Conduit offset math practice
Given a rise and a bend angle, work out the centre-to-centre distance between the two bends. Type it in and it is marked, with the multiplier and the working shown when you miss. A field rounding still counts.
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The maths behind it
An offset is two equal bends that step the pipe over by a rise. The distance you mark between them is the rise times the offset multiplier, where the multiplier is 1 ÷ sin of the bend angle. For a 6"150 mm offset at 30°, the multiplier is 1 ÷ sin 30° = 2.0, so the marks are 6 × 2.0 = 12"150 × 2.0 = 300 mm apart. The multiplier has no unit, so it works the same in inches or millimetres. At 45° the multiplier is about 1.414; at 22.5°, about 2.613. The trade carries rounded versions of these in its head, and they are close enough to mark a pipe by.
Three worked
The drill asks one thing: rise times multiplier. These three are that one thing at the three angles you will meet most, written out in full.
1. A 5" rise at 30°
- Multiplier for 30° is 1 ÷ sin 30°. sin 30° = 0.5, so 1 ÷ 0.5 = 2.
- Distance between bends = rise × multiplier = 5 × 2 = 10".
Why 30° is the friendly one
A multiplier of exactly 2 means you just double the rise — no calculator, no rounding. That is why so much of the trade defaults to 30° offsets: the arithmetic disappears.
2. An 8" rise at 45°
- Multiplier for 45° is 1 ÷ sin 45°. sin 45° ≈ 0.7071, so 1 ÷ 0.7071 ≈ 1.414.
- 8 × 1.414 = 11.31", or 11 5/16" on the tape.
The field rounding
Most people carry 1.4 for 45°. 8 × 1.4 = 11.2", a tenth off — inside what a bender can hold. The drill marks against 1.414 with tolerance, so the rounded answer counts. Just know which number you used.
3. A 3 1/2" rise at 22.5°
- Turn the rise into a decimal: 3.5".
- Multiplier for 22.5° is 1 ÷ sin 22.5°. sin 22.5° ≈ 0.3827, so the multiplier is ≈ 2.613.
- 3.5 × 2.613 = 9.15" — call it 9 1/8" on the tape.
The thing to notice
A shallower angle means a bigger multiplier: the same rise takes more pipe to achieve. Steeper angle, smaller multiplier, tighter offset. If your multiplier got bigger as the angle got steeper, you have it upside down.
1. A 125 mm rise at 30°
- Multiplier for 30° is 1 ÷ sin 30°. sin 30° = 0.5, so 1 ÷ 0.5 = 2.
- Distance between bends = rise × multiplier = 125 × 2 = 250 mm.
Why 30° is the friendly one
A multiplier of exactly 2 means you just double the rise — no calculator, no rounding. That is why so much of the trade defaults to 30° offsets: the arithmetic disappears.
2. A 200 mm rise at 45°
- Multiplier for 45° is 1 ÷ sin 45°. sin 45° ≈ 0.7071, so 1 ÷ 0.7071 ≈ 1.414.
- 200 × 1.414 = 282.8 mm — mark it at 283 mm.
The field rounding
Most people carry 1.4 for 45°. 200 × 1.4 = 280 mm, 3 mm short — inside what a bender can hold. The drill marks against 1.414 with tolerance, so the rounded answer counts. Just know which number you used.
3. A 90 mm rise at 22.5°
- Multiplier for 22.5° is 1 ÷ sin 22.5°. sin 22.5° ≈ 0.3827, so the multiplier is ≈ 2.613.
- 90 × 2.613 = 235.2 mm — call it 235 mm.
The thing to notice
A shallower angle means a bigger multiplier: the same rise takes more pipe to achieve. Steeper angle, smaller multiplier, tighter offset. If your multiplier got bigger as the angle got steeper, you have it upside down.
The slips
- Used sin instead of 1 ÷ sin5 × 0.5 = 2.5"125 × 0.5 = 62.5 mm for a 30° offset is the giveaway. The distance between bends is always more than the rise, never less.
- Multiplier bigger at steeper anglesWrong way round. 22.5° → 2.6, 30° → 2, 45° → 1.4, 60° → 1.15. The multiplier shrinks as the angle grows.
- Forgot the rise was a fraction3 1/2 × 2 is 7, not 6.5 or 7.5. Convert the fraction to a decimal before you multiply — that is the other drill.
- Mixed centimetres and millimetresA 12.5 cm rise at 30° is 25 cm, which is 250 mm — not 25 mm. Put everything in millimetres before you multiply; the bender marks and the tape both read in them.
Try one before the drill: a 6 1/4" rise at 45°
6.25 × 1.414 = 8.84", or about 8 13/16" — with the field 1.4 it is 8.75", 8 3/4". Both inside tolerance. Then the sanity check: bigger than the rise, and a 45° offset is tighter than a 30° one would be (which would be 12.5"). Passes.
Try one before the drill: a 160 mm rise at 45°
160 × 1.414 = 226.2 mm, so 226 mm — with the field 1.4 it is 224 mm. Both inside tolerance. Then the sanity check: bigger than the rise, and a 45° offset is tighter than a 30° one would be (which would be 320 mm). Passes.
Questions fréquentes
What is the offset multiplier?
It is 1 ÷ sin of the bend angle, and it turns the offset you need into the distance between the two bends. At 30° it is 2.0, at 45° about 1.414, at 22.5° about 2.613, at 60° about 1.155. Multiply your rise by it and you have the centre-to-centre mark spacing.
Where does the field rule of thumb fit?
The trade rounds the multipliers to memorable numbers — 2 for 30°, 1.4 for 45°, and so on. The drill marks against the exact trig value but a rounded field answer still lands inside the tolerance, which is the honest way to practise it.
Is this the same as the bending calculators elsewhere?
No — this drills the maths, it does not do your job for you. It generates a rise and an angle and asks you for the spacing. When you are actually at the bender and want the numbers worked for a real pull, that is what bentright.com is for.
Voir aussi
- Formula sheets — the multipliers and shrink, printable for your pouch.
- Glossary: shrink — the other half of the offset bookkeeping.
- At the bender for a real pull? bentright.com works the numbers.
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