Why stair math rounds the riser count, then splits the rise
Why the riser count is rounded
A stair has to land on the upper floor with every step the same height. You do not pick a favorite riser and hope the last one fits. You take the finished-floor-to-finished-floor rise, divide by a target (usually 7 to 7.5 inches), and round to a whole number of risers. The exact riser is that total rise divided by the rounded count. The target is only a starting guess.
Tread count is risers minus one on a straight run that dies into a landing. The last vertical face is that landing, so a 15-riser flight needs 14 treads of floor. Stringer length is the hypotenuse of rise and run — the board you cut. Blondel's (24 to 25.5 inches) is a stride check. Inspectors read R311.7. The IBC 7-11 pair is commercial; using it on a house will reject a legal 7.5 / 10.5 layout.
Worked field example
Finished floor to finished floor, including both floor finishes and any landing buildup: 109.25 inches. Target 7.5 inches. Tread 10.5 inches. This is not the 108-inch default on the form.
109.25 ÷ 7.5 = 14.57, which rounds to 15 risers. Exact rise is 109.25 ÷ 15 = 7.283 inches — under the 7.75 inch IRC cap and well above 4 inches. Fourteen treads at 10.5 inches give 147 inches of run (12 feet 3 inches of floor). Stringer length is inches, so a 16-foot 2x12 still has waste for plumb and seat cuts. Blondel is 2 × 7.283 + 10.5 = 25.07 inches, inside the comfort band.
Measuring only the rough opening drops the finish thickness and leaves a short last riser after flooring — the usual fail, not a rounding error.
Code limits this tool checks — and those it does not
The engine flags riser height and tread depth. It does not check headroom, grip, or landing plan. IBC 1011 is listed so you do not apply 7-11 to a dwelling.
IRC R311.7 / IBC 1011 — geometry only, not structural design
- Provision
- IRC R311.7.5.1
- Limit
- Riser 4 in min, 7.75 in max; 3/8 in max variation
- Failure mode
- One tall closer after flooring; trip at the landing
- Provision
- IRC R311.7.5.2
- Limit
- Tread 10 in min (nosing rules in R311.7.5.3)
- Failure mode
- Short tread, heel slip, especially without nosing
- Provision
- IRC R311.7.1
- Limit
- 36 in clear width
- Failure mode
- Handrail or drywall eating the walking width
- Provision
- IRC R311.7.8
- Limit
- Handrail 34–38 in above nosing
- Failure mode
- Rail mounted to skirt, not to the nosing line
- Provision
- IRC R311.7.6
- Limit
- Landing ≥ stair width; 36 in in direction of travel
- Failure mode
- Door swing eating the landing; 147 in is run, not the landing rule
- Provision
- IBC 1011 (7-11)
- Limit
- Commercial 7 in rise / 11 in tread
- Failure mode
- Rejecting a legal IRC 7.5 / 10.5 house stair
| Provision | Limit | Failure mode |
|---|---|---|
| IRC R311.7.5.1 | Riser 4 in min, 7.75 in max; 3/8 in max variation | One tall closer after flooring; trip at the landing |
| IRC R311.7.5.2 | Tread 10 in min (nosing rules in R311.7.5.3) | Short tread, heel slip, especially without nosing |
| IRC R311.7.1 | 36 in clear width | Handrail or drywall eating the walking width |
| IRC R311.7.8 | Handrail 34–38 in above nosing | Rail mounted to skirt, not to the nosing line |
| IRC R311.7.6 | Landing ≥ stair width; 36 in in direction of travel | Door swing eating the landing; 147 in is run, not the landing rule |
| IBC 1011 (7-11) | Commercial 7 in rise / 11 in tread | Rejecting a legal IRC 7.5 / 10.5 house stair |
Where the field usually goes wrong
Measuring the framed box and ignoring finish is the first miss. Hardwood, underlayment, and tile upstairs can add more than an inch — enough to fail the 3/8-inch variation or force an extra riser after the well is hung. A 2x12 is 11-1/4 inches deep; after a 7-1/4 by 10-1/2 notch you still need 3.5 inches of throat. 2x10 stock usually cannot. Open risers still need a 4-inch sphere test (R311.7.5.1). This tool does not test that sphere or the 6 foot 8 inch headroom in R311.7.2.
Frequently asked questions
Why is the number of treads one less than the number of risers?
The last vertical face is the landing. A 15-riser flight has 14 treads and 14 × tread of run.
Do I measure to the subfloor or to the finished floor?
Finished floor to finished floor, including both finishes and landing buildup. Rough opening only leaves a short last riser after flooring.
What does the 3/8-inch riser variation rule actually limit?
R311.7.5.1 limits tallest-to-shortest in a flight to 3/8 inch. It is not a tolerance on the 7.75-inch cap.
Is the 7-11 rule IRC or IBC?
IBC commercial: 7-inch rise, 11-inch tread. IRC dwellings allow 7.75 and 10. This tool checks IRC.
How much throat has to remain on a 2x12 stringer?
At least 3.5 inches behind the notch. A 2x12 is 11-1/4 deep; 2x10 usually cannot hold that on a house stair.
Are open risers allowed in a dwelling?
Yes if a 4-inch sphere cannot pass (R311.7.5.1). Geometry limits still apply. This tool does not test the sphere.
What landing size does IRC require?
R311.7.6: at least as wide as the stair and 36 inches in the direction of travel. The 147 inches in the example is run, not the landing.
Does an inspector use Blondel’s 2R+T?
No. Blondel is comfort (24–25.5 inches). Inspectors read rise, tread, variation, width, rail, and headroom.