Conduit Bend Offset Calculator

Offset multiplier and derived shrink table

Rounded multiplier: how much error?

Charts often round the multiplier to one decimal place. The table shows the exact value to 3 decimals, a value rounded to one decimal place (labelled “rounded value”; it is not what any particular manufacturer prints), and the relative error that rounding puts into the bend-to-bend distance.

Formula and shrink derivation

Let h be the rise, d the sloped distance between ideal bend vertices, and θ the angle of either bend. Since sin(θ) = h/d, d = h/sin(θ). The horizontal projection is x = d cos(θ).

Derived: shrink s = d − x = d(1 − cos(θ)) = h(1 − cos(θ))/sin(θ) = h tan(θ/2). Shrink per inch of offset is therefore tan(θ/2), in inches per inch. This is the extra straight length consumed compared with the horizontal projection in a zero-radius, sharp-vertex model.

Theoretical bend vertices are not automatically the marks on a real bending shoe. Finite bend radius and tool calibration change the practical layout. This calculator supplies geometry, not a manufacturer-specific bending procedure.

Offset: how to lay out and mark

1. Measure the offset height h (the perpendicular distance the conduit must move).
2. Pick the bend angle θ and read the multiplier 1/sin(θ).
3. Distance between bends = h × multiplier.
4. Mark the first bend point, then measure the distance from step 3 along the conduit and mark the second bend point.
5. Make both bends at the same angle, in opposite directions, with the bender marks aligned to the layout marks as the bender’s instructions require.
6. If the end-to-end length matters, add the derived shrink h × tan(θ/2) to the starting length.

Assumed example, inches: a 6 in offset at 30° gives d = 6 ÷ 0.5 = 12 in and derived shrink = 6 × tan(15°) ≈ 1.608 in. Assumed example, inches (45°): a 4 in offset at 45° gives d = 4 × 1.4142 ≈ 5.657 in and shrink ≈ 1.657 in.

Assumed example, metric: a 100 mm offset at 45° gives d = 100√2 ≈ 141.421 mm and shrink = 100(√2 − 1) ≈ 41.421 mm. The ratios are dimensionless, so the same method works in any unit.

Kick: a single bend

A kick is one bend of angle θ that raises the conduit by h. The distance travelled along the conduit from the bend until the rise reaches h is h/sin(θ), and the horizontal run over that rise is h/tan(θ). This is ideal geometry with a sharp vertex.

Assumed example: a 3 in kick at 30°: along the conduit 3 ÷ sin(30°) = 6.000 in; horizontal run 3 ÷ tan(30°) ≈ 5.196 in.

3-point saddle

A 3-point saddle clears an obstacle of height h with a 45° centre bend and two 22.5° outer bends. Each sloped leg rises h at 22.5°, so each outer bend sits d = h/sin(22.5°) = 2.6131h from the centre bend.

Derivation of shrink. The path has two sloped legs of length d. Their horizontal projection is 2d cos(22.5°), so total shrink = 2d(1 − cos 22.5°) = 2h(1 − cos 22.5°)/sin 22.5° = 2h tan(11.25°) = 0.3978h. In this page’s definition the centre-mark shift is one side of that, h tan(11.25°) = 0.1989h. These are two different quantities; do not use one where the other is meant. The same derivation works for a general saddle with a centre bend of 2θ and outer bends of θ.

Assumed example: a 4 in obstacle: outer marks 10.453 in on each side of the centre mark (4 × 2.6131); centre-mark shift 0.796 in; total shrink 1.591 in. Field charts often round the multiplier (for example 2.5 or 2.6), which shortens the distance slightly; this page uses the exact value and does not quote any manufacturer’s numbers.

Rolling offset

When the conduit must move both vertically and sideways, the offset happens in a tilted plane. The true offset is √(rise² + roll²); the distance between bends is then the true offset × 1/sin(θ), exactly as for a plain offset. The tilt of that plane from vertical is atan2(roll, rise).

Assumed example: rise 6 in and roll 8 in make a 6-8-10 triangle, so the true offset is 10 in. At 30° the distance between bends is 10 × 2 = 20 in, and the derived shrink is 10 × tan(15°) ≈ 2.679 in. The plane is tilted about 53.1° from vertical.

Stub-up

For a 90° stub-up, the bender removes some length from the straight section, called take-up. The mark position from the conduit end is desired stub height − take-up. Take-up depends on your bender and conduit size, so you enter it yourself; this page does not contain a take-up table.

Assumed example: a 12 in stub with a take-up of 5 in puts the mark at 12 − 5 = 7 in from the end. The 5 in take-up is an assumed value, not a bender spec.

Definitions

Multiplier

The ratio 1/sin(θ) that turns offset height into the distance between bends.

Shrink

The straight length consumed by an offset or saddle compared with the horizontal projection; here h × tan(θ/2) per offset, derived from ideal geometry.

Take-up

The length a bender uses up in a 90° bend, taken from the bender’s own instructions.

Gain

The straight length saved by a rounded bend compared with the sharp-corner path. It depends on the bend radius and is not modelled here.

Common mistakes

• Mixing up the angle of each bend with the total change of direction.
• Forgetting shrink when the end-to-end length matters.
• Marking from the wrong reference, such as the wrong end or the wrong edge of the bender mark.
• Using the take-up from a different bender or conduit size.
• Bending the second bend in the wrong direction, which turns an offset into a dog-leg.
• Treating the centre-mark shift and the total shrink of a saddle as the same number.
• Using a rounded multiplier without checking the error it adds.