Calculators/Rolling offset

Rolling offset calculator

When a run has to rise and roll at the same time: the true offset, the travel between fittings, and the run it consumes along the original line.

The offset

Centre-line geometry. Fitting take-off still has to come off the travel — how much depends on the fitting and the joint, and it is not something this can know.

The travel

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Check the result against the job. This does the arithmetic from the values you enter. Building codes, span tables and material specifications vary by jurisdiction and by product, and the governing document is your approved drawings and your local code. Nothing you type here leaves your browser: the calculation runs on your device and we never receive it.

How this is checked. Every formula on this site runs from one shared calculation engine, and every one has a test fixture whose expected answer was worked out from the geometry or taken from a figure the trade already publishes — never from what the code returns. Some land exactly, which is the point: a 6/12 hip over a 144″ run is exactly 216″, a 45° crown in a square corner bevels at exactly 30°, and a 30° conduit bend has a multiplier of exactly 2. Where a number is a convention rather than arithmetic — a code limit, a waste allowance, bundles per square — it is an input with a stated default rather than a hidden constant. Found something wrong? Tell us at [email protected] and we will fix it and say so.

Two moves become one

A rolling offset looks like two problems and is actually one. The pipe moves up by some amount and sideways by another, and those two displacements are at right angles to each other — so they combine by Pythagoras into a single true offset along the diagonal.

Once you have that, it is an ordinary offset. Travel is the true offset divided by the sine of the fitting angle, exactly as it would be if the pipe only moved one way.

The number that surprises people is how much bigger the true offset is than either measurement. Nine up and twelve across is a true offset of fifteen — larger than both, because the pipe is cutting a diagonal through the room.

Why 45 is the default

At 45° the tangent is one, so the run equals the true offset, and the travel is just the true offset times 1.414. Two of the three numbers you need are the same number. Nothing else lays out that cleanly.

Shallower fittings — 22.5° and below — travel further but consume much more run, and that trade is usually decided by what is in the way rather than by preference.

These are centre-line figures. Cut length is the travel less the take-off at each fitting, and take-off is a property of the fitting, the pipe size and the joint method. Get it from the fitting's own data, not from a general calculator.

Common questions

How do you calculate a rolling offset?

Combine the vertical rise and the horizontal roll into one true offset using Pythagoras, then divide that by the sine of the fitting angle to get the travel. A pipe rising 9 inches and rolling 12 has a true offset of 15, and at 45 degree fittings the travel is 15 times 1.414, or about 21 1/4 inches.

What is the difference between an offset and a rolling offset?

A plain offset moves in one plane, either up or sideways. A rolling offset moves in both at once, so the pipe is travelling diagonally through space. The two displacements have to be combined into a single true offset before the fitting angle is applied.

What is the travel for a 45 degree rolling offset?

The true offset multiplied by 1.414, which is one over the sine of 45 degrees. At 45 degrees the run also equals the true offset exactly, because the tangent of 45 is one — which is what makes 45 the easiest fitting angle to lay out.

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