Parallel offset calculator
When a rack of conduits all take the same offset, each pipe has to start its jog a little further along than the one beside it, or the diagonal sections crowd together. Enter the centre-to-centre spacing and the bend angle; the calculator gives the stagger to advance each successive conduit, and the offset mark spacing if you add a rise.
Worked examples
Four conduits, 3" (76 mm) apart, at 30°
Say four conduits run 3" (76 mm) apart and all offset at 30°. The stagger is the spacing times the shrink constant for 30°, 3 × 0.268 ≈ 13/16" (20 mm). So if the first conduit starts its offset at some mark, the second starts about 13/16" (20 mm) further along, the third about 1 5/8" (41 mm), the fourth about 2 7/16" (61 mm). Every conduit gets the identical 30° offset; only the starting point walks along by 13/16" (20 mm) each time, and the group stays 3" (76 mm) apart through the jog.
Four conduits at 75 mm centres, 30°, metric job
A metric rack: four runs at 2 15/16" (75 mm) centres jog 5 7/8" (150 mm) at 30°. The stagger is 75 × 0.268 = 20.1 → 13/16" (20 mm) per pipe, so pipe 2 starts 13/16" (20 mm) along from pipe 1, pipe 3 1 9/16" (40 mm) and pipe 4 2 3/8" (60 mm) (3 × 20.1 = 60.3). The offset is the same on all four: marks 150 × 2 = 11 13/16" (300 mm) apart, shrink 150 × 0.268 = 40.2 → 1 9/16" (40 mm) added to each pipe's length.
A rack of six, 2 1/2" (64 mm) apart, at 45°
Six runs on a rack at 2 1/2" (64 mm) centres all have to jog 8" (203 mm) to clear a column, and the run is short so it is a 45° offset. The stagger is 2 1/2 × 0.414 = 1.035 → 1 1/16" (26 mm) per pipe. Pipe 1 starts its offset on the layout mark; pipe 2 starts 1 1/16" (26 mm) further along; pipe 6, five staggers on, starts 5 × 1.035 = 5.18 → 5 3/16" (132 mm) further along. Work the offset itself once — 8 × 1.414 = 11 5/16" (287 mm) between marks, shrink 8 × 0.414 = 3 5/16" (84 mm) — and it is the same on all six. Lay the six first marks out across the rack with a straightedge before you touch a bender: they should fall on a straight line angled across the group, not a step.
Why the sixth pipe cares about the rounding
On the same rack with the shop shrink (7/16 per inch of spacing) the stagger would be 2 1/2 × 7/16 = 1.09 against the exact 1.04. That is only 1/16" (1.6 mm) off per pipe — but by pipe 6 it has compounded five times to 5 1/2" (139 mm) against the exact 5 3/16" (132 mm), nearly 5/16" (7.9 mm) out across the group, and the outside pipe visibly crowds the one beside it through the jog. The wider the rack, the more the exact constant earns its place.
Laying the rack out
- Measure. The centre-to-centre spacing (it must be the same between every pair), the rise all of them jog, and the layout measurement to where the first pipe's offset starts — mark 1 goes on it unchanged, with each pipe's shrink added to that pipe's overall length.
- Mark. Pipe 1: mark 1 on the layout measurement itself — nothing added — and mark 2 the between-marks distance past it. Pipe 2: both marks one stagger further along than pipe 1's. Pipe 3: two staggers. And so on, always in the same direction. The shrink never moves a mark; it goes on each pipe's cut length.
- Line up. Each pipe is an ordinary offset on its own marks, arrow on mark 1.
- Bend. Bend them all to the same angle, in the same plane. A batch goes faster if you pull every first bend, then every second bend.
- Check. Lay them side by side on the floor. The bends should line up on a slanted line across the group and the spacing should be the same before, through and after the jog.
What went wrong
The runs fan apart through the jog
What you see: the group is evenly spaced before and after, but the diagonals spread out or pinch together in the middle.
Why: the stagger went the wrong way — each pipe was started earlier than the one beside it instead of later, or the direction flipped halfway across the rack.
Fix: the pipe on the inside of the jog starts first; every pipe further out starts one stagger later. Pick the direction once and keep it for the whole rack.
The stagger is far too big
What you see: the bends step along the rack by several inches (tens of millimetres) per pipe and the diagonals do not line up at all.
Why: the multiplier was used instead of the shrink constant. At 45° with 2 1/2" (64 mm) spacing that is 3 9/16" (90 mm) per pipe instead of 1 1/16" (26 mm) — more than three times too much.
Fix: the stagger is spacing × the shrink constant (0.414 at 45°), never spacing × multiplier. The multiplier only ever meets the rise.
The rises are all different
What you see: the pipes come out of the jog at different heights.
Why: the stagger was applied to the between-marks distance instead of to the starting point, so each pipe got a different offset.
Fix: every pipe gets the identical between-marks distance. Only where mark 1 sits moves, by one stagger per pipe.
The formula
The stagger per conduit is the spacing times the offset shrink constant for the angle:
stagger = spacing × (1 ÷ sin θ − 1 ÷ tan θ)
That shrink constant is also tan(θ/2), which is what falls out of the geometry: to hold the same perpendicular spacing between two parallel runs once they tilt by θ, adjacent bend points have to sit tan(θ/2) of the spacing apart along the run. It is the same constant as offset shrink, so the number you already know for the angle does this job too.
Exact vs. the shop rule
The stagger constant is the shrink figure for the angle, so the same rounding applies: the shop rules are close on tight spacing and drift as the group gets wider. On a big rack it is worth taking the stagger off the exact constant, because the error compounds across every conduit in the run.
| Ángulo | Stagger × spacing | Shop rule (in per in) |
|---|---|---|
| 10° | 0.087 | 1/16 |
| 22.5° | 0.199 | 3/16 |
| 30° | 0.268 | 1/4 |
| 45° | 0.414 | 3/8 |
| 60° | 0.577 | 1/2 |
Which bender these numbers assume — and how to adjust
The geometry belongs to the angle, not the tool. What the tool changes — its reference marks, its take-up and gain, its springback — is yours to read off the shoe or prove on scrap, never a table here (the four test bends). For this bend:
Every mark goes on the arrow, and the calculator assumes all the conduits are the same size bent on the same shoe, so the same angle and the same multiplier apply to each. Mixing sizes on a rack changes the radius of each bend but not the offset multiplier — the geometry is still the angle. What mixing sizes does change is take-up and gain, so if a rack has a 90 in it, work each size on its own figures.
Preguntas
- Why stagger the marks on parallel offsets?
- So the runs stay parallel and evenly spaced through the jog. If every conduit bent at the same point, the diagonal sections would crowd closer together than the straight runs and the group would bunch. Advancing each successive pipe along its length by the right amount keeps the spacing constant all the way through.
- How much do I advance each conduit?
- By the centre-to-centre spacing times the shrink constant for the bend angle — the same 1 ÷ sin θ − 1 ÷ tan θ that gives offset shrink. At 30° with 3" (76 mm) spacing that is 3 × 0.268 ≈ 13/16" (20 mm) per pipe. Each conduit further along the group starts its offset that much later than the one before it.
- Which conduit starts first?
- The one on the inside of the jog — the side the run steps toward. Each conduit further out is advanced by one more stagger, so the group fans along a common line and the diagonals come out parallel. As long as you step them consistently in one direction it comes out right.
- Is the offset itself the same on every conduit?
- Yes. Every conduit gets the same rise, the same angle, and the same distance between its own two marks. Only the starting point moves. Work the offset once with the offset calculator, then apply the stagger to place where each run begins.