A 12 ft (144" level) stair railing on a 7-1/2" × 10" stair takes 27 wood balusters at 3-11/16" gaps or 32 metal at 3-5/8", under the 4-3/8" stair sphere. Along the 36.9° rail every mark is the level one × 1.250: wood balusters sit 6-1/2" apart on the rail.
144" between posts (level)
Hook the tape on the post face at the top of the bottom rail and mark down the slope. Marks along the rail are the level positions × 1.2500 (1 ÷ cos 36.9°).
| Done | # | Mark | Along rail | Centre (rail) | Level |
|---|---|---|---|---|---|
| 1 | Leading edge | 4-5/8" | 5-9/16" | 3-11/16" | |
| 2 | Leading edge | 11-1/8" | 12-1/16" | 8-7/8"+ | |
| 3 | Leading edge | 17-5/8" | 18-9/16" | 14-1/16"+ | |
| 4 | Leading edge | 24-1/8"− | 25-1/16"− | 19-5/16"− | |
| 5 | Leading edge | 30-5/8"− | 31-9/16"− | 24-1/2"− | |
| 6 | Leading edge | 37-1/8"− | 38-1/16"− | 29-11/16" | |
| 7 | Leading edge | 43-5/8"− | 44-9/16"− | 34-7/8" | |
| 8 | Leading edge | 50-1/16"+ | 51"+ | 40-1/16" | |
| 9 | Leading edge | 56-9/16"+ | 57-1/2"+ | 45-1/4"+ | |
| 10 | Leading edge | 63-1/16"+ | 64"+ | 50-7/16"+ | |
| 11 | Leading edge | 69-9/16" | 70-1/2" | 55-11/16"− | |
| 12 | Leading edge | 76-1/16" | 77" | 60-7/8"− | |
| 13 | Leading edge | 82-9/16" | 83-1/2" | 66-1/16" | |
| 14 | Leading edge | 89-1/16" | 90" | 71-1/4" | |
| 15 | Leading edge | 95-9/16" | 96-1/2" | 76-7/16" | |
| 16 | Leading edge | 102-1/16" | 103" | 81-5/8"+ | |
| 17 | Leading edge | 108-9/16" | 109-1/2" | 86-13/16"+ | |
| 18 | Leading edge | 115-1/16"− | 116"− | 92-1/16"− | |
| 19 | Leading edge | 121-9/16"− | 122-1/2"− | 97-1/4"− | |
| 20 | Leading edge | 128-1/16"− | 129"− | 102-7/16" | |
| 21 | Leading edge | 134-1/2"+ | 135-7/16"+ | 107-5/8" | |
| 22 | Leading edge | 141"+ | 141-15/16"+ | 112-13/16" | |
| 23 | Leading edge | 147-1/2"+ | 148-7/16"+ | 118"+ | |
| 24 | Leading edge | 154"+ | 154-15/16"+ | 123-3/16"+ | |
| 25 | Leading edge | 160-1/2" | 161-7/16" | 128-7/16"− | |
| 26 | Leading edge | 167" | 167-15/16" | 133-5/8"− | |
| 27 | Leading edge | 173-1/2" | 174-7/16" | 138-13/16" |
Fillets: cut each one to the gap along the rail with the rake angle on both ends.
4x4 posts at 147-1/2" on centre exceed the 6'-0" AWC DCA 6 guard-post limit: add a middle post or use a railing kit rated for this length.
