Free Stair Calculator
Plan risers, treads, total run, stringer length, stair angle, and opening fit for a straight interior, basement, or garage-entry stair layout.
Last updated: April 4, 2026
Enter Stair Details
36 in is the usual residential minimum.
Measure from the finished lower floor to the finished upper floor.
10 in is the IRC minimum. 10.5-11 in usually feels better indoors.
Use this if you already know the opening length or the run available on site.
Adds extra length to the stringer board order only. It does not change riser height, tread count, or total run.
Stair Layout Summary
Long interior stairs usually work best with a consistent 11-inch tread and full-size 2×12 stringer stock.
Project Examples
Basement Stair
108 in rise · 10.5 in tread · 147 in opening
A full-height basement stair often works on paper before it works in the opening, so run and fit matter just as much as the rise math.
Main Floor Stair
120 in rise · 11 in tread · 176 in opening
A full interior run usually needs a longer opening, a shallower angle, and a layout that stays comfortable from top to bottom.
Garage Entry Stair
36 in rise · 12 in tread · 60 in opening
Short utility stairs usually fit more easily, but a wider tread still makes them safer and easier to use.
How to Use This Calculator
- Measure total rise from the finished lower floor to the finished upper floor.
- Pick a tread depth based on comfort and the run you can actually fit.
- Check opening fit if you already know the opening length or available run.
- Use the layout summary to lock the risers, treads, stringer length, and board order before cutting.
How to Turn Floor Height Into a Stair Layout
Measure the full floor-to-floor height first. Every other layout decision starts from that vertical number.
Divide total rise by a comfortable target riser height, then round up to get a workable riser count.
Once risers are fixed, tread count becomes risers minus one. Multiply by tread depth to see how much floor space the stair really needs.
Compare total run against the opening or available run. Then use the diagonal stringer length to round into practical 2×12 stock.
Calculation Steps
Formula basis: IRC R311.7 rise/run limits and AWC stair stringer guidance.
Stair Planning Decisions
10 in vs 11 in tread
Ten inches is the code floor. Eleven inches usually feels better indoors if the opening can absorb the extra run.
Opening length is often the real constraint
A lot of layouts fit vertically but fail on run. Check the opening length before you lock the tread depth.
Route exterior and concrete work elsewhere
Use the deck stair page for exterior lumber and railing takeoff. Use concrete steps when the project is poured, reinforced, and priced by yardage.
What This Page Covers vs Next
This page covers
- Straight stair rise/run layout
- Riser and tread count
- Stringer length and stair angle
- Opening-fit planning warning
Estimate separately next
- Deck railing and exterior tread choices on the deck stair page
- Concrete volume, rebar, and pour cost on the concrete steps page
- Permit-specific structural review with your local building department