Conduit shrink calculator
Shrink is what an offset takes out of the run, and it is the number people forget. Enter the rise of your offset and the bend angle; the calculator gives the shrink to add to the overall length, so the pipe still reaches the far fitting after both bends pull the end back.
Where the shrink goes on the pipe
Shrink is a number you add to a length, not to a mark. Where you add it is the part people get wrong.
- Measure. From the end of the stick to the point where the offset must start — the near edge of whatever it is clearing, or the point where the run has to step over. That is the layout measurement.
- Mark. Mark 1 goes on the layout measurement itself. The pipe between the end and mark 1 is dead straight and loses nothing, so nothing is added to it. Mark 2 goes the between-marks distance past mark 1 — from the offset calculator, rise × multiplier.
- Add. The shrink is consumed between the two marks, so everything past mark 2 pulls back by it. Add the shrink to the overall length — the stick you cut, or the measurement to the far box — before you cut anything.
- Bend. The two bends as for any offset. The jog lands where the layout point was, and the far end finishes exactly the shrink nearer than the bare length would suggest.
- Check. Measure from the end of the stick to the first bend on the finished pipe: it should still read the layout measurement, because that stretch never changed. Then measure end to end: the piece is the shrink shorter than the pipe you started with.
Worked examples
One offset on the way to a box
You are running to a box 40" (1016 mm) away and the jog has to start 20" (508 mm) along — a 6" (152 mm) offset at 30°. The shrink for 30° is 0.268 × the rise, so 6 × 0.268 ≈ 1 5/8" (41 mm). Mark 1 still goes at the 20" (508 mm) you measured; what changes is the stick — cut it 41 5/8" (1057 mm) rather than 40" (1016 mm). Skip that and the run finishes about 1 5/8" (41 mm) short of the box.
A 100 mm offset at 45°, metric job
The box is 39 3/8" (1000 mm) away, the jog starts 19 11/16" (500 mm) along, and it has to step 3 15/16" (100 mm) at 45°. The 45° shrink is 0.414 × the rise, so 100 × 0.414 = 41.4 → 1 5/8" (41 mm). Mark 1 stays at 19 11/16" (500 mm); cut the stick 41" (1041 mm), not 39 3/8" (1000 mm).
Three bends in one stick — shrink adds up
A single piece has a 6" (152 mm) offset at 30° to get off the wall, a 2" (51 mm) three-point saddle at a 45° centre over a crossing pipe, and a 3" (76 mm) offset at 45° into the box. Each one takes something out of the run: 6 × 0.268 = 1 5/8" (41 mm); the saddle uses its 22.5° outer, and a saddle eats twice one offset's worth — 2 × 2 × 0.199 = 13/16" (20 mm); 3 × 0.414 = 1 1/4" (32 mm). Total 3 5/8" (93 mm) gone. If the box is 8' 0" (2.44 m) from the coupling, the stick has to be 8' 3 5/8" (2.53 m) long. Each bend's own marks still go where they were measured — but each bend pulls everything downstream of it back by its own shrink, so work down the pipe and carry the running total: the second bend's layout point moves along by the first bend's shrink, the third by the first two.
The pipe is already cut
The stick was cut to 40" (1016 mm) before anyone thought about the offset. A 6" (152 mm) offset at 30° will take 1 5/8" (41 mm) out of it, so it will finish at 38 3/8" (975 mm) — short. Your choices are the ones the geometry gives you: a shallower angle (at 22.5° the shrink is 6 × 0.199 = 1 3/16" (30 mm), still short), a coupling and a nipple, or a new stick. What the number buys you is finding that out before the bend, not after.
At a glance — shrink by rise
Rise down the side, angle across the top, shrink in the cell — to the nearest sixteenth in the inch table, to the millimetre in the metric one. Every figure is the rise times the constant for the angle, which is the same arithmetic the calculator does — this is a shortcut for the common cases, not a table to trust over your own rise.
| Rise (in) | 10° | 22.5° | 30° | 45° | 60° |
|---|---|---|---|---|---|
| 2" | 3/16 | 3/8 | 9/16 | 13/16 | 1 1/8 |
| 3" | 1/4 | 5/8 | 13/16 | 1 1/4 | 1 3/4 |
| 4" | 3/8 | 13/16 | 1 1/16 | 1 11/16 | 2 5/16 |
| 6" | 1/2 | 1 3/16 | 1 5/8 | 2 1/2 | 3 7/16 |
| 8" | 11/16 | 1 9/16 | 2 1/8 | 3 5/16 | 4 5/8 |
| 10" | 7/8 | 2 | 2 11/16 | 4 1/8 | 5 3/4 |
| 12" | 1 1/16 | 2 3/8 | 3 3/16 | 5 | 6 15/16 |
| Rise (mm) | 10° | 22.5° | 30° | 45° | 60° |
|---|---|---|---|---|---|
| 50 | 4 | 10 | 13 | 21 | 29 |
| 75 | 7 | 15 | 20 | 31 | 43 |
| 100 | 9 | 20 | 27 | 41 | 58 |
| 150 | 13 | 30 | 40 | 62 | 87 |
| 200 | 17 | 40 | 54 | 83 | 115 |
| 250 | 22 | 50 | 67 | 104 | 144 |
| 300 | 26 | 60 | 80 | 124 | 173 |
Read across a row and the point makes itself: the same 8" (203 mm) rise costs 11/16" (18 mm) at 10° and 4 5/8" (117 mm) at 60°. When the run is tight, the angle is the lever.
