Stair Calculator
Enter the floor-to-floor height to get risers, treads, total run, stringer length and angle, checked against IRC limits and the 2R + T comfort rule.
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About this toolWhat is the Stair Calculator?
Stair layout starts from one fixed number: the height from finished floor to finished floor. Everything else follows. Divide the height by the tallest riser you will accept and round up to get the number of risers; divide the height by that count to get the exact riser; and multiply the tread depth by one fewer than the risers to get the total run. This stair calculator does those steps, then works out the stringer length and the angle, and draws the flight to scale.
Every result is checked against the International Residential Code limits most US jurisdictions adopt: a maximum riser of 7 3/4 inches, a minimum tread of 10 inches and a nosing of 3/4 to 1 1/4 inches where treads are shallower than 11 inches. It also tests the comfort rule that twice the riser plus the tread should land between 24 and 25 inches, and flags angles outside the comfortable 30 to 37 degree band.
If the run is fixed by the room, enter the available length and the calculator fits the treads into it instead, telling you if they come out too shallow. A small table compares one riser more and one fewer, which is often the quickest way to fix a tight layout.
- Risers, treads, total run, stringer length and angle from the total rise
- Checks against IRC R311.7 riser, tread and nosing limits, plus 2R + T comfort
- Fit-to-run mode for stairs in a fixed space
- Scale side-view drawing and one-more, one-fewer riser comparison
- Inches or millimetres; copy the layout or share a link
How to use the Stair Calculator
- Measure the total rise from finished floor to finished floor, including flooring that is not yet installed.
- Enter the maximum riser height you want. 7 to 7 1/2 inches is comfortable; 7 3/4 inches is the IRC limit.
- Enter the tread depth, or leave it and enter an available run to have the treads fitted into that length.
- Read risers, treads, total run, stringer length and angle, and check the green or red code and comfort lines.
- Use the comparison table to try one more or one fewer riser, then copy the layout for your cut list.
Worked examples
Nine-foot floor to floor
Input
Total rise 108 in, maximum riser 7.5 in, tread 10.5 in
Output
15 risers of 7.2 in, 14 treads, total run 147 in (12 ft 3 in), stringer 182.4 in, angle 34.4°
This is the default example. 2R + T is 24.9 inches, inside the comfort band, and every IRC check passes.
Metric stair that fails on tread depth
Input
Total rise 2,700 mm, maximum riser 190 mm, tread 250 mm
Output
15 risers of 180 mm, 14 treads, run 3,500 mm, angle 35.8°
The 250 mm tread is just under the IRC 254 mm minimum, so the tread line turns red. A 260 mm tread fixes it for 140 mm more run.
Fitting a stair into 10 feet of run
Input
Total rise 108 in, maximum riser 7.75 in, available run 120 in
Output
14 risers of 7.71 in, treads only 9.23 in, angle 39.9°
The space is too short for a code-compliant straight stair. A landing or a turn is needed, which the table makes obvious.
Background and insight
The code numbers come from IRC section R311.7: R311.7.5.1 limits riser height to 7 3/4 inches (196 mm), R311.7.5.2 sets a 10 inch (254 mm) minimum tread, R311.7.2 requires 6 feet 8 inches of headroom measured from the sloped line of the nosings, and riser heights within a flight may differ by no more than 3/8 inch. Commercial stairs under the IBC are stricter, at a 7 inch maximum riser and 11 inch minimum tread, and local amendments are common, so confirm with your building department.
The 2R + T rule goes back to François Blondel, a 17th-century French architect who tied step size to the length of a natural stride; modern practice puts the target at 24 to 25 inches (610 to 635 mm). A 7 inch riser with an 11 inch tread scores 25, a classic comfortable stair. Steeper stairs save floor space but feel abrupt going down, which is when most stair falls happen.
Common use cases
Deck and porch stairs
Lay out stringers for the drop from a deck to grade, with the riser count and cut dimensions.
Basement and loft stairs
Check whether a replacement stair will fit the existing opening and pass inspection.
Planning a renovation
See how much floor space a new stair needs before moving walls.
Checking an existing stair
Measure a stair that feels steep and see which rule it breaks.
Tips and limitations
- Measure rise at the stair location with finished flooring thickness included. A 3/4 inch floor added later makes the top or bottom riser non-uniform.
- Stringer stock must be longer than the stringer line shown: add for the top cut and the plumb and level cuts at each end.
- Headroom is measured plumb from the line through the nosings, not from each tread. Check the opening above before you cut anything.
- Exterior stairs to grade change height as soil settles. Pour a solid landing at the bottom so the first riser stays the same as the others.
- Stairs with four or more risers generally need a handrail between 34 and 38 inches high under the IRC.
Frequently asked questions
How do I calculate the number of stairs I need?
Divide the floor-to-floor height by your maximum riser and round up. For a 108 inch rise and a 7.5 inch maximum, 108 / 7.5 = 14.4, so you need 15 risers of 7.2 inches and 14 treads.
What is the maximum stair riser height allowed by code?
The IRC limits residential risers to 7 3/4 inches (196 mm), with a minimum tread of 10 inches. Commercial buildings under the IBC allow at most 7 inches with an 11 inch tread. Check your local adopted code.
Why is there one fewer tread than risers?
The upper floor acts as the top step. A flight with 15 risers needs only 14 treads, so the total run is 14 times the tread depth.
How long should a stair stringer be?
The stringer line is the square root of total rise squared plus total run squared: 182.4 inches for a 108 inch rise over a 147 inch run. Buy stock at least a foot longer to allow for the end cuts.
What is the 2R + T stair rule?
Twice the riser plus the tread should be about 24 to 25 inches (610 to 635 mm), close to a natural walking stride. A 7 inch riser with an 11 inch tread scores 25.
What is the best angle for a staircase?
Between about 30 and 37 degrees feels comfortable for most people. Steeper stairs save space but are harder to descend; much shallower ones feel like a ramp with steps.
How much headroom does a staircase need?
The IRC requires at least 6 feet 8 inches (2,032 mm), measured straight up from the sloped line connecting the tread nosings.
Is there an in-depth guide to the Stair Calculator?
Yes. “Roof Pitch, Stair Layout, AC Sizing and Running Costs Explained” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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