Stair Calculator - Calculate Rise, Run, and Stringer Dimensions

What This Calculator Does and When to Use It

Building a staircase means dividing a fixed floor-to-floor height (the total rise) into a whole number of equal steps, then fitting those steps into whatever horizontal space (the total run) you have available. Get the riser height or tread depth wrong and the stairs feel awkward at best or become a fall hazard at worst — which is why residential building codes set specific limits on both. This calculator takes your total rise and available run and works out a code-conscious riser/tread split automatically, along with the stringer length you need to cut and the resulting stair angle.

Use it when framing a new staircase, replacing an existing one, or checking whether a space you're planning (a deck, a loft, a basement remodel) has enough headroom and run for a comfortable, code-compliant stair before you commit to a layout. It's a planning tool, not a substitute for your local building inspector — always confirm the final dimensions against your specific jurisdiction's adopted code, since exact maximums and minimums vary by location.

How the Calculation Works

The calculator starts from an ideal riser height of about 7 inches and works out how many risers are needed:

• Number of risers = round(Total rise ÷ 7), then increased if needed until riser height ≤ 7.75 in (a common residential maximum)
• Riser height = Total rise ÷ Number of risers
• Number of treads = Number of risers − 1 (the top step lands on the upper floor, which needs no tread of its own)
• Tread depth = Available run ÷ Number of treads (or a standard 10 in if no run is entered)
• Stringer length = √(Total rise² + Total run²) — the Pythagorean hypotenuse connecting the bottom of the stair to the top
• Stair angle = arctan(Total rise ÷ Total run), in degrees

The calculator also checks two widely used residential rules of thumb: a minimum 10-inch tread depth, and the "comfort rule" (2 × riser height + tread depth should fall between 24 and 25 inches) that many stair builders use as a quick ergonomics check.

Worked Example

Using the calculator's default inputs — a 9 ft 0 in total rise (108 in) and a 10 ft 0 in available run (120 in):

• Number of risers = round(108 ÷ 7) = 15; riser height = 108 ÷ 15 = 7.20 in (within the 7.75 in limit, so no adjustment needed)
• Number of treads = 15 − 1 = 14; tread depth = 120 ÷ 14 = 8.57 in
• Stringer length = √(108² + 120²) = √26,064 ≈ 161.4 in (13 ft 5.4 in)
• Stair angle = arctan(108 ÷ 120) ≈ 42.0°
• Comfort check: 2 × 7.20 + 8.57 = 23.0 in, just outside the ideal 24–25 in range, and the 8.57 in tread depth falls short of the common 10 in minimum

This example illustrates exactly the kind of trade-off the calculator is built to surface: with only 10 feet of run available for a 9-foot rise, the resulting stair is steeper and the treads shallower than ideal — the fix is either a longer run, a winder/landing to break up the flight, or accepting the steeper, code-borderline result.

Common Mistakes / How to Interpret the Result

• Confusing total rise with a single riser height. Total rise is the entire floor-to-floor height, not one step's height — measure it with a level and tape from finished floor to finished floor, not from the subfloor.
• Not leaving available run empty on purpose. If you leave the run fields blank, the calculator assumes a standard 10-inch tread depth and calculates whatever total run results — check the "Total Run" output against the actual space you have before finalizing a design.
• Ignoring headroom. This calculator checks riser and tread dimensions but doesn't check headroom clearance above the stair — codes typically require at least 6 ft 8 in of clear headroom measured vertically from any tread nosing, which you need to verify separately against your framing plan.
• Treating code maximums as targets. A 7.75-inch riser is the common legal maximum in many U.S. jurisdictions, not a design goal — stairs closer to 7 inches per riser with a 10–11 inch tread generally feel safer and more comfortable to climb.

Frequently Asked Questions

Common Tip Percentages

General Household Budget Ratios (50/30/20 Rule)

Common Measurement Conversions for Everyday Use