What went wrong
Short by exactly the shrink
What you see: a perfect offset and a coupling an inch and a half (about 40 mm) off the box.
Why: the shrink was known and never added to the length — the stick was cut to the flat distance.
Fix: the calculator's first line. Add it to the overall length before you cut, and write the cut length on the stick.
Long by twice the shrink
What you see: the far end overshoots the box by twice the figure you worked out.
Why: the shrink was taken off the length instead of put on. It is a loss, so you compensate by starting with more pipe, not less.
Fix: add. Always add. If the sign ever feels unclear, remember the finished piece comes out shorter than the stick, so the stick has to be longer.
Short by a little on a big offset
What you see: a 20" (508 mm) offset at 45° lands about 3/4" (19 mm) short even though shrink was added.
Why: an old imperial shop figure of 3/8 per inch was used where the exact ratio (0.414) mattered: 20 × 3/8 = 7 1/2" (191 mm) against the true 8 5/16" (210 mm). That is why the table below rounds the shop figure up (7/16) rather than down.
Fix: shop rules for small offsets; exact for anything over about 12" (305 mm) of rise, or where the far end has no slack.
The formula
Shrink as a ratio of the rise is one over the sine of the angle, less one over the tangent:
shrink = rise × (1 ÷ sin θ − 1 ÷ tan θ)
The pipe between the marks is the hypotenuse of a right triangle, length rise ÷ sin θ. The horizontal ground it covers is only rise ÷ tan θ. The difference between the sloped pipe and the flat ground it replaces is the run you lose — that is the shrink. It falls out of the same triangle as the multiplier, so there is no separate table to trust.
Shrink ratio, exact vs. the shop rule
The exact figure is a ratio: multiply it by your rise, in inches or millimetres. The shop rules are the rounded per-inch amounts a lot of benders keep in their head. They are close on a small offset and drift on a large one. The shop-shrink column is the exact figure rounded up to the next sixteenth, not down: a shrink figure that is always low lands the run short, which is the one failure worth avoiding. Rounded up it errs long, and pipe you can trim.
| Ángulo | Shrink × rise | Shop shrink (in per in) |
|---|---|---|
| 10° | 0.087 | 1/8 |
| 22.5° | 0.199 | 1/4 |
| 30° | 0.268 | 5/16 |
| 45° | 0.414 | 7/16 |
| 60° | 0.577 | 5/8 |
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:
Shrink is angle only — it is the same on a hand bender, a hickey, a mechanical bender or a hot box. What the bender changes is whether the angle you pulled is the angle you read: springback and a coarse degree scale both make the real angle a little less than the line, and a shallower real angle means a little less shrink. If a run keeps landing long, read the finished angle with a level and put it in "Other angle" on the offset page.
Preguntas
- What is shrink in conduit bending?
- Shrink is the length an offset pulls out of the run. Bending the pipe up and back over pulls the far end toward the bends, so the finished run is shorter than the flat length you measured. Shrink is how much shorter, and you add it to your measurement before the first mark so the end still lands where you want it.
- Do I add shrink or subtract it?
- Add it — to the length, not to a mark. The offset makes the finished piece shorter than the pipe you started with, so you lay the run out as if it were the shrink longer, or cut the stick the shrink longer. Mark 1 itself does not move: the pipe between the end of the stick and mark 1 is dead straight and loses nothing. Subtracting the figure leaves the far end short by twice it.
- Which angle shrinks the most?
- The steeper the bend, the more it shrinks for the same rise. A 60° offset shrinks about 0.58 × the rise; a 10° offset only about 0.09 × the rise — the same ratio in inches or millimetres. That is one reason a shallow angle is easier to place in a long run — it pulls the end back far less.
- Does shrink depend on the pipe size?
- No. Shrink as a ratio of the rise comes only from the bend angle, so it is the same for half-inch EMT and two-inch rigid. What changes with pipe size is take-up and gain on a 90, not the offset shrink.