ReckonBoxTurning a height into a staircase
A stair riser is the vertical height of one step, and the number of risers is the total rise divided by a comfortable riser near 7 inches. Start with the height you have to climb, in inches, and the rest of the staircase falls out of it.
Say the floor-to-floor height is 108 inches. Divide by 7 and you get about 15, so 15 risers of 7.2 inches each. The treads, the flat parts you step on, are one fewer than the risers, because the top riser lands on the upper floor and needs no tread of its own. So 15 risers means 14 treads. At an 11 inch tread depth, the staircase runs 154 inches horizontally and needs a stringer just over 188 inches long.
The limits that keep it legal and comfortable
Under the 2015 IRC, a riser can be no taller than 7.75 inches and a tread no shallower than 10 inches, with the total rise between landings capped at 147 inches. These are the numbers most stair calculators compute around but never show you.
The tool sizes the risers to stay at or under that 7.75 inch ceiling while aiming for the 7 inch mark, so the step is comfortable and not merely allowed. There is also an old carpenter's comfort rule: twice the riser plus the tread should land near 24 to 25 inches, matching a natural stride. A 7.2 inch riser with an 11 inch tread gives 25.4 inches. Get that number far off and the stairs feel either like a ladder or like a slog.
| 108 in rise, 11 in tread | Value | Limit |
|---|---|---|
| Risers | 15 | max 7.75 in each |
| Riser height | 7.2 in | pass |
| Treads | 14 | min 10 in each |
| Total run | 154 in | pass |
| Stringer length | 188.1 in | total rise max 147 in, pass |
Why the stringer is longer than you think
A stringer is the diagonal board the steps mount to, and its length is the hypotenuse of the rise and run. The Pythagorean theorem does the work: the square root of the total rise squared plus the total run squared. For a 108 inch rise and a 154 inch run, that is the square root of 11,664 plus 23,716, about 188 inches. People underestimate this because they picture the 108 inch height or the 154 inch run alone, not the slope that spans both, which is why a 16 foot board gets bought for a stair that looked like it needed 13.
What this does not cover
This sizes the step geometry and checks it against three core IRC limits: riser height, tread depth, and total rise. It leaves out the rest of the code, including the handrail height of 34 to 38 inches, the 6 foot 8 inch minimum headroom, the 36 inch minimum width, and the nosing and landing rules. It also assumes one straight flight with equal steps, not a winder, a spiral, or a run broken by a landing. Local codes can differ from the model IRC, so the building department has the final say.
Cut one riser wrong and every step above it is off, which is why the total rise gets measured before a single board does.
Frequently asked questions
How do I calculate the number of stairs? Divide the total rise by a comfortable riser height near 7 inches, then round to a whole number of risers. A 108 inch total rise divided by about 7 gives 15 risers of 7.2 inches each. The treads are one fewer than the risers, so 15 risers means 14 treads, because the top riser lands on the upper floor.
What is the maximum riser height and minimum tread depth? Under the 2015 IRC, the maximum riser height is 7.75 inches and the minimum tread depth is 10 inches. The total rise between landings cannot exceed 147 inches. This tool sizes the risers to stay at or under the 7.75 inch maximum while staying near the comfortable 7 inch mark, and it flags any input that falls outside these limits.
How do I calculate stringer length? A stringer length is the diagonal of the staircase, found with the Pythagorean theorem: the square root of the total rise squared plus the total run squared. For a 108 inch rise and a 154 inch run, that is the square root of 108 squared plus 154 squared, which is about 188 inches. That is the minimum board length before you account for the cut at each end.
What makes a staircase comfortable to climb? A common comfort rule is that twice the riser plus the tread should land near 24 to 25 inches, matching a natural stride. A 7.2 inch riser with an 11 inch tread gives 25.4 inches, which is comfortable. Steps that are too steep or too shallow both feel wrong, which is why the riser and tread are balanced against each other, not set alone.
Does this meet building code? This checks your layout against the core 2015 IRC limits for riser height, tread depth, and total rise, and it tells you when a value falls outside them. It does not cover every rule, such as handrail height, headroom clearance, or the minimum stair width, and local codes can differ from the model IRC. Confirm the full requirements with your local building department before you build.