Stair Calculator
Enter your total rise and this stair calculator works out the number of steps, exact riser height, total run and stringer length — and checks the result against standard residential code limits.
Vertical height, finished floor to finished floor/ground
7″ is the comfort standard; 7¾″ is the code max
Code minimum 10″; 10½–11″ is comfortable
—
You might also need:Board Foot Calculator·Rafter Calculator
How stair math works
actual riser = total rise ÷ steps
total run = (steps − 1) × tread depth (top landing counts as the last "tread")
stringer = √(total rise² + total run²)
The critical rule: every riser must end up identical, so you divide the total rise evenly rather than stacking fixed 7-inch steps. Note the run uses one fewer tread than there are risers when the stair lands flush at the top — deck stairs that finish at grade level with a final step work the same way.
Residential code limits (IRC — always confirm locally)
| Element | Requirement |
|---|---|
| Max riser height | 7¾″ |
| Min tread depth | 10″ |
| Max riser variation | ⅜″ within a flight |
| Min stair width | 36″ |
| Min headroom | 6′8″ |
| Handrail height | 34–38″ above nosing |
Building tips
- Account for finishes first: measure total rise from finished floor to finished floor. A 1-inch tread on an unfinished frame changes every riser.
- The first cut is a template: lay out one stringer with a framing square and stair gauges, cut it, test-fit it, then trace the rest from it.
- Drop the bottom: trim the tread thickness off the bottom of the stringer so the first step isn't taller than the rest once treads go on.
- Ordering lumber? Total your board feet with the board foot calculator.
Frequently asked questions
What is the standard rise and run for stairs?+
Comfortable stairs have a rise of about 7 inches and a run (tread depth) of 10–11 inches. The IRC residential code caps riser height at 7¾ inches and requires treads at least 10 inches deep. A classic comfort check: rise + run should total 17–18 inches.
How do I calculate the number of steps?+
Divide the total rise (floor-to-floor height) by your target riser height, round to the nearest whole number, then divide again to get the exact riser height. Example: 45 inches ÷ 7 = 6.4, round to 6 steps → 45 ÷ 6 = 7.5 inch risers.
How long a board do I need for stair stringers?+
Stringer length is the diagonal: √(total rise² + total run²), plus about a foot of extra for the top connection and cutting. A stair with 45 inches of rise and 60 inches of run needs 75 inches diagonally — buy a 2×12 at least 8 feet long.
Can all the risers be different heights?+
No — building codes allow at most ⅜ inch variation between the tallest and shortest riser in a flight. Uneven risers are the #1 cause of stair trips. This is why you divide the total rise evenly rather than building each step at a fixed height.
What size lumber do I use for stringers?+
Use 2×12s — after cutting the notches, at least 5 inches of solid wood ("throat") must remain. Space stringers 16 inches apart for 2× lumber treads (max three stringers for a 36-inch-wide stair) and closer for composite treads.