Parallel Pipe Cutback Calculations for Multi-Line Offsets

The Cutback Problem

When parallel pipes running at a constant spacing all need to offset at the same location (around a beam, through a wall penetration, or to a new elevation), each successive pipe’s travel piece must be shorter than the one before it. This length reduction is the cutback.

Without cutbacks, the parallel pipes would converge at the offset and lose their spacing. Cutbacks maintain the spacing through the offset so pipes arrive at the same parallel alignment on the other side.

The Formula

Cutback = Pipe Spacing × cos(fitting angle)

For 45° fittings: Cutback = Spacing × cos(45°) = Spacing × 0.7071 For 30° fittings: Cutback = Spacing × cos(30°) = Spacing × 0.8660 For 22.5° fittings: Cutback = Spacing × cos(22.5°) = Spacing × 0.9239

Worked Example: Three Parallel Pipes at 45°

Three pipes run parallel at 6-inch spacing (center to center). All three offset 10 inches vertically using 45° fittings.

First pipe (reference): Travel = 10 / sin(45°) = 14.14 inches. No cutback — this is the longest pipe.

Second pipe: Cutback = 6 × cos(45°) = 6 × 0.7071 = 4.24 inches. Travel = 14.14 - 4.24 = 9.90 inches.

Third pipe: Cutback from reference = 2 × 4.24 = 8.49 inches. Travel = 14.14 - 8.49 = 5.66 inches.

Each successive pipe’s travel is 4.24 inches shorter than the previous one — the cutback is constant because the spacing is constant.

Variable Spacing

When pipes aren’t equally spaced, calculate each cutback individually based on the distance between each pipe and the reference pipe. If pipe 1 is the reference, pipe 2 is 6 inches away, and pipe 3 is 10 inches away (4-inch gap between pipes 2 and 3):

Pipe 2 cutback: 6 × cos(45°) = 4.24 inches. Pipe 3 cutback: 10 × cos(45°) = 7.07 inches.

Always reference all cutbacks to the same pipe (the first or longest), not to the adjacent pipe, to avoid accumulating rounding errors.

Which Pipe Gets the Full Travel?

Convention: the pipe farthest from the obstruction (or the highest/outermost pipe) gets the full calculated travel length, and inner/lower pipes get progressively shorter. This ensures all pipes clear the obstruction while maintaining spacing.

Some fitters prefer to work from the inside out — cutting the shortest travel first and working outward. Either approach works as long as you’re consistent about which pipe is the reference.

Common Spacing Scenarios

Spacing	45° Cutback	30° Cutback	22.5° Cutback
4”	2.83”	3.46”	3.70”
6”	4.24”	5.20”	5.54”
8”	5.66”	6.93”	7.39”
10”	7.07”	8.66”	9.24”
12”	8.49”	10.39”	11.09”

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