A 40" deck needs 6 risers at 6-11/16" and 5 treads: a 50" run on 10" treads, cut from 8 ft 2x12s with 5-11/16" of throat. Gentler: 7 risers at 5-11/16" on 11" treads, a 66" run (2R + T = 22.4").
finished to finished
fewest legal
nosing to nosing
3/4–1-1/4 under 11″
DCA 6 max 18
40" 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"+ 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"+ | 12.019" | 305 mm | |
| 2 | Tread 2 nose | 24-1/16"− | 24.037" | 611 mm | |
| 3 | Tread 3 nose | 36-1/16" | 36.056" | 916 mm | |
| 4 | Tread 4 nose | 48-1/16" | 48.074" | 1221 mm | |
| 5 | Top plumb cut | 60-1/16"+ | 60.093" | 1526 mm |
Then from mark 0 measure 5-11/16" down the bottom riser line and square across for the level heel cut.
Shortest run, steepest legal stair
Gentler climb, longer run
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| 0 | Bottom riser line | 0" | 0.000" | 0 mm | 0" |
|---|---|---|---|---|---|
| 1 | Tread 1 nose | 12"+ | 12.019" | 305 mm | 1' 0" |
| 2 | Tread 2 nose | 24-1/16"− | 24.037" | 611 mm | 2' 1/16" |
| 3 | Tread 3 nose | 36-1/16" | 36.056" | 916 mm | 3' 1/16" |
| 4 | Tread 4 nose | 48-1/16" | 48.074" | 1221 mm | 4' 1/16" |
| 5 | Top plumb cut | 60-1/16"+ | 60.093" | 1526 mm | 5' 1/16" |
Step-off 12"+ (12.0185") · bottom cut 5-11/16" under 1" treads · last mark 5' 1/16" at the top plumb cut.
| 0 | Bottom riser line | 0" | 0.000" | 0 mm | 0" |
|---|---|---|---|---|---|
| 1 | Tread 1 nose | 12-3/8"+ | 12.396" | 315 mm | 1' 3/8" |
| 2 | Tread 2 nose | 24-13/16"− | 24.791" | 630 mm | 2' 13/16" |
| 3 | Tread 3 nose | 37-3/16" | 37.187" | 945 mm | 3' 1-3/16" |
| 4 | Tread 4 nose | 49-9/16"+ | 49.583" | 1259 mm | 4' 1-9/16" |
| 5 | Tread 5 nose | 62"− | 61.978" | 1574 mm | 5' 2" |
| 6 | Top plumb cut | 74-3/8" | 74.374" | 1889 mm | 6' 2-3/8" |
Step-off 12-3/8"+ (12.3957") · bottom cut 4-11/16" under 1" treads · last mark 6' 2-3/8" at the top plumb cut.
05 / Field guide
This sheet works a 40" deck two ways. Option A uses the fewest risers the IRC allows, 6 at 6-11/16" on 10" treads. Option B adds one riser and uses 11" treads for a gentler 5-11/16" step. Both come with hook-once tape marks for the stringer edge, below the calculator.
40" ÷ 7.75 = 5.16, rounded up to 6 risers, each 6-11/16"− (6.6667", 169.3 mm). On 5 treads of 10" the stair lands 50" (4' 2") out from the deck and climbs at 33.7°; 2R + T scores 23.33".
Each step is 12"+ along the stringer edge. Notches cut 5-9/16" deep, leaving 5-11/16" of throat in a 2x12; the cut length is 5' 3-1/4", so buy 8 ft stock.
One more riser drops each step to 5-11/16"+ (5.7143"). With 11" treads the run grows to 66" (5' 6"), 16" more than Option A, and the angle eases to 27.5° with 2R + T at 22.43".
The diagonal is 12-3/8"+, the throat 6-3/16", and the stringer cut length 6' 4-9/16" (8 ft).
Take the tread thickness off the bottom riser: 6-11/16" becomes 5-11/16" under 1" 5/4 or composite treads, 5-3/16" under 1-1/2" 2x6 treads. Every finished step then measures 6-11/16".
At the top, the last tread notch sits 7-11/16" below the finished deck for 1" treads. Hung flush from the rim, the stringer edge meets the rim face 1" below the finished deck surface.
6 risers means a handrail is required (four or more risers), 34" to 38" above the nosings.
At 40" the open sides need a guard: 34" above the nosings, no opening wider than a 4-3/8" sphere, and the tread-riser triangle under 6".
The 50" run is within the 6'-0" a cut 2x12 stringer may span under DCA 6, so no mid-support is needed.
Questions
6 risers at 6-11/16" with 5 treads, the fewest the IRC allows. For an easier climb, 7 risers at 5-11/16" with 6 treads.
About 5' 3-1/4" along the board for Option A, so 8 ft 2x12s; Option B needs 6' 4-9/16" (8 ft).
50" (4' 2") on 10" treads, or 66" (5' 6") on 11" treads with 7 risers. Leave a landing at least 36" deep beyond the last riser.
Yes. The total rise is 30" or more, so the open sides need a guard, and 6 risers also call for a handrail.
Not under AWC DCA 6, which asks for 2x12s. A 2x10 would keep only 3-11/16" of throat on Option A, under the 5" minimum.
Enter your exact rise in the calculator. 38" takes 5 risers and 42" takes 6; the riser count changes wherever the rise passes a multiple of 7-3/4".
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