How to use it
Enter the total rise, measured from finished floor to finished floor. Set the maximum riser height you are allowed and the tread depth you want. The calculator gives the riser count, the exact height of each riser, the number of treads, the total run, the stringer length, the incline and the 2R + T comfort check.
Worked example
A 9' 1 1/8" rise with a 7 1/2" maximum riser and 10" treads needs 15 risers at about 7 1/4" each (7.275" exactly) and 14 treads. The total run is 11' 8", the stringer is about 15' 5 1/2" and the incline is 36.04°. 2R + T comes to about 24 9/16".
How it is worked out
- Risers = total rise ÷ maximum riser, rounded up. That is the fewest risers that keep every riser at or under your maximum.
- Riser height = total rise ÷ risers, so every step is the same height.
- Treads = risers − 1, because the upper floor is the last step.
- Total run = treads × tread depth.
- Stringer length is measured along the slope for the full rise. Treat it as the stock length before the top and bottom cuts.
- 2R + T is twice the riser plus the tread. A stair is generally comfortable when it falls between 24" and 25".
Riser and tread settings
The calculator starts with a 7 1/2" maximum riser and a 10" tread. Change either one to suit your project and the layout updates.
Typing measurements
Every box takes feet, inches and fractions the way you would write them: 9' 1 1/8", 7 1/2", 2.5' or 300mm. Put a space between whole inches and a fraction. A box also takes a whole calculation, so 14' 6" / 2 works anywhere.
Questions
How do I calculate the number of stair risers?
Divide the floor-to-floor rise by the maximum riser height and round up. A 9' 1 1/8" rise (109.125 inches) divided by 7 1/2" is 14.55, so you need 15 risers, each 7.275 inches high.
How many treads does a stair have?
One less than the number of risers, because the upper floor acts as the last step. Fifteen risers means fourteen treads.
How long a stringer do I need?
Measure along the slope: the number of risers times the square root of riser squared plus tread squared. For 15 risers of 7.275" with 10" treads that is about 15' 5 1/2", so buy 16-foot stock.
What is the 2R + T rule?
Twice the riser height plus the tread depth should come to roughly 24 to 25 inches. It is a rule of thumb for a comfortable stride.
How precise is the answer?
The math is done at full precision and only the displayed fraction is rounded, to the nearest 1/16 inch by default. You can change that from 1/2 down to 1/64. When rounding changes the value, the answer is marked with ≈.
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