HowMuchIt / Stairs
How do I calculate stairs?
Start from one measurement, the total rise from floor to floor. Divide it into equal risers of about 7 inches, and the calculator gives the treads, the run, the stringer length and a check against the code.
Work out your stairs
| Rule | Value |
|---|---|
| Riser | at most 7 3/4 in; many builders aim for 7 to 7 1/2 |
| Tread | at least 10 in deep |
| Difference within a flight | at most 3/8 in |
| Headroom | at least 6 ft 8 in |
| Width | at least 36 in |
| Nosing, where required | 3/4 to 1 1/4 in |
| Handrail | 34 to 38 in high |
| Comfort | 2 risers + 1 tread = about 24 to 25 in |
Risers = total rise / about 7 in, rounded up; each riser = total rise / that number. Treads = risers minus one. Stringer = square root of (rise x rise + run x run).
Every step the same
Divide the total rise by a target of about 7 inches and round up: that is the number of risers. Then divide the total rise by that whole number, so every riser comes out the same height. A straight stair has one tread fewer than risers, because the upper floor is the last step. The code allows no more than 3/8 inch of difference between the tallest and shortest riser in a flight, which is why the division matters.
The code and the comfort rules
Under the International Residential Code, a riser may be at most 7 3/4 inches, a tread at least 10 inches deep, headroom at least 6 feet 8 inches, and a house stairway at least 36 inches wide; a handrail sits 34 to 38 inches high. Local codes can add to that, so check with your building department. Builders also use two rules of thumb: two risers plus one tread make about 24 to 25 inches, and one riser plus one tread about 17 to 18.
Questions people ask
How many steps for a 9 foot rise?
108 inches / 7 = 15.4, so 16 risers of 6.75 inches and 15 treads.
What is the maximum riser height?
7 3/4 inches under the IRC.
How long is the stringer?
The square root of rise squared plus run squared: about 15.4 feet for a 108 inch rise and 15 treads of 10 inches.
Good to know
- Under the International Residential Code (IRC), a stair riser may be at most 7 3/4 inches high.1,2,3,4,5
- Under the IRC, each tread must be at least 10 inches deep.1,3,4,6
- Within one flight, the tallest and shortest riser (and the deepest and shallowest tread) may differ by no more than 3/8 inch.1,3,7
- Headroom over a stair must be at least 6 feet 8 inches, measured up from the line of the tread nosings.1,7,8
- A house stairway must be at least 36 inches wide.1,7,9
- Where a nosing is required, it overhangs the riser below by at least 3/4 inch and no more than 1 1/4 inches.1,2,7
- A stair handrail is set 34 to 38 inches high.1,6,7
- To find how many risers you need, divide the total rise (floor to floor) by a target riser height of about 7 inches and round up to a whole number.2,4,9
- The exact height of each riser is then the total rise divided by that whole number of risers, so every riser is the same.2,9
- A straight stair has one fewer tread than it has risers, because the upper floor acts as the last step.4,6,9
- Stringer length comes from the Pythagorean theorem: square the total rise and the total run, add them, and take the square root.5,9
- Comfort rule of thumb: two risers plus one tread should add up to about 24 to 25 inches.9,10,11
- A second rule of thumb: one riser plus one tread should add up to about 17 to 18 inches.4,9,10
- For comfort, many builders aim for risers of about 7 to 7 1/2 inches, below the 7 3/4 inch code maximum.7,9
How we checked this page
Checked on October 27, 2026. We kept a fact only when two independent publishers agree, and each one is numbered to its sources below. Where sources give different numbers, we keep only what they share. We never write a price. This page was written with an AI assistant and checked against the sources below.
Sources
- 2018 Residential Stair Guide (reproduces IRC Section R311.7 Stairways), Town of Timnath, Colorado (building department)
- How to Measure Stair Stringers, The Home Depot
- Tread Depth and Risers (stair code), Viewrail
- Stair Stringers: Calculation and Layout (Tim Healey), JLC - Journal of Light Construction
- Laying Out the Deck Stair Stringers, Fine Homebuilding
- How to Build a Staircase, The Home Depot
- JLC Field Guide: Stairs, JLC - Journal of Light Construction
- How To Add Headroom to a Stairway, This Old House
- Stair Stringer Layout (The Pro Reference, Blue Book), Swanson Tool Co.
- 2 Rules for Building Comfortable Stairs Like a Pro (Mike Maines), Fine Homebuilding
- Rule-of-Thumb Formulas for Stairs (Revit 2014 help), Autodesk