Two displacements, one diagonal piece
A rolling offset is the pipefitting situation where a new pipe centerline must move in two directions at once — over (the roll) and up or down (the offset) — to reach its continuation. Seen end-on, the displacement is the diagonal of a rectangle whose sides are the roll and the offset; that diagonal is called the true offset. A simple offset moves in only one plane; a rolling offset 'rolls' the fitting out of plane to cover both displacements with a single diagonal piece of pipe.
The formula: true offset, travel and run
The true offset is the hypotenuse of the roll and offset seen end-on: √(roll² + offset²). The travel divides the true offset by the sine of the fitting angle — sin 45° ≈ 0.7071, sin 30° = 0.5, sin 60° ≈ 0.8660 — and the run divides it by the tangent, giving the along-pipe length the offset consumes. The multipliers are trade constants: at 45° the travel is 1.414 × the true offset; at 30° it is 2.000 × the true offset; at 60° it is 1.155 ×.
- True offset = √(Roll² + Offset²)
- Travel = True offset ÷ sin(fitting angle)
- Run = True offset ÷ tan(fitting angle)
Worked example: 30/40 cm offset with 45° fittings
A 30 cm roll with a 40 cm offset gives a true offset of √(30² + 40²) = √(900 + 1600) = √2500 = 50 cm — the classic 3-4-5 right triangle scaled up by 10. With 45° fittings, the travel is 50 ÷ sin 45° = 50 ÷ 0.7071 ≈ 70.71 cm and the run equals 50 cm. With 30° fittings the travel would be 100 cm instead, and with 60° fittings 57.74 cm.
| Fitting angle | Travel multiplier | Travel (50 cm true offset) |
|---|---|---|
| 45° | 1.414 | 70.71 cm |
| 30° | 2.000 | 100.00 cm |
| 60° | 1.155 | 57.74 cm |
Travel is center-to-center — remember the take-off
Travel is a center-to-center dimension between the fittings, not the length to cut the physical pipe piece. The actual cut length subtracts the fitting allowance (fitting take-off) at each end — the distance from the fitting's center to where the pipe seats — which depends on the fitting size, type and joining method, and comes from the fitting manufacturer's take-off tables.
45° fittings are the general-purpose default, balancing travel length against the sharpness of the direction change. 30° fittings turn more gently — favored where flow resistance matters or space allows — but need the longest travel; 60° fittings fit the offset into the shortest space at the cost of sharper turns. The geometry is identical for any piping material; what varies by trade is the joining allowance, not the trigonometry.
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How do you calculate a rolling offset?
First combine the roll and offset into the true offset with the Pythagorean theorem: true offset = √(roll² + offset²). Then divide by the sine of the fitting angle to get the travel — the center-to-center length of the diagonal piece. A 30/40 cm rolling offset has a 50 cm true offset and, with 45° fittings, a 70.71 cm travel.
What is the 45 degree offset multiplier?
1.414 — the reciprocal of sin 45°. Multiply the true offset by 1.414 to get the travel with 45° fittings. For 30° fittings the multiplier is 2.0, and for 60° fittings 1.155.
What is the difference between travel, run and true offset?
True offset is the straight-line displacement between the two pipe centerlines (combining roll and offset). Travel is the length of the diagonal pipe piece, center-to-center of its fittings. Run is the length the offset assembly consumes along the original pipe direction — true offset ÷ tan(angle).
Do I still subtract fitting allowances from the travel?
Yes. Travel is a center-to-center dimension; the physical pipe piece is cut shorter by each fitting's take-off, which depends on fitting size and joining method. Manufacturers publish take-off tables for their fittings.
संदर्भ
- Graves, Thomas W. — Pipe Fitter's and Pipe Welder's Handbook: rolling offset formulas, travel multipliers and fitting take-off practice.
- IPT's Pipe Trades Handbook (IPT Publishing) — offset and rolling-offset trigonometry tables for standard fitting angles.
- Standard trigonometry — Pythagorean theorem and right-triangle sine/tangent relationships underlying the true offset, travel and run formulas.