Stair Calculator
Enter your finished floor-to-floor height and the calculator works out how many risers you need, their exact height, the total run, and the stringer length — then checks all of it against IRC limits before you cut anything.
Start with total rise, and measure it right
Everything else follows from one number: the vertical distance from finished floor to finished floor.
The most expensive mistake in stair building is measuring to the subfloor. If the upstairs is getting 3/4" hardwood and the downstairs is getting tile over mortar, and you measured bare subfloor to bare subfloor, every riser will be wrong — and because the error concentrates at the top and bottom steps, it usually fails inspection.
Measure to where the finished surfaces will be, not where they are now.
Why the riser count is always a whole number
You cannot have 14.7 risers. So the real calculation is: divide total rise by your preferred riser height, round to a whole number, then divide back to get the actual riser height.
That is why entering 7½" as your preference does not give you 7½" risers. It gives you the closest achievable height that divides evenly into your total rise. If the result comes out above the 7¾" code maximum, this calculator adds another riser automatically.
Treads are always one fewer than risers. The top floor surface is the last step, so it needs no tread of its own. Cutting an extra tread is a classic first-time error.
The first riser is cut shorter
When you lay out a stringer, the bottom riser must be reduced by the thickness of the tread material. Otherwise, once you install the treads, the first step ends up a full tread-thickness taller than every other step.
IRC allows only 3/8" of variation between the tallest and shortest riser in a flight. A 1" error is both a code failure and a genuine trip hazard, because people climb stairs by rhythm rather than by looking.
The "first riser cut" output already has this subtracted.
Reading the comfort check
Centuries of practice converge on 2 × riser + tread = 24 to 25 inches. This tracks the way stride length shortens as you climb.
Inside that band, stairs feel natural. Below it, steps feel cramped and people catch their toes. Above it, the stride feels stretched and descending feels unsafe.
A stair can pass code and still feel wrong — code sets outer limits, not comfort. If you have the floor space, aim for the comfort band rather than the code maximum.
Space and headroom
The total run is how much floor the stair eats. A 9-foot ceiling typically needs 12 to 13 feet of horizontal run, which surprises people planning a basement stair into a tight space.
Headroom must be at least 6 feet 8 inches, measured vertically from a line along the tread nosings. This is where basement stairs most often fail — a duct or beam crossing above the flight.
This calculator is for planning. Confirm every dimension against your local adopted code and have the work inspected.
Frequently asked questions
What is the maximum riser height allowed?
The IRC sets the maximum riser height at 7 3/4 inches for residential stairs, and the greatest riser in a flight may not exceed the smallest by more than 3/8 inch. Commercial stairs under the IBC are stricter at 7 inches. Local amendments can differ, so confirm with your building department.
How many stairs do I need for a given height?
Divide the total finished floor-to-floor rise by your preferred riser height and round to the nearest whole number, then divide the total rise back by that count to get the actual riser height. For a 108 inch rise at roughly 7 1/4 inches, that is 15 risers at 7.2 inches each, with 14 treads.
Why is the first riser cut shorter than the others?
Because the tread material sits on top of it. If you cut the bottom riser to full height, installing a one inch tread makes the first step one inch taller than the rest — a code violation and a trip hazard. Subtract the tread thickness from the bottom riser when laying out the stringer.
What is the 2R plus T rule?
Two times the riser height plus the tread depth should land between 24 and 25 inches. It reflects how stride length shortens when climbing. Stairs outside that range can still meet code but tend to feel cramped or stretched underfoot.
How much floor space will my staircase take up?
Multiply the number of treads by the tread depth. A typical stair for a 9 foot floor height needs roughly 12 to 13 feet of horizontal run, plus landing space at top and bottom. This is usually the constraint that decides whether a straight run fits or you need a landing and a turn.