Four-point saddle calculator
A four-point saddle carries a run over a flat-topped obstruction with two matched offsets and a flat run between them. Enter the depth you need to clear, the width across the top, and the bend angle; the calculator gives the four mark positions measured along the pipe and the total shrink to add so the far end still lands right.
Worked examples
A 6" (152 mm) duct, 3" (76 mm) proud, at 30°
Say a 6" (152 mm) wide duct sits 3" (76 mm) proud of your run and you cross it at 30°. The 30° multiplier is 2, so each offset spaces its two marks 3 × 2 = 6" (152 mm). The four marks fall at 0, 6" (152 mm), 12" (305 mm) and 18" (457 mm) from the first: up over 6" (152 mm), flat across the 6" (152 mm) top, down over 6" (152 mm). Two offsets of 3" (76 mm) at 30° shrink the run by 2 × 3 × 0.268 ≈ 1 5/8" (41 mm), which you add to your measurement before the first mark.
A 200 mm duct at 30°, metric job
A duct 7 7/8" (200 mm) wide, and the run has to lift 1 15/16" (50 mm) to clear it, crossed at 30°. Each offset spaces its marks 50 × 2 = 3 15/16" (100 mm). Measured from mark 1, the marks fall at 0, 3 15/16" (100 mm), 11 13/16" (300 mm) and 15 3/4" (400 mm): up over 100, flat across the 200 top, down over 100. Shrink is two offsets' worth, 2 × 50 × 0.268 = 26.8 → 1 1/16" (27 mm), added to the overall length.
A cable tray with flanges, at 45°
The tray is 8" (203 mm) wide but its flanges stand 1" (25 mm) proud each side, so the flat has to span 10" (254 mm) — the width you type is the width the pipe has to clear, not the width of the thing underneath. Depth to clear is 4" (102 mm). At 45° the multiplier is 1.414, so each offset is 4 × 1.414 = 5.66 → 5 11/16" (144 mm) between its marks. The four marks, each measured from the first: 0, 5 11/16" (144 mm), 15 11/16" (398 mm) and 21 5/16" (541 mm). Step the tape off the rounded figures instead — 5 11/16 + 10 + 5 11/16 — and you land on 21 3/8, a sixteenth out, because two roundings add up; measure every mark from mark 1. Shrink is two offsets' worth, 2 × 4 × 0.414 = 3.31 → 3 5/16" (84 mm), added to the overall length, not to mark 1.
A deep one, where the numbers get big
A 12" (305 mm) beam that the run has to clear by 5" (127 mm), crossed at 30°. Each offset is 5 × 2 = 10" (254 mm); the marks sit at 0, 10" (254 mm), 22" (559 mm) and 32" (813 mm). That is nearly 3' 0" (914 mm) of pipe in bends and flat, so measure the stick before you start — 32" (813 mm) plus the tails either side has to be there. Shrink is 2 × 5 × 0.268 = 2.68 → 2 11/16" (68 mm). At this depth the imperial shop shrink (5/16 per inch, so 2 × 5 × 5/16 = 3 1/8) is 7/16" (11 mm) long. It errs the safe way — the stick comes out a shade over rather than under — but that is most of 1/2" (13 mm) of slack to take up at the far end, so use the exact figure when you can.
Doing it at the bender
- Measure. The depth to clear (top of the obstruction to where the pipe runs now, plus any clearance you want), and the width across the top including anything that sticks out. And the layout measurement from the end of the pipe to where the first offset should start.
- Mark. Mark 1 at the layout measurement — the pipe before it is straight, so the shrink does not move it, and adding it here would carry the flat past the obstruction. Marks 2, 3 and 4 at the three distances the calculator gives, each measured from mark 1 rather than stepped off the one before. Lines all the way round.
- Add the shrink to the length. Two offsets eat two offsets' worth out of the finished run, so add the shrink figure to the overall measurement before you cut.
- Line up. Arrow on mark 1, free end away from you.
