How this calculation works
Stubs and take-up
A bender does not turn the pipe at a point. It turns it over an arc, and that arc consumes length. Take-up is how much. Subtract it from the stub height you want and that is where the mark goes.
Common EMT take-up: 5" on 1/2", 6" on 3/4", 8" on 1", 11" on 1-1/4". Rigid and IMC take more because the bend radius is larger.
Offsets and multipliers
An offset is two equal bends in opposite directions. The distance between them is the obstacle height times a multiplier that comes from the angle.
The multiplier is the cosecant of the angle. At 30 degrees the cosecant is exactly 2, which is why 30 is the angle everybody defaults to — the math is doable on a ladder.
Shrink, the thing that gets forgotten
Bending an offset pulls the end of the pipe back toward you. If you cut to your original measurement and then bend, the pipe comes up short. Shrink per inch of rise: about 1/16" at 10 degrees, 3/16" at 22.5, 1/4" at 30, 3/8" at 45, and 1/2" at 60.
A 6 inch rise at 30 degrees shrinks about 1-1/2 inches. On a run to a fixed connection, that is the difference between landing in the box and coming up short of it.
Three-point saddles
A saddle crosses an obstacle and comes back to the original line. The center bend is twice each outer bend — a 45 degree center with two 22.5 degree outers is the standard. Mark the center of the obstacle, then measure out to each side by the obstacle height times 2.5 for a 45 degree center.
Above all this sits one hard limit: 360 degrees of bend between pull points. Four 90s, or any combination that adds to a full circle, and you need a box.
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Frequently asked questions
Why is my stub always short?
Take-up. The bend consumes pipe over its arc, so you have to subtract the take-up from the height you want before you mark. If your stubs are short by exactly the take-up figure, you are marking at the finished height instead of the deducted one.
What is shrink and do I have to account for it?
Shrink is the length an offset pulls back from the straight-line run. Yes, account for it any time the pipe has to land somewhere specific. At 30 degrees it is about a quarter inch for every inch of rise, so a 6 inch offset costs you an inch and a half.
Why does everybody use 30 degrees?
The multiplier is exactly 2, which you can do in your head on a ladder. It also produces a gentle enough bend to pull through easily. Steeper angles like 45 and 60 take less length but make the pull harder and add more shrink.
How many bends can I put between boxes?
The equivalent of four quarter bends — 360 degrees total — between pull points. Offsets count. Two 30 degree offsets and a 90 is 150 degrees, so you have room for one more 90 and not much else.
Are these take-up numbers right for my bender?
They are typical values, not universal ones. Take-up varies between manufacturers and between shoe designs. Check the marks stamped on your bender, and bend a test piece before committing a full stick of large pipe.
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