A 90" floor-to-floor stair needs 12 risers at 7-1/2" and 11 treads: a 110" run on 10" treads, cut from 12 ft 2x12s with 5-1/4" of throat. Gentler: 13 risers at 6-15/16" on 11" treads, a 132" run (2R + T = 24.8").
finished to finished
fewest legal
nosing to nosing
3/4–1-1/4 under 11″
DCA 6 max 18
90" total rise
Hook the tape once at mark 0, where the bottom riser line meets the top edge of the 2x12, and pencil every mark from that same hook. Each step is 12-1/2" along the edge; marks are rounded once each, so nothing drifts.
| Done | # | Mark | Tape | Decimal | mm |
|---|---|---|---|---|---|
| 0 | Bottom riser line | 0" | 0.000" | 0 mm | |
| 1 | Tread 1 nose | 12-1/2" | 12.500" | 318 mm | |
| 2 | Tread 2 nose | 25" | 25.000" | 635 mm | |
| 3 | Tread 3 nose | 37-1/2" | 37.500" | 953 mm | |
| 4 | Tread 4 nose | 50" | 50.000" | 1270 mm | |
| 5 | Tread 5 nose | 62-1/2" | 62.500" | 1588 mm | |
| 6 | Tread 6 nose | 75" | 75.000" | 1905 mm | |
| 7 | Tread 7 nose | 87-1/2" | 87.500" | 2223 mm | |
| 8 | Tread 8 nose | 100" | 100.000" | 2540 mm | |
| 9 | Tread 9 nose | 112-1/2" | 112.500" | 2858 mm | |
| 10 | Tread 10 nose | 125" | 125.000" | 3175 mm | |
| 11 | Top plumb cut | 137-1/2" | 137.500" | 3493 mm |
Then from mark 0 measure 6-7/16" down the bottom riser line and square across for the level heel cut.
Shortest run, steepest legal stair
Gentler climb, longer run
Floor to floor = ceiling + joists + subfloor. With 12-1/2" of floor framing and ceiling, the stairwell opening over Option A starts 0" from the bottom riser and runs 9' 2" for 6'-8" of headroom.
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| 0 | Bottom riser line | 0" | 0.000" | 0 mm | 0" |
|---|---|---|---|---|---|
| 1 | Tread 1 nose | 12-1/2" | 12.500" | 318 mm | 1' 1/2" |
| 2 | Tread 2 nose | 25" | 25.000" | 635 mm | 2' 1" |
| 3 | Tread 3 nose | 37-1/2" | 37.500" | 953 mm | 3' 1-1/2" |
| 4 | Tread 4 nose | 50" | 50.000" | 1270 mm | 4' 2" |
| 5 | Tread 5 nose | 62-1/2" | 62.500" | 1588 mm | 5' 2-1/2" |
| 6 | Tread 6 nose | 75" | 75.000" | 1905 mm | 6' 3" |
| 7 | Tread 7 nose | 87-1/2" | 87.500" | 2223 mm | 7' 3-1/2" |
| 8 | Tread 8 nose | 100" | 100.000" | 2540 mm | 8' 4" |
| 9 | Tread 9 nose | 112-1/2" | 112.500" | 2858 mm | 9' 4-1/2" |
| 10 | Tread 10 nose | 125" | 125.000" | 3175 mm | 10' 5" |
| 11 | Top plumb cut | 137-1/2" | 137.500" | 3493 mm | 11' 5-1/2" |
Step-off 12-1/2" (12.5000") · bottom cut 6-1/2" under 1" treads · last mark 11' 5-1/2" at the top plumb cut.