- Bend. Mark 1 lifts the run. Mark 2 turns it back level — the opposite way, so the pipe goes in the shoe end for end, as on any offset. Mark 3 turns it back down, the same way as mark 2, and mark 4 levels it out again, the same way as mark 1. Up, down, down, up. Sight along the pipe before every pull so all four stay in one plane.
- Check. Both tails lie flat on the floor, the flat top sits level, and the gap under it is the depth. If it rocks, one bend is out of plane.
What went wrong
The pipe rides on the edge of the obstruction
What you see: the saddle clears the middle but sits on a flange, a lip or a bolt head at one side.
Why: the width typed in was the body of the thing, not the full width the pipe has to clear.
Fix: measure the widest thing under the pipe, and add a little. The flat can always be longer than the obstruction; it cannot be shorter.
The end lands short by about one offset's shrink
What you see: the saddle is right and the far coupling is an inch or so (25 mm or more) off.
Why: only one offset's shrink was added. A four-point saddle is two offsets, and each one pulls the run back.
Fix: use the shrink figure from this calculator, which is already doubled, not the one from the plain offset page.
It comes out as a big Z or a step
What you see: the pipe goes up and keeps going, or comes back down too early.
Why: the bend directions got mixed up. The pattern is up, down, down, up — the two middle bends go the same way as each other, which is not what your hands expect after two offsets' worth of flipping.
Fix: before each pull, hold the pipe against the job and say which way this bend takes it. Four bends is enough to lose count.
The formula
Each offset spaces its marks by the depth times the multiplier; the flat between the inner marks is the obstruction width:
offset spacing = depth ÷ sin θ
flat = obstruction width
shrink = 2 × depth × (1 ÷ sin θ − 1 ÷ tan θ)
The two outer marks and the two inner marks are just two ordinary offsets back to back, so each obeys the same rise ÷ sin θ. The flat top is measured straight across — the pipe does not bend there — so the middle gap equals the width exactly. Shrink is the offset shrink counted twice, once for each jog.
Exact vs. the shop rule
The multiplier and shrink are the ordinary offset figures, applied to the depth and then doubled for the shrink. Rounded shop multipliers are fine on a shallow saddle; on a deep one over a wide obstruction the doubled rounding adds up, so take the spacing off the exact figure when the depth is large.
| Angle | Multiplier | Shop rule | Shrink × depth, one offset | Both offsets |
|---|---|---|---|---|
| 22,5° | 2,613 | 2,6 | 0,199 | 0,398 |
| 30° | 2,000 | 2 | 0,268 | 0,536 |
| 45° | 1,414 | 1,4 | 0,414 | 0,828 |
| 60° | 1,155 | 1,15 | 0,577 | 1,155 |
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:
All four marks go on the arrow; it is two offsets back to back and nothing more. What the bender adds is springback: the two inner bends face away from each other, and if each relaxes a degree the flat across the top tilts. Pull each bend a touch past the line, then lay the pipe on the floor and check the top flat sits parallel to the two ends before you trust it.
Questions
- When do I use a four-point saddle instead of a three-point?
- A four-point saddle goes over something with a flat top — a beam, a duct, a tray. It is two offsets with a flat run across the top, so the pipe follows the shape of a squared-off obstruction. A three-point saddle peaks to a single centre bend and suits a round crossing pipe. Match the saddle to the top you are clearing.
- How many bends is a four-point saddle?
- Four, all the same angle. The first two make an offset up to the top of the obstruction; the second two make an offset back down the far side. Between them the pipe runs flat across the top, which is where the obstruction width comes in.
- Why is the shrink bigger than on a plain offset?
- Because there are two offsets, not one. Each offset pulls the run in by its own shrink, and a four-point saddle has one going up and one coming down, so the shrink is doubled. Add the full figure to your measurement or the far end lands short.
- Does the flat width change the mark spacing?
- It sets the gap between the two middle marks — the flat run across the top is exactly the obstruction width. The two offset spacings on either side come only from the depth and the angle, and the width does not change them.