Use this Stair Calculator to turn your total rise into a practical straight-stair layout. Calculate equal risers, tread count, total run, stair angle and approximate stringer length in US or metric units, then review the proportional stair diagram before planning your build.
Enter stair dimensions
Use the measurements you actually have. Fractional inches such as 7 3/4 are accepted.
Your stair layout
Results update after you calculate and include a visual geometry check.
Transparent calculation
How the stair calculation works
The calculator uses your maximum riser height to choose a whole number of equal risers, then calculates the horizontal stair run and straight-line stringer length.
ceil(total rise ÷ max riser)total rise ÷ risersrisers − 1treads × tread depthatan(total rise ÷ total run)√(rise² + run²)Important: this is a planning and estimating tool, not structural approval or a substitute for adopted local building codes, manufacturer instructions, or professional review where required.
Step-by-step
How to use the Stair Calculator
Start with the vertical height your stair must cover, measured from the lower finished floor to the upper finished floor. Then choose the riser-height limit and tread depth you want the calculator to use. The calculator divides the rise into equal risers and reports the resulting geometry.
- Measure the total rise. Measure finished floor to finished floor, not rough framing to rough framing.
- Set the maximum riser height. The calculator rounds the number of risers up so the calculated riser does not exceed the limit you entered.
- Set the tread depth. This is the horizontal run used for each tread in the geometry calculation.
- Add available run if you know it. This optional field tells you whether the calculated flight fits the horizontal space you have.
- Review the diagram and results. Check the number of risers, exact riser height, treads, total run, angle and approximate sloped stringer length.
For a measurement walkthrough, see How to Measure Stair Rise and Run.
Read your result
What each stair result means
Number of risers
The number of equal vertical steps needed to cover the total rise without exceeding your selected maximum riser height.
Exact riser height
Total rise divided by the number of risers. BuildCalcForge keeps full precision internally and can display US results as common fractions or decimal inches.
Number of treads
For a simple straight flight between two floor levels, this calculator uses one fewer tread than risers because the upper floor acts as the final walking surface.
Total run
The horizontal distance created by the calculated number of treads multiplied by the tread depth you entered.
Stair angle
The angle of the overall rise/run envelope, calculated from total rise and total horizontal run. It describes flight geometry; it is not a separate code approval.
Stringer length
The straight sloped distance from the bottom to the top of the calculated flight. Actual stock length, cuts, bearing and structural sizing require additional planning.
Worked example
Example: stairs for a 9 ft total rise
Suppose the finished-floor-to-finished-floor rise is 9 ft (108 in). Using a maximum riser setting of 7¾ in and a tread depth of 10 in, the calculator first divides 108 by 7.75 and rounds up to a whole number of risers.
ceil(108 ÷ 7.75) = 14108 ÷ 14 ≈ 7.714 in14 − 1 = 1313 × 10 = 130 inThe resulting flight is approximately 39.7°, with a straight-line sloped length of about 14 ft 1 in. Use the live calculator above for your actual measurements rather than copying this example.
Measure carefully
Stair measurement tips
- Measure total rise between the final finished surfaces whenever those thicknesses are known.
- Take more than one measurement if floors or landings may be out of level.
- Keep all risers in a flight consistent; small measurement errors can become noticeable across many steps.
- Check that the calculated total run fits before finalizing tread depth or layout.
- Account separately for landings, nosings, headroom, handrails, guards and structural support.
Know the limits
Assumptions and limitations
The current Stair Calculator models a straight stair flight with equal risers and equal tread runs. It assumes the upper floor is the final walking surface, so tread count is calculated as risers minus one. It does not design winders, curved stairs, landings, headroom, handrails, guards, framing connections or structural stringer capacity.
The displayed stringer length is the geometric sloped distance of the flight. A real stringer may require additional stock length for cuts, bearing, top/bottom details and site conditions. Use the result for planning, then verify construction details against the requirements that apply to your project.
Planning reference
About the default stair dimensions
The default US inputs use a 7¾ in maximum riser and a 10 in tread depth. Those values match the 2021 International Residential Code model-code provisions in Sections R311.7.5.1 and R311.7.5.2 for the stair conditions covered by those sections. The IRC is a model code: states and local jurisdictions can adopt different editions or amendments, and special stair types can have different requirements.
That is why BuildCalcForge treats these values as a planning reference rather than a pass/fail code checker. Always verify the code and amendments adopted by the authority having jurisdiction for the actual project.
International Code Council — 2021 IRC, Chapter 3 ↗Common questions
Stair Calculator FAQ
How does the calculator decide how many risers I need?
It divides the total rise by the maximum riser height you entered and rounds up to the next whole riser. The total rise is then divided equally across that number of risers.
Why are there usually fewer treads than risers?
For a simple straight flight between floor levels, the upper floor serves as the final walking surface. That is why this calculator uses one fewer tread than risers.
Can I enter fractional inches?
Yes. US fields accept common entries such as 7 3/4, 7 1/2 and decimal values such as 7.75. You can also change result display rounding to 1/16, 1/8, 1/4 or decimal inches.
Does the stair angle tell me whether the stair is code compliant?
No. The angle is a geometry result based on the overall total rise and total horizontal run. Code compliance depends on the adopted rules for the project and can include many requirements that are not represented by one angle value.
Can I use this calculator for deck stairs?
You can use it for basic straight-flight geometry, but the dedicated Deck Stair Calculator will be the better tool for deck-specific planning and related details.
Is the stringer length the board length I should buy?
Not necessarily. The calculator reports the straight sloped length of the flight. Actual stock length can require extra material for cuts, bearing and connection details.
Sources & methodology
How this page is checked
The formulas are standard stair geometry. BuildCalcForge separates the mathematical result from code and construction decisions, shows the assumptions used, and cites the model-code reference behind the default riser/tread values.