How a pipe offset is calculated
An offset is a right triangle. The offset is the side across the two pipe lines, the run is the side along them, and the travel is the angled piece you cut. The fitting angle fixes the triangle's shape, so one multiplier per angle turns the offset into the travel and another turns it into the run.
true offset = √(offset² + roll²) (rolling offsets only)
travel = offset × travel multiplier = offset ÷ sin (fitting angle)
run = offset × run multiplier = offset ÷ tan (fitting angle)
cut length = travel − 2 × fitting take-off
Measure the offset center to center, from the centerline of one pipe to the centerline of the other. Travel and run come out center to center too, measured between the centers of the two fittings.
At 45° the two short sides of the triangle are equal, which is why the run equals the offset and the travel is the offset times √2. Steeper fittings shorten the run; shallower ones stretch both the run and the travel. The table gives the multipliers for every angle the calculator offers, so you can check a result by hand with a tape and a phone calculator.
| Fitting angle | Travel multiplier | Run multiplier |
|---|---|---|
| 45° | 1.4142 | 1.0000 |
| 60° | 1.1547 | 0.5774 |
| 30° | 2.0000 | 1.7321 |
| 22.5° | 2.6131 | 2.4142 |
| 11.25° | 5.1258 | 5.0273 |
Worked example: 18 in offset with 45° fittings
The calculator opens on a 18 in offset made with 45° fittings and no take-off.
- Travel multiplier for 45°: 1 ÷ sin 45° = 1.4142.
- Travel: 18.000 × 1.4142 = 25.456 in, or 25-7/16 in to the nearest 1/16.
- Run: 18.000 × 1.0000 = 18.000 in, so the offset advances 18 in along the line.
- Cut length: with no take-off entered it equals the travel, 25-7/16 in. Enter your fitting's take-off under advanced options to get the actual piece to cut.
Measuring a rolling offset
A rolling offset moves the pipe in two directions at once, for example up and over to clear a beam and a duct. Measure the two offsets square to each other: the set (vertical) and the roll (horizontal), both center to center. A framing square or two levels and a tape make this easy.
For a 12 in set and a 9 in roll, the true offset is √(12² + 9²) = 15 in. At 45°, the travel is 15.000 × 1.4142 = 21.213 in (21-3/16 in) and the run is 15 in. Roll the two fittings so the travel piece points along the diagonal of the box formed by the set and the roll.
- Take every measurement from centerline to centerline, not from the outside of the pipe.
- Use the same fitting angle at both ends of the travel, or the two pipe runs will not end up parallel.
- Check the run against the space you have. A shallow offset needs much more room along the line than a 45° offset.
Fitting take-off and cut length
Travel is measured between fitting centers, but the pipe stops short of each center. Take-off is the distance from the fitting's center to the point where the pipe end stops: the face of a butt-weld fitting, or the bottom of the socket on a glued or soldered fitting. For threaded fittings it is the center-to-end dimension minus the thread engagement. Read it from the manufacturer's chart for your exact size, material and fitting pattern; it is not the same for every brand.
For example, a 10 in offset with 22.5° fittings has a travel of 26.131 in. With a take-off of 1-1/2 in at each end, the cut length is 26.131 − 2 × 1.500 = 23.131 in, or 23-1/8 in. Its run is 24-1/8 in, 2.4 times the offset.
If the calculator reports that the take-off is larger than the travel, the offset is too small for those fittings at that angle. Try a steeper fitting angle or a different fitting pattern.
Sources
- Right-triangle trigonometry: travel = offset ÷ sin θ, run = offset ÷ tan θ
- Rolling offset: true offset = √(offset² + roll²) (Pythagorean theorem in the plane of the two offsets)
- Standard fitting angles 45°, 60°, 30°, 22.5° and 11.25°; multipliers computed in full precision
- Fitting take-off (center to face): manufacturer fitting dimension charts for the pipe size and material in use