How do you calculate a rolling offset?

First find the true offset: the square root of rise squared plus roll squared. Then divide the true offset by the sine of the fitting angle to get the travel. For a rise of 12 inches, a roll of 9 inches and 45 degree fittings, the true offset is 15 inches and the travel is 21.21 inches.

What is the difference between travel and run?

Travel is the length of the angled piece, measured center to center between the two fittings. Run is how far that piece moves forward along the straight line of the pipe. The run equals the true offset divided by the tangent of the fitting angle.

How do I find the cut length of the pipe?

Subtract the take-off of each fitting from the travel. Take-off is the part of the fitting that does not count toward the pipe, which is the center to the bottom of the socket for glued or soldered fittings. Pick a pipe type and size in the tool and it does the subtraction for you. The table values come from maker catalogs for 45 degree fittings. For other angles, type your own take-off.

How do I use this for EMT conduit?

Switch to EMT conduit. Pick the bend angle and the tool shows the multiplier, the distance between bends, the shrink and the tilt of the bend plane. Both bends lie in that one plane. Make the first bend, turn the conduit 180 degrees, and make the second bend at the second mark.

What is shrink and why does it matter?

When you bend an offset, the conduit gets shorter along its length. That loss is the shrink. For each inch of offset it is about 3/8 inch at 45 degrees, 1/4 inch at 30 degrees and 3/16 inch at 22.5 degrees. Add the total shrink to your length before you cut.

Which fitting angle should I use?

Use 45 degrees for most pipe work because 45 degree fittings are easy to find and give a short run. Use 22.5 degrees for a long, gentle offset. Use 60 degrees when you need a short, steep offset. In conduit, 30 degrees and 45 degrees are the most common bender angles.

Does this work on my phone?

Yes. The buttons are large, the answers update as you type, and the fraction buttons set 1/8 inch steps without a full keyboard, and you can type feet and inches. Nothing you type leaves your device.

What is the multiplier for a 45 degree rolling offset?

It is 1.414. Multiply the true offset by it to get the travel, or the distance between bends in conduit. A 30 degree bend uses 2.0, and a 22.5 degree bend uses 2.613.

How do I measure rise and roll?

Measure from the center line of the first pipe or conduit to the center line of the second. The rise is the vertical distance. The roll is the sideways distance. Measure to the center, not to the edge.

Rolling Offset Calculator for Pipe and EMT Conduit

What is a rolling offset?

A rolling offset moves a run of pipe or conduit up and sideways at the same time. The run ends on a new line that is parallel to the old one. You need it when a beam, a duct, a wall or another pipe is in the way and the path is not flat or square. This rolling offset calculator gives you every number you need to lay it out: the true offset, the travel, the run and the cut length.

What each number means

Rise is the vertical distance between the two center lines.
Roll is the sideways distance between the two center lines.
True offset (the set) is the straight distance between the center lines. It is the diagonal across the rise and the roll.
Travel is the center to center length of the angled piece between the two fittings.
Run is how far the offset moves along the pipe. It is the length the offset uses up on the straight line.

How to use the calculator

Measure the rise and the roll from center line to center line, in inches.
Type them in. Use decimals such as 12.5, or fractions such as 12 1/2. You can also type feet and inches, such as 2'-3 1/2". The fraction buttons set the fraction on the field you used last.
Pick the fitting angle. For pipe: 22.5, 45 or 60 degrees. For conduit: 10, 22.5, 30, 45 or 60 degrees. You can also type your own.
For pipe, pick the pipe type and size to subtract the fitting take-off. The tool then shows the cut length.
For EMT, switch to conduit mode and pick the bend angle. The tool shows the distance between bends and the shrink.

Which fitting angle should you use?

Most plumbers use 45 degree fittings. They are easy to stock, and they keep the offset short. A 22.5 degree fitting makes a long, gentle offset that is good for drain lines. A 60 degree fitting makes a short, steep offset when space is tight, but it slows the flow more. A smaller angle always makes a longer travel for the same offset. Try each angle in the tool and compare the run.

Rolling offset formula

The math is right-triangle geometry. These are the steps the calculator follows.

True offset = the square root of (rise squared + roll squared)
Travel = true offset / sin(angle)
Run = true offset / tan(angle)
Cut length = travel - 2 x take-off

Worked example

A copper line must clear a duct. The rise is 12 inches and the roll is 9 inches. You use 45 degree fittings.

True offset = the square root of (144 + 81) = 15 inches.
Travel = 15 / sin 45 = 21.21 inches, or about 21 3/16 inches.
Run = 15 / tan 45 = 15 inches.
The 3/4 inch copper 45 degree fitting has a take-off of 11/32 inch (0.344). Cut length = 21.21 - 2 x 0.344 = 20.53 inches, or about 20 1/2 inches.

