What are concentric bends?

Several conduits bent around the same corner on shared centres, so they stay parallel and evenly spaced the whole way round. Each run sits a fixed distance outside the last, which means each has a larger radius and takes a bit more pipe than the one inside it.

Why is the outer conduit longer?

Because it follows a bigger radius. Developed length is radius times the angle, so a larger radius over the same turn is more pipe. Each step out adds the spacing to the radius, which adds spacing times the angle in radians to the length — the gain the calculator lists per run.

Which conduit do I bend first?

The innermost, on the tightest radius, then work outward. Each successive run is the same shape on a larger radius, so once you have the inner one set, the rest follow by adding the spacing to the radius and the gain to the pipe length.

Is a concentric bend the same as a parallel offset?

They are cousins. A parallel offset keeps runs even through a step; a concentric bend keeps them even around a sweep. The offset uses a stagger along the pipe; the concentric bend uses a growing radius. Use the parallel offset calculator for the step and this one for the corner.

Concentric bend calculator

Inner radius

Radius of the tightest (innermost) conduit. Inches, or feet-inches.

Spacing between conduits

Centre-to-centre, added to each successive radius.

Number of conduits

How many parallel runs follow the curve.

Bend angle

The turn the whole run makes, in degrees.

4 11/16 in longer per conduit out

inner developed length 18.85 in · outer 28.27 in

Bend the innermost first; each run out is this much longer of pipe.

Run	Radius	Developed length	Gain

Concentric bends carry several conduits around one corner on shared centres, each a fixed distance outside the last, so the group stays parallel and evenly spaced. Enter the inner radius, the spacing between runs, how many there are, and the angle of the turn; the calculator lists each run's radius, its developed length, and how much longer it is than the innermost.

Worked example

Say three conduits turn 90°, the inner one on a 12 inch radius, spaced 3 inches apart. The inner run is 12 × (90° in radians) ≈ 18.85 inches of pipe. The middle run sits on a 15 inch radius, 15 × the same angle ≈ 23.56 inches; the outer on 18 inches ≈ 28.27. Each step out adds 3 × (90° in radians) ≈ 4 11/16 inches of pipe — that constant gain is the same for every step because the spacing is constant.

The formula

Each run's radius grows by the spacing; its developed length is radius times the angle in radians:

radiusₙ = inner + n × spacing · developedₙ = radiusₙ × angle(rad) · gain = spacing × angle(rad)

Developed length is just how far you travel along a circle of that radius through the turn, which is radius times the angle in radians. Step the radius out by the spacing and the length grows by the spacing times that same angle — a fixed gain per run. Bend the inner one, then add the gain for each run further out.

Spacing and count only — no capacity

This tool lays out the geometry of the group: radii, lengths, and the gain between runs. It says nothing about how many conductors a raceway may carry or how a rack should be supported — those are code and engineering questions, not trigonometry, and they are not what this is for. Keep every run in the same plane so the group turns flat.

Questions

What are concentric bends?: Several conduits bent around the same corner on shared centres, so they stay parallel and evenly spaced the whole way round. Each run sits a fixed distance outside the last, which means each has a larger radius and takes a bit more pipe than the one inside it.
Why is the outer conduit longer?: Because it follows a bigger radius. Developed length is radius times the angle, so a larger radius over the same turn is more pipe. Each step out adds the spacing to the radius, which adds spacing times the angle in radians to the length — the gain the calculator lists per run.
Which conduit do I bend first?: The innermost, on the tightest radius, then work outward. Each successive run is the same shape on a larger radius, so once you have the inner one set, the rest follow by adding the spacing to the radius and the gain to the pipe length.
Is a concentric bend the same as a parallel offset?: They are cousins. A parallel offset keeps runs even through a step; a concentric bend keeps them even around a sweep. The offset uses a stagger along the pipe; the concentric bend uses a growing radius. Use the parallel offset calculator for the step and this one for the corner.

Related tools

Parallel offset calculator — the same idea through a step.
Segmented bend calculator — how each large-radius run is actually bent.
Conduit bending formulas — developed length and gain in the full reference.