Conduit Bending Calculator

Gain (also called 'deduct' or 'take-up') is the length of conduit saved by a bend compared with running two straight pieces into a square corner. For a 90° bend the gain is NOT the full bend radius — it is 2r − πr/2 = 0.4292 × the former radius, because the curved quadrant arc (πr/2) replaces 2r of square-corner length. For example a 20mm conduit on a 75mm former gives a gain of 2×75 − π×75/2 ≈ 32mm, not 75mm. Subtract the gain from your measured lengths to get the correct cutting length, otherwise the conduit ends up too long.

BS 7671:2018 Regulation 522.8.3 requires every bend to be made so cables are not damaged; the IET On-Site Guide expresses this as a minimum INTERNAL bend radius of 2.5 × the outside diameter — so 16mm conduit = 40mm, 20mm = 50mm, 25mm = 62.5mm, 32mm = 80mm. In practice a bending machine former produces a larger radius than the code minimum (this calculator uses typical former radii of 50/75/100/125mm for 16/20/25/32mm — confirm against your own bender), and a larger radius makes cables easier to draw in. A proper bender with the correct former keeps the radius constant; hand-bending risks kinks and flat spots that breach 522.8.3.

An offset uses two equal-angle bends (typically 45°) to shift the conduit run sideways by a set distance. The travel (distance between the two bend marks along the conduit) = offset ÷ sin(bend angle): for 45° bends travel = offset × 1.414 (√2), for 30° bends travel = offset × 2, for 60° bends travel = offset × 1.155. The shrink — how much shorter the run's straight-line advance becomes — is offset × (1 − cos θ)/sin θ, NOT offset ÷ tan θ (that is the horizontal advance of the offset section). Per unit of offset the shrink is roughly 0.27 at 30°, 0.41 at 45° and 0.58 at 60°; for a 50mm offset at 45° the shrink is about 20.7mm. Mark the first bend, measure the travel along the conduit, mark the second, and shift the far fixture along by the shrink to keep the run on target.

A saddle bend (also called a 'bridge' or 'crossover') is used when the conduit run needs to pass over an obstacle — such as another conduit, a pipe, or a structural member. It consists of three bends: the centre bend (typically 60°) and two equal outer bends (30° each). To mark it, place the centre bend over the middle of the obstacle, then measure each outer mark away from the centre by the obstacle depth × the cosecant of the outer angle — for the 60°/30° saddle that multiplier is 1 ÷ sin 30° = 2.0 (a 30mm-deep obstacle gives 60mm each side, NOT depth ÷ tan 30° = 52mm). A shallower 45°/22.5° saddle uses a 2.6 multiplier instead. The saddle lifts the conduit over the obstacle and returns it to the original level — precise marking is essential, so measure the obstacle depth and width carefully.

Good practice limits a conduit run to two 90° bends (or equivalent) between draw-in points. More bends make cable pulling extremely difficult and risk damaging cable insulation. If your route requires more than two 90° bends, install an additional draw-in box or inspection fitting. Each 90° bend approximately doubles the pulling force required. For long runs with multiple bends, consider using trunking instead. Use our Conduit Fill Calculator to check cable capacity for your conduit size.