Conduit bending is an aspect of electrical work that I have finally become more comfortable with. I found a great site that explains the math of bending really well.
The photo above shows an offset bend. The conduit here must enter the existing hole in the box, which is not flush with the wall. An offset bend is a three part process.
1) Measure the distance between the first plane and the second plane, center of conduit to the center of the whole or space to be entered. In the case of the photo shown above, our distance is 1 1/2".
2) Determine the angle of the bend. For small offsets like the one shown here, a typical angle used is 22.5 degrees.
3) Find the 'cosecant' of the determined angle (yes! We are using geometry, my high school math teacher would be soo proud). In this case, the cosecant of a 22.5 degree angle bend is 2.6. The cosecant is multiplied by the size of the offset (the distance between the two planes found in the first step). If the size of the offset is 1 1/2", then we multiply 1.5 times 2.6= 3.9". This is the distance between our two 22.5 degree bends. For an excellent table and illustration of this process, check out this page.
In my experience, the offset bend is the most common bend used, so it is important to get this process down by heart. The other bend I use all the time is a basic 90 degree bend. This is a two step process:
1) Find the correct 'deduction' for the size conduit you are bending. Most hand benders for 1/2" to 1" conduit come with an arrow mark or 'B' mark built into the bender. This mark is placed at the beginning of the 90 degree bend.
2) Lay out the conduit and mark the beginning of the bend, having deducted the correct number of inches from the center of the 90 degree bend. For example, if you are bending 3/4" conduit and the 90 degree bend needs to be 32" from the end of the conduit, we will find that the deduction for 3/4" conduit is 6". 32-6 = 26". We place the arrow marker on the conduit bender at the 26" mark....and bend! Keep bending until it looks like the conduit is a perfect, or close to perfect right angle. You can place a bubble level on the conduit piece sticking up in the air to make sure that it is level.
Here is a photo of a hand bender:
