FabLayout

Back to back 90 conduit calculator

Enter the first stub height, the back-to-back distance, your take-up and centre-line radius. You get both marks, from the bend or from the end, and the shortest pipe it needs.

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Back to backBack to back3636″″1122
Scale drawing

First mark from the end

7in

Exact geometry

Second mark from the back of the first
In the app
Second mark from the end
In the app
Shortest pipe for this
In the app

Every result is in Truemark

The free web version shows the main result and the drawing. Truemark gives you every mark and measurement, and exports a cut sheet. It is free to download.

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How to lay it out

  1. Bend the first 90°

    Mark the first stub height minus the take-up from the end. Put the arrow on the mark, conduit under the hook, and bend to 90°.

  2. Measure from the back of the first bend

    Measure the back-to-back distance from the back of the finished 90° and wrap the mark.

  3. Bend the second 90° on the star

    Put the star (back of 90°) on the mark, hook toward the free end, and bend to 90°. No star on your bender: mark the distance minus the take-up instead, turn the hook toward the first bend and use the arrow.

  4. Or mark both on straight pipe

    To mark both before bending, use the second mark from the end. It includes the arc of the first bend.

Good to know

  • FabLayout runs the same code as Truemark; the result is exact geometry.
  • Source: Greenlee 840A instruction manual and Use Guide 52034125; Klein Tools Conduit Bender Guide 56203–56211; Gardner Bender Hand Bender How-To

Method and formula

A back-to-back is two 90° bends on one pipe with a set distance between their backs, as in a U from a tray down a wall and back up. The field method needs no radius at all.

  • First mark = first stub height − take-up, from the end, on the arrow.
  • Second mark from the back of the first = the back-to-back distance, on the star. Without a star: back-to-back − take-up, on the arrow with the hook toward the first bend.
  • Second mark from the end, on straight pipe = first mark + ¼ arc of the centre-line radius (πR ÷ 2) + back-to-back − take-up.
  • Shortest pipe = first mark + πR + back-to-back − 2 × take-up, with the second leg as short as the take-up.

The back-to-back distance cannot be less than two take-ups: the two bends would overlap.

Worked example

The page defaults: a 12″ first stub, 36″ back to back, 5″ take-up and a 4⅝″ radius. The first mark is 12 − 5 = 7″. After that bend, mark 36″ from its back. On straight pipe, the second mark is 7 + πR ÷ 2 + 36 − 5 = 45.2649″ from the end, and the shortest pipe is 47.5299″.

Questions

Why is the second mark from the end not just first mark plus back-to-back?

The first 90° turns part of the pipe into an arc. Measured on straight pipe, the second mark has to include the quarter arc of the centre-line radius, minus the take-up.

What if my bender has no star?

Measure the back-to-back distance minus the take-up from the back of the first bend, turn the hook toward the first bend and bend on the arrow.

What is the shortest back-to-back I can bend?

Two take-ups. Each 90° ends one take-up inside its back, so at two take-ups there is no straight between the bends, and anything shorter overlaps.

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