Stair Stringer Calculator with Code Check and Cut List
Stair Stringer Calculator with Code Check
Enter the total rise floor to floor and the tread depth you intend to use. The tool lays out the flight, checks it against the commonly adopted residential limits, and prints a stringer cut list you can carry to the saw.
Risers always outnumber treads by one, because the top tread is the floor you are arriving at. The 2R plus T rule of thumb, twice the riser plus the tread landing between 24 and 25 inches, is what separates a comfortable flight from a tiring one, and it is why a shallow riser wants a deeper tread. Riser height must be consistent within 3/8 in across the whole flight: uneven risers are the single most common cause of stair falls.
How to use this tool
Enter the total rise measured from finished floor to finished floor, the tread depth you plan to use, and the headroom you have. The tool divides the rise into equal risers, checks the result against the limits that most residential codes adopt, and prints a stringer cut list with the rise and run at every step.
The one measurement people get wrong is the total rise. It is finished floor to finished floor, so if the upper floor is getting tile and the lower one is getting carpet, both finished thicknesses belong in the number. Laying out to subfloor and then adding finishes afterwards is how a flight ends up with one odd riser at the top.
Why the top riser is the one that fails inspection
A flight always has one more riser than it has treads, because the last riser lands you on the floor above and that floor is the final tread. Miscounting this is the classic error: it produces a flight where every riser is right except the last one, which absorbs the leftover.
That matters because riser consistency is itself a rule. Most adopted codes allow no more than 3/8 inch of variation between the largest and smallest riser in a flight, and the reason is human, not structural. Walking a stair is a rhythm, and your foot places itself for the height it just learned. A single riser that differs by half an inch is the one that catches a toe, which is why uneven risers are among the most common causes of stair falls.
The 2R plus T rule, and what it is really telling you
Twice the riser plus the tread should land between 24 and 25 inches. It is a rule of thumb rather than a code requirement, and it encodes something real about how people climb: a stride has a roughly fixed length, and rise and run trade against each other within it. Steep risers with shallow treads exceed what the stride can absorb and the flight becomes tiring; shallow risers with deep treads break the rhythm the other way and the flight feels stretched.
This is why the tool reports the sum rather than hiding it. A flight can pass every hard limit and still be unpleasant to use, and the 2R plus T figure is usually the first sign of it.
A worked check you can repeat on paper
A 108 inch total rise, a common 9 foot floor to floor with finishes, at a 10 inch tread. Dividing by a 7 inch target gives 15.43, which rounds to 15 risers. Each riser is 108 divided by 15, exactly 7.2 inches, comfortably under the 7.75 limit. Fifteen risers means 14 treads, so the total run is 140 inches, and the stringer is the hypotenuse of 140 and 108, which is 176.8 inches, at a pitch of 37.6 degrees. The comfort sum is 2 times 7.2 plus 10, or 24.4 inches, inside the 24 to 25 band. Every one of those numbers is checkable with a calculator, which is the point.
Mistakes that catch people out
Counting treads and risers as equal. Risers are one more than treads, always. Fix: the tool derives treads from risers rather than asking twice.
Measuring rise to the subfloor. Finishes on either floor change the riser height across the whole flight. Fix: measure finished to finished, or add the finish thicknesses before entering.
Rounding the riser to a fraction. Rounding 7.2 to 7.25 across 15 risers loses three quarters of an inch, which has to go somewhere, usually into the top riser. Fix: carry the decimal and mark the stringer from a running dimension, which is why the cut list gives rise from base rather than step by step.
Forgetting headroom on the flight below. Headroom is measured plumb from the tread nosing line, and a stair that fits in plan can still fail it. Fix: enter the available headroom and let the check catch it.
How this was checked
The layout was verified against hand arithmetic on the 108 inch case above, giving 15 risers at exactly 7.2 inches, 14 treads, a 140 inch run and a 176.8 inch stringer at 37.6 degrees. The rounding path was checked at 120 inches, which yields 17 risers at 7.06 inches, and the limit path was checked separately: the smallest riser count that keeps a 120 inch rise under a 7.75 inch maximum is 16. The failure branch was confirmed by entering a 9 inch tread, which the tool rejects against the 10 inch minimum rather than quietly accepting.
Related Calculators and Reference
Stairs framed as part of a larger job belong with the Deck Material Takeoff Calculator, which counts the framing they land on. The geometry underneath the stringer is the Distance and Pythagoras Calculator, and angles in other forms are the Angle Converter.