Stair calculator
Work out a straight stair from the floor-to-floor height: how many risers, how tall each one is, the treads, the total run and the stringer length, with the building code checks.
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Your stair
How the stair is worked out
The floor-to-floor height is divided into the fewest equal risers the code allows. The residential code (IRC) allows risers up to 7 3/4", so a 9' (108") floor-to-floor needs 14 risers of about 7 11/16". The commercial code (IBC) allows 7", which takes 16.
A straight stair has one tread fewer than it has risers, because the top riser lands on the upper floor. If you leave the tread depth blank, the calculator picks a comfortable one: two risers plus one tread close to 25", never under the code minimum (10" IRC, 11" IBC). The total run is the treads times their depth.
The stringer length is measured along the slope from the bottom of the first riser to the top of the last, so buy the next longer board. A flight can only rise so far before it needs a landing (151" IRC, 144" IBC), and the IRC wants stairs at least 36" wide.
These are the same code presets and sizing vPlan AR's editor uses when you place a stair between two levels: it sizes the stair to the height between them and shows these checks on the stair.
Tips
- Measure floor to floor with the finished flooring in place, or add its thickness to the subfloor measurement: an uneven first or last riser is a common trip hazard and a code failure.
- Risers within a flight should match: the IRC allows a 3/8" difference between the tallest and shortest.
- Check headroom too: the IRC wants 80" (6'8") measured up from the line of the nosings.
- Local codes can differ from the model codes: check with your building department before you cut stringers.
Do this from a scan instead
Scan each floor or draw them, and when you place a stair between two levels vPlan AR sizes it to the height between them, checks risers, treads, width, landings and headroom against the IRC or IBC, cuts the opening in the floor above, and draws the stair on both levels of the plan set.
Frequently asked questions
How many steps do I need for a 9 foot ceiling?
Use the floor-to-floor height, not the ceiling: a 9' ceiling plus a typical floor structure is often around 10'. For exactly 9' (108") floor to floor, the IRC needs 14 risers of about 7 11/16" and 13 treads; with 10" treads the run is 10' 10".
What is the maximum riser height?
7 3/4" under the IRC for homes, and 7" under the IBC for commercial buildings. Spiral stairs have their own, higher limit.
What is the minimum tread depth?
10" under the IRC and 11" under the IBC, measured nosing to nosing. Deeper treads are more comfortable but make the stair longer.
How long should stair stringers be?
At least the length along the slope from the bottom of the first riser to the top of the last, which the calculator shows. Buy the next longer standard board to allow for the cuts at each end.
Guides
- How to Create an As-Built Floor Plan for a Building Permit ApplicationMost permit rejections trace back to the existing-conditions plan, not the design. Here's how to scan, verify, and export an as-built floor plan that a plan reviewer can actually approve.
- Floor Plan Symbols and Abbreviations: The Complete Reference GuideFrom wall weights to MLS shorthand, here's what every line, arc, and abbreviation on a floor plan actually means — and why a scanned plan sidesteps most of it. Bookmark it for the next plan that lands on your desk.