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.
On the bender
Two 90s in one stick, and the whole trick is that the second one uses a different reference on the shoe from the first.
- Measure. The height of the first leg, and the distance between the backs of the two 90s — outside to outside of the finished U.
- Mark. Mark 1 at height minus take-up from the end — a stub mark, a line all the way round. Do not mark 2 yet.
- Line up. First bend: arrow on mark 1, hook toward the end you measured from. Pull a full 90 and check it stands square.
- Bend. Hook the tape on the back of the first 90 and mark the back-to-back distance along the pipe: that is mark 2. Turn the bender round so the hook faces the first bend, put the back-of-bend mark — the star or notch — on mark 2, keep the pipe in the same plane, and pull the second 90 the same direction so the legs come up parallel. The star places the back of the bend on the mark.
- Check. Lay the U on the floor: both legs parallel, and a tape across the outside of the two backs reads the distance you asked for. A constant error on every U means the second bend went on the arrow instead of the star, or mark 2 was measured from mark 1 instead of the back of the first 90.
Worked examples
24" (610 mm) between the backs, 12" (305 mm) first leg
Say you want 24" (610 mm) between the backs, a 12" (305 mm) first leg, and your bender is stamped 5 (5" (127 mm)). The first 90 is an ordinary stub: 12 − 5 = 7" (178 mm) from the end, arrow on the mark, bend up. Now hook the tape on the back of that 90 and mark 24" (610 mm) along the pipe. 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" (610 mm) apart. (Marking both first? Say this bender's gain is 2 5/8" (67 mm): the second mark goes at 12 + 24 − 2 5/8 = 33 3/8" (848 mm) from the end — not 7 + 24 = 31, which lands the U 2 3/8" (60 mm) narrow.)
600 mm between the backs, metric job
A metric drawing wants the backs 23 5/8" (600 mm) apart with a 11 13/16" (300 mm) first leg, and you have measured your bender's take-up for this pipe at 125 mm. First mark: 300 − 125 = 6 7/8" (175 mm) from the end, arrow, bend. Second mark: hook the tape on the back of that 90 and mark 23 5/8" (600 mm) along the pipe, star on the mark, bend the same way. The backs finish 600 apart — no multiplier to round, so the millimetres carry straight through. With a measured gain of 65 mm you could mark both first: 300 + 600 − 65 = 835 mm from the end.
A 4' 0" (1219 mm) U between two boxes on a wall
Two boxes sit 4' 0" (1219 mm) apart, outside edge to outside edge, and the run drops from one, along a ledge, and up into the other. The first leg into the box is 18" (457 mm); say this bender is stamped 6. First mark: 18 − 6 = 12" (305 mm) from the end, arrow, bend. Second mark: 4' 0" (1219 mm) from the back of that 90, star on the mark, bend the same way. Before you bend anything, check the stick: the pipe runs about first leg + distance + second leg, less the gain of each 90, so with an 18" (457 mm) second leg this U takes a little under 7' 0" (2.13 m) of pipe. On a 10' 0" (3.05 m) stick that is plenty; on an offcut it may not be, and it is better to find out with a pencil than a bender.
When the U has to fit a rack, not a wall
A drop over a rack wants the backs 15 1/2" (394 mm) apart so both legs clear the rack sides, first leg 10" (254 mm), bender stamped 5 1/4. First mark 10 − 5 1/4 = 4 3/4" (121 mm); second mark 15 1/2" (394 mm) from the back of the first 90. The fractions carry straight through — there is no multiplier in a back-to-back to round, so the shop rule and the exact figure are the same thing here.
Try one — a back-to-back
Work it on paper, then type the marks. A sixteenth either way counts — that is as close as a tape reads. Two millimetres either way counts.
Loading a bend…
Escriba pulgadas como 12, 1 5/8 o 1' 3 1/2".
What went wrong
The backs are off by the same amount every time
What you see: a clean U, but the distance between the backs is wrong — and every one you make is wrong by the same amount, in the same direction.
Why: the second 90 was bent with the arrow on the second mark instead of the star. The arrow locates the start of a bend; the star locates its back. A constant error is always a fixed distance on the tool, and this one is the gap between those two marks on your shoe — not the take-up, not the gain, a figure of its own.
Fix: arrow for the first bend, star (or notch, whatever your shoe calls its back-of-bend reference) for the second. Say it out loud if you have to. If the shoe has no back-of-bend mark at all, bend one U on a scrap with the arrow on both marks, measure it, and carry that difference as this shoe's arrow-to-star gap.
A Z instead of a U
What you see: the two legs point opposite ways.
Why: the pipe was turned over between the bends, the way it would be for an offset. A back-to-back is the one bend where you do not flip the pipe.
Fix: keep the pipe the same way up in the shoe for both pulls. A pencil line along the pipe stays on top for both.
The second leg is too short to reach
What you see: the layout is right, the U is right, and the second leg finishes below the box.
Why: nobody checked the stick length before bending. The two legs and the distance between them were more pipe than the offcut had in it.
Fix: add first leg + distance + second leg before the first pull (the gain of the two 90s comes off that, so it is a little less). If it does not fit, use a full stick, or cut the first leg long and couple the second.
The formula
The first mark is the stub mark; the second is measured from the back of the first 90, or pre-marked with your gain:
first mark = height − take-up · second mark = distance from the back of the first 90 (or height + distance − gain from the end)
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, a mark that distance from the back of the first 90 puts the second back exactly there. What does not work is measuring it from mark 1 on the straight stick: the back of the first 90 ends up take-up − gain past its mark (the curve uses less pipe than the two legs to the corner), so "mark 1 + distance" makes the U narrow by that much. No multiplier, no shrink.
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.
Which bender these numbers assume — and how to adjust
The geometry belongs to the angle, not the tool. What the tool changes — its reference marks, its take-up and gain, its springback — is yours to read off the shoe or prove on scrap, never a table here (the four test bends). For this bend:
These numbers assume two references on the shoe: the arrow for the first 90 and the back-of-bend mark (a star on most hand benders, a notch on some) for the second, so the second mark is the finished distance between the backs with nothing to add. If your shoe has no back-of-bend mark, bend the second 90 on the arrow instead and correct the second mark by the gap between the arrow and the back of the bend on that shoe — a fixed distance on the tool, like take-up, and one you get by laying out a single U on a scrap, bending both 90s on the arrow, and measuring what actually comes out.
Preguntas
- 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. Hook your tape on the back of the first 90, mark the distance along the pipe, line that mark up with the star 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?
- No shrink — there is no offset in it. But if you mark both bends on the straight stick before bending, the second mark is not the first mark plus the distance: the back of the first 90 ends up past its mark by the take-up less the gain. Either measure the second mark from the back of the first 90 after bending it, or pre-mark it at height + distance − gain from the end.
- 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.