| 0 | Bottom riser line | 0" | 0.000" | 0 mm | 0" |
|---|---|---|---|---|---|
| 1 | Tread 1 nose | 13" | 12.997" | 330 mm | 1' 1" |
| 2 | Tread 2 nose | 26" | 25.995" | 660 mm | 2' 2" |
| 3 | Tread 3 nose | 39" | 38.992" | 990 mm | 3' 3" |
| 4 | Tread 4 nose | 52" | 51.989" | 1321 mm | 4' 4" |
| 5 | Tread 5 nose | 65" | 64.986" | 1651 mm | 5' 5" |
| 6 | Tread 6 nose | 78"− | 77.984" | 1981 mm | 6' 6" |
| 7 | Tread 7 nose | 91"− | 90.981" | 2311 mm | 7' 7" |
| 8 | Tread 8 nose | 104"− | 103.978" | 2641 mm | 8' 8" |
| 9 | Tread 9 nose | 117"− | 116.975" | 2971 mm | 9' 9" |
| 10 | Tread 10 nose | 130"− | 129.973" | 3301 mm | 10' 10" |
| 11 | Tread 11 nose | 143"− | 142.970" | 3631 mm | 11' 11" |
| 12 | Top plumb cut | 155-15/16"+ | 155.967" | 3962 mm | 12' 11-15/16" |
Step-off 13" (12.9973") · bottom cut 5-15/16" under 1" treads · last mark 12' 11-15/16" at the top plumb cut.
05 / Field guide
This sheet works a 90" floor-to-floor rise two ways. Option A uses the fewest risers the IRC allows, 12 at 7-1/2" on 10" treads. Option B adds one riser and uses 11" treads for a gentler 6-15/16" step. Both come with hook-once tape marks for the stringer edge, below the calculator.
90" ÷ 7.75 = 11.61, rounded up to 12 risers, each 7-1/2" (7.5000", 190.5 mm). On 11 treads of 10" the stair lands 110" (9' 2") out from the upper floor edge and climbs at 36.9°; 2R + T scores 25.00".
Each step is 12-1/2" along the stringer edge. Notches cut 6" deep, leaving 5-1/4" of throat in a 2x12; the cut length is 11' 9-3/8", so buy 12 ft stock.
One more riser drops each step to 6-15/16" (6.9231"). With 11" treads the run grows to 132" (11' 0"), 22" more than Option A, and the angle eases to 32.2° with 2R + T at 24.85".
The diagonal is 13", the throat 5-3/8", and the stringer cut length 13' 3-1/8" (14 ft).
Take the tread thickness off the bottom riser: 7-1/2" becomes 6-1/2" under 1" 5/4 or composite treads, 6" under 1-1/2" 2x6 treads and 6-7/16" under 1-1/16" oak treads. Every finished step then measures 7-1/2".
At the top, the last tread notch sits 8-1/2" below the finished floor for 1" treads. Seated on a 2x header, the header top goes 16-1/4" below the finished floor with oak treads.
12 risers means a handrail is required (four or more risers), 34" to 38" above the nosings.
The 110" run is more than the 6'-0" a cut stringer may span under DCA 6: set 1 4x4 support post on 12" footings under the stringers, or build a landing.
The IRC wants 6'-8" measured plumb from the nosing line. With 12-1/2" of floor framing and ceiling, the opening above Option A must start 0" out from the bottom riser and run 110" (9' 2") to the top of the stair.
Floor to floor is the ceiling height plus the joists and subfloor: 90" is a 78" ceiling over 2x12 joists, or 80" over 2x10s.
Questions
12 risers at 7-1/2" with 11 treads, the fewest the IRC allows. For an easier climb, 13 risers at 6-15/16" with 12 treads.
About 11' 9-3/8" along the board for Option A, so 12 ft 2x12s; Option B needs 13' 3-1/8" (14 ft).
110" (9' 2") on 10" treads, or 132" (11' 0") on 11" treads with 13 risers. Leave a landing at least 36" deep beyond the last riser.
A 78" ceiling over 2x12 joists and a 3/4" subfloor, or 80" over 2x10s.
Not under AWC DCA 6, which asks for 2x12s. A 2x10 would keep only 3-1/4" of throat on Option A, under the 5" minimum.
Enter your exact rise in the calculator. 84" takes 11 risers and 96" takes 13; the riser count changes wherever the rise passes a multiple of 7-3/4".
Raised decks, basements and storeys