Box offset calculator
A box offset is the little step at the end of a run that lets a connector seat square into a box knockout. Enter the small rise to the knockout and the bend angle; the calculator gives the distance between your two marks and the shrink, so the pipe enters straight and still reaches.
Worked examples
A 1/2" (13 mm) box at 22.5°
Say the box stands off the wall 1/2" (13 mm) and you make the offset at 22.5°. The 22.5° multiplier is 2.613, so the two marks go 0.5 × 2.613 ≈ 1 5/16" (33 mm) apart. The shrink is 0.5 × 0.199 ≈ 1/8" (2.5 mm) — small, but on the short tail into the box worth adding so the connector lands on the knockout. Bend the first mark, then the second the same angle the opposite way, and the pipe steps up level into the box.
A 12 mm box at 22.5°, metric job
The knockout centre sits 1/2" (12 mm) off the surface the pipe runs along. At 22.5° the marks go 12 × 2.613 = 31.4 → 1 1/4" (31 mm) apart, and the shrink is 12 × 0.199 = 1/8" (2.4 mm) — call it 1/16" (2 mm) or 1/8" (3 mm) on the tape, and add it only if the tail is tight. Pull the second bend to the same line as the first.
A 3/4" (19 mm) raised cover at 10°
A box with a raised cover puts the knockout 3/4" (19 mm) off the surface, and the run has a long straight tail so a 10° offset will do it gently. The 10° multiplier is 5.759 — the shallow angle spreads the jog — so the marks go 0.75 × 5.759 = 4.32 → 4 5/16" (110 mm) apart. Shrink is 0.75 × 0.087 ≈ 1/16" (1.7 mm), which you can add or ignore depending on how much tail there is. Ten degrees is barely a bend; the check is that the pipe lies flat on the wall and the connector sits square.
The same box at 30° — when the tail is short
Same 1/2" (13 mm) box, but the run comes off a 90 close by and there is no room for a 1 5/16" (33 mm) jog. At 30° the multiplier is 2, so the marks go 0.5 × 2 = 1" (25 mm) apart and the shrink is 0.5 × 0.268 ≈ 1/8" (3.4 mm). Steeper packs it tighter, and a box offset this small never gets steep enough for the shrink to hurt you.
Twenty boxes on one wall
A run of boxes at the same depth wants twenty identical box offsets. Work the marks once — say 5/8" (16 mm) at 22.5°: 5/8 × 2.613 = 1.63 → 1 5/8" (41 mm) between marks, shrink 1/8" (3.1 mm) — then make a template: a short offcut with the two marks on it, held against each stick to transfer them. Pull every first bend, then every second bend, and hold each finished one against the first. Twenty box offsets made one at a time are twenty chances to be different.
Doing it
- Measure. From the wall to the centre of the knockout — that is the rise. Not the depth of the box, unless the knockout is in the back.
- Mark. Mark 1 where the offset should start — nothing added; the pipe before it is straight. Mark 2 the between-marks distance further along. The shrink comes off the far end, so add it to the cut length if the tail is tight.
- Line up. Arrow on mark 1. On a dedicated box-offset tool, its own reference.
- Bend. A gentle pull to the angle. Flip end for end, arrow on mark 2, the same angle back. Small angles are easy to over-pull.
- Check. Hold it against the box: the tail lies on the wall and the end sits square in the knockout. If it cocks, one bend is heavier than the other.
What went wrong
The connector cocks in the knockout
What you see: the pipe enters the box at an angle and the locknut will not draw up square.
Why: the two bends are not the same angle, so the end is not parallel to the run. On a bend this small a couple of degrees is all it takes.
Fix: match the second pull to the first by feel and by eye, then lay it on the floor: both ends flat, no rock.
It stands proud, or does not lift enough
What you see: the end sits above the knockout, or the tail still touches the box edge.
Why: the rise was the box depth, not the distance from the wall to the knockout centre.
Fix: measure to the centre of the hole the pipe goes into, from the surface the pipe runs along.
The jog is too long to fit
What you see: the box offset runs into the 90 or the next fitting.
Why: a shallow angle on a short tail. At 10° a 1/2" (13 mm) rise needs nearly 2 7/8" (73 mm) between marks; at 30° it needs 1" (25 mm).
Fix: steepen the angle. The calculator shows the between-marks distance change as you press the chips.
The formula
A box offset is an ordinary offset with a small rise, so the same trigonometry applies:
between marks = rise ÷ sin θ
shrink = rise × (1 ÷ sin θ − 1 ÷ tan θ)
The rise here is just the depth the box stands off the surface. The pipe between the marks is the hypotenuse of the little triangle, rise ÷ sin θ, and the shrink is the same difference between that sloped pipe and the flat run it replaces. Nothing about the box changes the math — it only sets how small the rise is.
Matching a fixed-angle tool
Many box offsets are made with a dedicated tool or a hickey that bends a set angle rather than one you choose. If yours does, read the angle it makes and enter it in the other-angle field, and the marks will match the tool. If you are pulling it on a standard bender, a shallow angle keeps the jog short and the connector easy to seat.
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:
A box offset on a hand bender is two small bends on the arrow at a shallow angle — the calculator opens on 22.5°, which is what most shoes can actually be read at, with 10° on the chips for the shallow tools that carry a line that low. Whichever chip you pick is the angle the numbers assume; if your shoe carries no line that shallow, read the angle with a level and type it into “Other angle”. A dedicated box-offset tool (the little lever type) bends a fixed rise at a fixed angle, so with one of those the calculator is only for checking what the tool gives you; the "matching a fixed-angle tool" section below covers it.
Questions
- What is a box offset?
- A box offset is a small double bend right at the end of a conduit that steps it up so the connector seats square into a box knockout. The box stands off the surface by its own depth, and the little offset lifts the pipe that much so it enters straight instead of at an angle.
- What angle is a box offset bent at?
- A shallow one — often around 10° to 30°, because the rise is small and a gentle angle keeps the jog short and easy to seat. Some benders have a dedicated box-offset tool or a hickey that makes a set small angle; if yours does, enter that angle to match it.
- How much rise does a box offset need?
- Exactly the depth the box or its raised ring stands off the surface, which is a thing to measure rather than assume: put a tape from the surface the pipe runs along to the centre of the knockout and use that as the offset depth. Too little and the connector cocks; too much and it stands proud.
- Does a box offset shrink the pipe?
- A little, because it is still an offset. The shrink is small at this depth but the calculator shows it, and on a short tail into the box it is worth adding so the pipe still reaches the knockout cleanly.