Common rise and roll values

These pairs give a whole number for the true offset. The travel is for 45 degree fittings.

True offset and travel for common rise and roll pairs at 45 degrees
Rise	Roll	True offset	Travel at 45°
3	4	5	7.07
6	8	10	14.14
5	12	13	18.38
12	9	15	21.21
8	15	17	24.04
24	7	25	35.36

Fitting take-off chart

Take-off is the distance from the center of the fitting to the bottom of the socket (for butt weld, to the weld end). The calculator subtracts it twice, once for each fitting. These are catalog values for 45 degree fittings, in inches. Other makers differ, so measure the fitting in your hand on jobs where an error costs money.

Take-off per 45 degree fitting by pipe size, in inches
Pipe size	PVC Sch 40	Copper, solder	Steel, butt weld
1/2"	11/32 (0.344)	9/32 (0.281)	-
3/4"	7/16 (0.438)	11/32 (0.344)	-
1"	15/32 (0.469)	3/8 (0.375)	7/8 (0.875)
1 1/4"	-	17/32 (0.531)	1 (1.000)
1 1/2"	19/32 (0.594)	19/32 (0.594)	1 1/8 (1.125)
2"	11/16 (0.688)	25/32 (0.781)	1 3/8 (1.375)
2 1/2"	-	29/32 (0.906)	1 3/4 (1.750)
3"	27/32 (0.844)	1 1/8 (1.125)	2 (2.000)
4"	1 3/32 (1.094)	1 9/16 (1.563)	2 1/2 (2.500)
6"	-	-	3 3/4 (3.750)

PVC Sch 40: Spears series 417, 45° socket elbow, column J. Spears PVC Sch 40 fittings catalog (parts 417-005 to 417-100, column J).
Copper, solder: NIBCO 606, 45° wrought elbow, dimension C. NIBCO 606 wrought copper 45° elbow, dimension C (catalog page 11, revision 5/9/2013).
Steel, butt weld: ASME B16.9 long radius 45° elbow, center to end. Texas Flange table, 45° long radius elbow (inches). Wermac table, elbow dimension A (mm).

The copper 1 inch value (3/8) is as NIBCO prints it, but it sits low between the 3/4 and 1-1/4 inch values. Measure that fitting before you rely on it. PVC 1-1/4 and 2-1/2 inch are not listed because the printed catalog values did not fit the size pattern. Steel at angles other than 45 degrees is scaled from the 45 degree value, so treat it as approximate. Butt weld cut lengths also lose about 1/8 inch per joint to the root gap.

Rolling offset in EMT conduit

Conduit is bent, not fitted, so the rules change a little. The bender angle sets two numbers. The multiplier is one divided by the sine of the angle. Multiply the true offset by it to get the distance between the two bends. Shrink is the length the conduit loses when you bend it. It equals the true offset times the tangent of half the angle.

Multiplier and shrink per inch of offset by bend angle
Bend angle	Multiplier	Shrink per inch of offset
10°	5.76	0.087 in
22.5°	2.61	0.199 in (about 3/16)
30°	2.00	0.268 in (about 1/4)
45°	1.41	0.414 in (about 3/8)
60°	1.15	0.577 in

Example: the true offset is 15 inches and you bend at 30 degrees. The bends go 30 inches apart. The shrink is about 4 inches. Many field charts round the 60 degree shrink to 1/2 inch per inch of offset. The tool shows the exact value, 0.577.

How to bend a rolling offset in EMT

Both bends of a rolling offset lie in one plane. That plane is tilted from the rise (vertical) direction by an angle that depends on the rise and the roll. The tool shows it as "Bend plane tilt from rise".

Measure the rise and the roll from center line to center line.
Switch the tool to EMT conduit and pick the bender angle.
Read the distance between bends, the total shrink and the bend plane tilt.
Mark the first bend on the conduit. Mark the second bend the distance between bends from the first one.
Make bend 1 with the bend plane at the tilt shown.
Turn the conduit 180 degrees, so the second bend lies in the same plane, and make bend 2 at the second mark.
Add the shrink to your overall length before you cut, then check the shape against the job.

Tips for a clean job

Measure to the center of the pipe, not to the edge. For conduit, measure to the center of the tube.
Use the same fitting angle on both ends of the offset.
Check the take-off against the fittings in your hand. Sizes differ between makers.
Dry fit the piece before you glue, solder or weld.
If the cut length comes out zero or negative, the fittings are too large for that offset. Use a smaller angle or a longer rise and roll.

Rolling Offset Calculator