Back-to-Back 90 Calculator
A back-to-back 90 is two 90-degree bends in opposite directions on the same conduit, forming a U shape. It is one of the most common bends for transitioning conduit between two parallel surfaces. Enter the outside-to-outside distance and your conduit specs to get the mark distance and total deduct.
How to Make a Back-to-Back 90
First, make your first 90-degree bend as a standard stub-up. Then measure the outside-to-outside distance between the two surfaces and subtract twice the deduct. Mark that distance from the back of the first bend and make the second bend in the opposite direction. The result is a clean U-shaped conduit that fits perfectly between the two surfaces.
Mark Distance = Outside Distance − (2 × Deduct)
Two deducts are subtracted because each 90 consumes one deduct
Deduct Values by Conduit Size
| Conduit Size | EMT Deduct | Double Deduct |
|---|---|---|
| 1/2" | 5" | 10" |
| 3/4" | 6" | 12" |
| 1" | 8" | 16" |
| 1-1/4" | 11" | 22" |
| 1-1/2" | 13" | 26" |
Frequently Asked Questions
What is a back-to-back 90 bend?
A back-to-back 90 is two 90-degree bends made in opposite directions on the same piece of conduit, creating a U shape. It is commonly used when conduit needs to go up a wall, across a ceiling, and back down another wall, or when transitioning between two parallel surfaces.
How do I calculate the mark distance for a back-to-back 90?
Measure the outside-to-outside distance between the two surfaces. Subtract twice the deduct for your conduit size and bender to get the mark distance. The mark distance is the space between the two bend marks on the conduit. For example, with a 24-inch distance and a 5-inch deduct, the mark distance is 24 minus 10, which equals 14 inches.
Why do I subtract two deducts for a back-to-back 90?
You subtract two deducts because each 90-degree bend consumes one deduct worth of conduit. Since a back-to-back 90 has two bends, the total deduct is doubled. This accounts for the conduit taken up by both bends and ensures the finished piece fits the measured distance exactly.