Stair Calculator — Rise, Run & Stringers
Calculate equal risers, treads, total run, stair angle, diagonal stringer length, and a planning lumber list.
Stair geometry starts with the finished floor-to-floor rise. Dividing that rise into equal risers determines the actual riser height; the number of treads is normally one fewer than the number of risers, and tread depth determines total run and the diagonal stringer length.
7-in actual riser · 10-in tread · 50-in total run · 5.44-ft diagonal · 40° stair angle.
- Continuous stringer blanks3 12-ft pieces
- Tread stock5 8-ft boards
This is a geometry and shopping-planning tool, not a structural stair or stringer design. A qualified professional or approved plan must determine lumber, notches, supports, connections, guards, handrails, landings, headroom, loads, and permits.
The 2021 IRC planning reference uses a maximum 7 3/4-in riser and minimum 10-in tread, but your state or local jurisdiction may use a different edition or amendments.
Step by step
- 1
Measure the finished vertical rise, then enter a target riser and tread depth for an early geometry check.
- 2
Use the approved stair plan—not this calculator—to enter stringer count, lumber, supports, connections, landings, guards, handrails, and other construction details.
- 3
Compare the equal riser height, tread depth, total run, headroom, landings, and complete assembly with the code edition and amendments adopted by the local jurisdiction before purchasing or cutting lumber.
Check the estimate before you buy
Calculate equal stair risers, tread count, total run, stair angle, diagonal stringer length, and planning tread lumber from finished floor-to-floor rise.
A 42-in finished rise divided into six equal risers produces 7-in risers. Five 10-in treads create a 50-in total run. The rise-and-run diagonal is about 65.3 in, or 5.44 ft, at an angle near 40 degrees.
How the calculation works
The tool rounds the riser count up so equal risers do not exceed the selected planning height, with 7 3/4 in used as an upper planning reference. Treads are one fewer than risers for the common floor-to-floor arrangement. The Pythagorean theorem gives the uncut diagonal—not a notch or span design.
Stair Calculator — Rise, Run & Stringers FAQ
How many steps do I need?
Divide the finished rise into equal risers that satisfy the locally adopted code. The calculator rounds the planning riser count up and shows both risers and treads.
Why are there fewer treads than risers?
In a common stair ending at an upper floor or landing, that surface acts as the final step, so the flight has one fewer constructed tread than risers. Other configurations need an approved plan.
Does the calculator design or size stair stringers?
No. It shows the rise-and-run diagonal and a user-entered shopping count. Notches, spans, lumber, supports, loads, connections, and construction require an approved design.
Are 7 3/4-in risers and 10-in treads legal everywhere?
No. They are 2021 IRC planning references. States and local jurisdictions adopt different editions and amendments, and the complete stair must satisfy many other requirements.
Reference: 2021 International Residential Code R311.7.5 planning references. Results are practical estimates; complete labels, package directions, product specifications, and qualified professional advice take priority.