread. Total run is the tread count multiplied by the tread depth.
The stringer length is the hypotenuse of the triangle formed by the total rise and the total run, so it is the square root of rise squared plus run squared. Check the result against IRC R311.7: risers may not exceed 7 3/4 in and treads must be at least 10 in. Also apply the comfort rule — riser plus tread should land around 17 to 18 in — since a flight can pass code and still feel steep.
Tips for an accurate stair layout
Small errors in a stair layout compound across every step, so a careful start saves rework later.
- Measure total rise to finished floor height on both levels — account for the upper subfloor plus its finish and the lower finished surface, or the bottom step ends up the wrong height.
- Treat the IRC limits as a hard pass/fail: max 7 3/4 in riser, min 10 in tread, and confirm local amendments since some jurisdictions are stricter.
- Use the comfort rule alongside code — aim for riser plus tread near 17 to 18 in so the stride feels natural.
- Buy stringer stock a foot or two longer than the calculated length to leave material for the plumb and seat cuts at top and bottom.
- Confirm you have the headroom and floor space for the total run before committing — a code stair still needs somewhere to land.
From a stair layout to a priced estimate
A clean layout tells you the flight will work; it does not tell the client what it costs. GreenlitBid takes the framing and finish quantities behind a stair like this, applies your labor and material rates, adds your markup, and produces an itemized estimate ready to send. The calculator confirms the build is sound — GreenlitBid turns it into a number the homeowner can sign.
Questions
How do I calculate the number of stairs from a floor height?+
Divide the total rise — the finished floor-to-floor height — by your preferred riser height and round to the nearest whole number. That gives the number of risers. Dividing the total rise back by that count gives the exact riser height each step must be so they all come out equal.
What is the maximum riser height allowed by code?+
The International Residential Code (IRC R311.7) caps riser height at 7 3/4 in and requires a minimum tread depth of 10 in. This calculator checks your layout against both limits and flags any value that fails. Always confirm your local amendments, since some jurisdictions are stricter.
Why is there one fewer tread than risers?+
The top of the stair lands on the finished upper floor, so that level acts as the final tread. A flight with eight risers has seven treads. Total run is the number of treads multiplied by the tread depth.
What is the riser-plus-tread comfort rule?+
A comfortable stair keeps riser height plus tread depth between 17 and 18 in, and twice the riser plus the tread between 24 and 25 in. These rules of thumb keep the stride natural — a stair that passes code can still feel steep or cramped if it ignores them.
How do I find the stringer length?+
The stringer is the hypotenuse of the triangle formed by the total rise and the total run, so its length is the square root of rise squared plus run squared. Buy stock a foot or two longer to allow for the plumb and seat cuts at each end.
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