Bending multipliers & shrink
| Angle | Multiplier (1/sin θ) | Field rule | Shrink per inch of rise | Field rule |
|---|---|---|---|---|
| 10° | 5.759 | 6 | 0.0875" | 1/16" |
| 22.5° | 2.613 | 2.6 | 0.1989" | 3/16" |
| 30° | 2.000 | 2 | 0.2679" | 1/4" |
| 45° | 1.414 | 1.4 | 0.4142" | 3/8" |
| 60° | 1.155 | 1.15 | 0.5774" | 1/2" |
Distance between the two bends = rise × multiplier. Shrink = rise × the shrink figure; add it to your measurement so the fitting lands where you wanted it.
Multiplier = 1 ÷ sin θ. Shrink per unit of rise = 1 ÷ sin θ − 1 ÷ tan θ. Both are pure trigonometry — nothing here is code-book data. Bender take-up is stamped on your tool; read it there.
The field rules are memorised roundings, and they drift. The multiplier columns stay close at every angle, but the shrink rules do not: 1/16" at 10° is 0.025" per inch of rise short of the computed 0.0875", and 1/2" at 60° is 0.077" per inch short of 0.5774". On a 10° offset with a 12" rise that is about 5/16" of shrink you never allowed for; on a 60° offset with a 6" rise, about 7/16". Use the rule for a quick offset in a short run; use the computed figure when the run is long, when several offsets stack, or when the pipe has to land in a fitting.
Print this small and it lives in the pouch. Practise the maths on the offset drill, and when you are at the bender working a real pull, bentright.com runs the numbers.
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