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| 27 | 3-11/16" | 5-3/16" · 6-1/2" | 4-5/8" | 30 | ||
| 32 | 3-5/8" | 4-3/8" · 5-1/2" | 4-9/16" | 36 | ||
| 30 | 3-11/16" | 4-11/16" · 5-7/8" | 4-5/8" | 33 | ||
| 20 | 3-1/2"+ | 7" · 8-3/4" | 4-3/8" | 22 |
| 1 | 3-11/16" | 4-7/16" | 4-5/8" | 4.621" |
|---|---|---|---|---|
| 2 | 8-7/8"+ | 9-5/8"+ | 11-1/8" | 11.116" |
| 3 | 14-1/16"+ | 14-13/16"+ | 17-5/8" | 17.612" |
| 4 | 19-5/16"− | 20-1/16"− | 24-1/8"− | 24.107" |
| 5 | 24-1/2"− | 25-1/4"− | 30-5/8"− | 30.603" |
| 6 | 29-11/16" | 30-7/16" | 37-1/8"− | 37.098" |
| 7 | 34-7/8" | 35-5/8" | 43-5/8"− | 43.594" |
| 8 | 40-1/16" | 40-13/16" | 50-1/16"+ | 50.089" |
| 9 | 45-1/4"+ | 46"+ | 56-9/16"+ | 56.585" |
| 10 | 50-7/16"+ | 51-3/16"+ | 63-1/16"+ | 63.080" |
| 11 | 55-11/16"− | 56-7/16"− | 69-9/16" | 69.576" |
| 12 | 60-7/8"− | 61-5/8"− | 76-1/16" | 76.071" |
| 13 | 66-1/16" | 66-13/16" | 82-9/16" | 82.567" |
| 14 | 71-1/4" | 72" | 89-1/16" | 89.063" |
| 15 | 76-7/16" | 77-3/16" | 95-9/16" | 95.558" |
| 16 | 81-5/8"+ | 82-3/8"+ | 102-1/16" | 102.054" |
| 17 | 86-13/16"+ | 87-9/16"+ | 108-9/16" | 108.549" |
| 18 | 92-1/16"− | 92-13/16"− | 115-1/16"− | 115.045" |
| 19 | 97-1/4"− | 98"− | 121-9/16"− | 121.540" |
| 20 | 102-7/16" | 103-3/16" | 128-1/16"− | 128.036" |
| 21 | 107-5/8" | 108-3/8" | 134-1/2"+ | 134.531" |
| 22 | 112-13/16" | 113-9/16" | 141"+ | 141.027" |
| 23 | 118"+ | 118-3/4"+ | 147-1/2"+ | 147.522" |
| 24 | 123-3/16"+ | 123-15/16"+ | 154"+ | 154.018" |
| 25 | 128-7/16"− | 129-3/16"− | 160-1/2" | 160.513" |
| 26 | 133-5/8"− | 134-3/8"− | 167" | 167.009" |
| 27 | 138-13/16" | 139-9/16" | 173-1/2" | 173.504" |
Along-rail marks are the level distance ÷ cos 36.9° (× 1.2500).
| 1 | 3-5/8" | 4" | 4-9/16"− | 4.545" |
|---|---|---|---|---|
| 2 | 8"+ | 8-3/8"+ | 10"+ | 10.028" |
| 3 | 12-7/16"− | 12-13/16"− | 15-1/2" | 15.511" |
| 4 | 16-13/16"− | 17-3/16"− | 21" | 20.994" |
| 5 | 21-3/16" | 21-9/16" | 26-1/2"− | 26.477" |
| 6 | 25-9/16" | 25-15/16" | 31-15/16"+ | 31.960" |
| 7 | 29-15/16"+ | 30-5/16"+ | 37-7/16" | 37.443" |
| 8 | 34-5/16"+ | 34-11/16"+ | 42-15/16" | 42.926" |
| 9 | 38-3/4"− | 39-1/8"− | 48-7/16"− | 48.409" |
| 10 | 43-1/8" | 43-1/2" | 53-7/8"+ | 53.892" |
| 11 | 47-1/2" | 47-7/8" | 59-3/8" | 59.375" |
| 12 | 51-7/8" | 52-1/4" | 64-7/8"− | 64.858" |
| 13 | 56-1/4"+ | 56-5/8"+ | 70-5/16"+ | 70.341" |
| 14 | 60-11/16"− | 61-1/16"− | 75-13/16" | 75.824" |
| 15 | 65-1/16"− | 65-7/16"− | 81-5/16" | 81.307" |
| 16 | 69-7/16" | 69-13/16" | 86-13/16"− | 86.790" |
| 17 | 73-13/16" | 74-3/16" | 92-1/4"+ | 92.273" |
| 18 | 78-3/16"+ | 78-9/16"+ | 97-3/4" | 97.756" |
| 19 | 82-9/16"+ | 82-15/16"+ | 103-1/4" | 103.239" |
| 20 | 87"− | 87-3/8"− | 108-3/4"− | 108.722" |
| 21 | 91-3/8" | 91-3/4" | 114-3/16"+ | 114.205" |
| 22 | 95-3/4" | 96-1/8" | 119-11/16" | 119.688" |
