What is a back-to-back bend?

Two 90° bends in one length of conduit, both turned the same way, so the pipe makes a squared-off U. The distance between the backs of the two bends is the dimension you are setting — a run down one wall, across, and up the other, for instance.

Why bend the second 90 off the back-of-bend mark?

Because the dimension you care about is between the backs of the two bends, not their arrows. Most benders have a second reference — often a star or a notch — that sits at the back of a 90. Line your second mark up with that reference and bend, and the back of the second 90 lands exactly your distance from the back of the first.

Do I need to add shrink to a back-to-back?

Not to the back-to-back distance itself — that is set by the two marks and the back-of-bend reference, and it comes out true. You do still account for take-up on each 90 when you work the leg heights, which the calculator does when it places the first mark.

Which way do I bend the second 90?

The same direction as the first, so both legs point the same way and you get a U rather than a Z. Keep the conduit oriented the same in the bender for both pulls; flipping it turns the second bend the wrong way.

Back-to-back bend calculator

A back-to-back bend puts two 90s in one stick so the pipe drops, runs, and rises again as a squared U. Enter the distance you want between the backs of the two bends, the height of the first leg, and your bender take-up; the calculator gives both marks measured from the end and tells you which bender reference to use for the second.

Worked example

Say you want 24 inches between the backs, a 12 inch first leg, and your take-up is 5 inches. The first 90 is an ordinary stub: 12 − 5 = 7 inches from the end, arrow on the mark, bend up. The second mark goes 7 + 24 = 31 inches from the end. Line that mark up with the back-of-bend reference on the shoe and bend the second 90 the same way, and the two backs sit exactly 24 inches apart.

The formula

The first mark is the stub mark; the second is that plus the back-to-back distance:

first mark = height − take-up · second mark = first mark + back-to-back distance

The first bend is a plain stub-up, so its mark is the leg height less the take-up the curve eats. The back-to-back distance is measured between the two backs, and because you bend the second 90 off the back-of-bend reference rather than the arrow, the along-the-pipe distance between the two marks is exactly that distance. No multiplier, no shrink — just two subtractions and a sum.

The one thing to get right

The second bend uses a different reference from the first. The arrow places a stub; the back-of-bend mark — a star or a notch, depending on the bender — places the heel of a 90. Use the arrow for the first bend and the back-of-bend mark for the second, keep the pipe oriented the same, and the U comes out true. Mixing the two references is the usual reason a back-to-back finishes long or short.

Questions

What is a back-to-back bend?: Two 90° bends in one length of conduit, both turned the same way, so the pipe makes a squared-off U. The distance between the backs of the two bends is the dimension you are setting — a run down one wall, across, and up the other, for instance.
Why bend the second 90 off the back-of-bend mark?: Because the dimension you care about is between the backs of the two bends, not their arrows. Most benders have a second reference — often a star or a notch — that sits at the back of a 90. Line your second mark up with that reference and bend, and the back of the second 90 lands exactly your distance from the back of the first.
Do I need to add shrink to a back-to-back?: Not to the back-to-back distance itself — that is set by the two marks and the back-of-bend reference, and it comes out true. You do still account for take-up on each 90 when you work the leg heights, which the calculator does when it places the first mark.
Which way do I bend the second 90?: The same direction as the first, so both legs point the same way and you get a U rather than a Z. Keep the conduit oriented the same in the bender for both pulls; flipping it turns the second bend the wrong way.

Related tools

90-degree stub-up calculator — each 90 in the U is a stub.
Bender take-up — find the take-up figure this uses.
Kick 90 calculator — a 90 with a kick in the tail.