| 23 | 100-1/8" | 100-1/2" | 125-3/16"− | 125.170" |
| 24 | 104-1/2"+ | 104-7/8"+ | 130-5/8"+ | 130.653" |
| 25 | 108-15/16"− | 109-5/16"− | 136-1/8" | 136.136" |
| 26 | 113-5/16"− | 113-11/16"− | 141-5/8" | 141.619" |
| 27 | 117-11/16" | 118-1/16" | 147-1/8"− | 147.102" |
| 28 | 122-1/16" | 122-7/16" | 152-9/16"+ | 152.585" |
| 29 | 126-7/16"+ | 126-13/16"+ | 158-1/16" | 158.068" |
| 30 | 130-13/16"+ | 131-3/16"+ | 163-9/16" | 163.551" |
| 31 | 135-1/4"− | 135-5/8"− | 169-1/16"− | 169.034" |
| 32 | 139-5/8" | 140" | 174-1/2"+ | 174.517" |
Along-rail marks are the level distance ÷ cos 36.9° (× 1.2500).
05 / Field guide
This sheet lays out a 12 ft (144") stair railing between post faces, measured level along a 7-1/2" × 10" stair. It compares 1-1/2" wood with 3/4" metal, gives every mark from one hook of the tape, and checks the 4-3/8" sphere rule and post spacing.
144" ÷ (1-1/2" + gap) fits 27 balusters with 28 equal gaps of 3-11/16" (3.696"), 5-3/16" on centre level and 6-1/2" along the rail.
From the post face the leading edges fall at 4-5/8", 11-1/8", 17-5/8", 24-1/8" and on in 6-1/2" steps. Buy 30 to allow 10% for splits and miscuts.
Thinner balusters need more of them: 32 at 3-5/8" gaps, 4-3/8" on centre (5-1/2" along the rail). The first four marks: 4-9/16", 10", 15-1/2", 21".
1" metal or composite takes 30 at 3-11/16"; 3-1/2" flat pickets take 20 at 3-1/2".
On a 7-1/2" rise and 10" run the rail climbs 108" over this 144" run and measures 180" along its length. Clear gaps are measured level, square to the plumb balusters; every tape mark along the rail is the level distance ÷ cos 36.9° = × 1.2500.
Cut fillet strips to 4-5/8" for wood or 4-9/16" for metal, with 36.9° on both ends. Keep the bottom rail within 1-1/2" of the nosing line so the tread-riser triangle stays under a 6" sphere.
With 4x4 posts the centres would be 12' 3-1/2" apart, past the 6'-0" DCA 6 limit for prescriptive guard posts. Add 1 intermediate post (2 bays of about 70-1/4" clear), or use a railing kit rated for this section length.
The rail sits 34" above the nosings (34" to 38" where it is also the handrail).
Questions
27 of 1-1/2" wood or 32 of 3/4" metal for 144" between posts, centred with equal gaps. Buy about 10% more.
3-11/16" clear between 1-1/2" balusters (5-3/16" on centre) or 3-5/8" between 3/4" balusters (4-3/8" on centre), measured level; along the sloped rail the centres are 6-1/2" and 5-1/2".
Its leading edge sits one gap from the post face: 4-5/8" for 1-1/2" wood or 4-9/16" for 3/4" metal, measured along the rail.
Under AWC DCA 6, yes: guard posts may be at most 6'-0" apart, and 144" clear on 4x4 posts is 12' 3-1/2" on centre. Railing kits rated for longer sections are the exception.
30 at 3-11/16" gaps; 3-1/2" flat pickets take 20 at 3-1/2".
Raised decks, basements and storeys