Measuring and marking conduit
To measure and mark conduit for a bend: hook the tape on a square, reamed end, read the figure straight down, carry a thin line all the way round the pipe, and write the figure beside it with an arrow pointing back to the end you measured from. Measure every mark on the stick from that same end. Most bends that come out wrong were wrong before the bender touched them, and it was nearly always the tape or the pencil.
Hooking the tape
The end of the pipe is your zero, so make it a good one. A cut out of square or still burred moves the zero. Ream it, hook the tape over the end, and run the blade along the top of the pipe, never wrapped round the curve.
The hook is loose on purpose — it slides by its own thickness so pulled or pushed reads true — but a bent hook lies. Burn an inch: hold the 1" (25 mm) line on the end, read the figure, and take one inch off. A metric crew burns 100 mm the same way. Forgetting the subtraction is the classic way to put a bend exactly an inch out.
What each bend measures to
- A 90° stub-up: the height is from the end of the stub to the back of the bend — the outside of the other leg. On a stub out of the floor that is floor to the end. The 90 guide has the rest.
- An offset: the rise, centre to centre (or bottom to bottom). Edge to edge counts the pipe's thickness.
- A saddle: from the end of the pipe to the centre of the obstruction, not its near edge.
- Into a fitting: to where the end of the pipe will actually sit inside the connector or coupling, not to the face of the box.
Marking a stick, start to finish
- Square the end. Check the end you will measure from is cut square and reamed. That end is your zero; a ragged or angled end moves every mark on the stick.
- Hook the tape. Hook the tape over the cut end and run it along the top of the pipe, pulled straight. A tape that sags or wraps round the pipe reads long.
- Read and tick. Look straight down on the graduation, not at an angle, and put a small tick exactly on it with a sharp pencil or fine marker.
- Wrap the line. Carry the tick all the way round the pipe with a straight edge: a strip of paper wrapped square, or a short piece of angle.
- Label it. Write the figure beside the line with an arrow pointing to the end you measured from. With more than one mark, number them, or letter the saddle centre C.
- Next mark from the same end. Hook the tape back on the same end for every mark on the stick. Do not measure a mark from the one before it.
- Check before you bend. Re-tape each mark from the end, and the distance between marks against your layout.
The line is the mark, not the figure. Keep it thin and line the arrow up on the line itself; a fat marker line is 1/8" (3.2 mm) wide, and you will use a different edge of it each time.
Reading the tape
In imperial, read to the sixteenth. If 11/16 still takes you a second, tape measure fractions has the conversion and a drill. Read from directly above; from an angle the blade's thickness shifts the line.
A metric crew reads the tape in millimetres and writes whole millimetres on the pipe with no units: 1245, not 124.5 cm and not 1.245 m. Half a millimetre is lost in the pencil line anyway, and centimetres mixed with millimetres on one stick is how a mark ends up ten times out. If the drawings are in metres, convert once and write millimetres from then on. Round-metric jobs come out tidily: a 5 7/8" (150 mm) rise at 30° puts the marks 11 13/16" (300 mm) apart.
Measure every mark from one end
When a stick has more than one mark, measure each from the end, not from the mark before it. That is cumulative measuring, and it keeps every mark's error to one reading.
Chaining, each from the last, adds the errors together. Each reading can be a hair out, and the tape has no hook to hold on a pencil line. Four chained marks on a segmented sweep can put the last one 1/8" (3.2 mm) out with no single mistake anywhere. Work out the running totals on paper first — mark 2 is mark 1 plus the between-marks distance — and tape each of them from the end.
Marking from the right end
- A 90: from the end that will stand up.
- An offset: mark 1 at the layout measurement from the end nearest the offset, mark 2 the between-marks distance further along. See how to bend an offset.
- A saddle: from the end to the centre of the obstruction, then the centre mark C moved along by the shrink, and the outer marks either side of C. See how to bend a saddle.
A figure from the other end — the far box, say — gets subtracted from the stick length on paper. Never half the marks from each end.
The layout measurement vs the stick length
The layout measurement is where a bend starts, from the end. The stick length is what you cut. Bends in the middle of a run pull the far end back, so the stick has to be longer than the flat distance it covers by the shrink.
Say the run is 96" (2.44 m) box to box on the flat and needs a 6" (152 mm) offset at 30°, starting 24" (610 mm) from the first box. Shrink at 30° is 0.268 × the rise: 0.268 × 6 = 1.61, call it 1 5/8" (41 mm). Cut the stick at 97 5/8" (2.48 m). Mark 1 stays at 24" (610 mm) and mark 2 goes at 36" (914 mm) (the rise × 2 further along). On a metric job: 94 1/2" (2.4 m) on the flat, a 5 7/8" (150 mm) offset at 30° starting 23 5/8" (600 mm) out — shrink 150 × 0.268 = 40 mm, so cut at 96 1/16" (2.44 m), mark 1 at 23 5/8" (600 mm) and mark 2 at 35 7/16" (900 mm).
A run round a 90 goes the other way: it needs less pipe than the two legs add up to, by the gain — see deduct and gain.
What goes wrong
Bend exactly an inch out
What you see: one bend, or one stick, off by a round inch or 100 mm.
Why: burned an inch and forgot to take it off.
Fix: write the burned figure on the pipe, then the real one, before you draw the line.
Bent from the wrong end
What you see: the bend is the right distance from the end you did not measure from.
Why: no arrow on the mark, and the stick got turned round.
Fix: figure and arrow on every mark, every time.
The last mark is out
What you see: the first bends of a multi-bend stick are fine and the last one is late or early.
Why: the marks were chained, each from the last.
Fix: running totals on paper, every mark from the end.
Offset lands short
What you see: the jog is right and the end is short of the box.
Why: the shrink went on mark 1 instead of the stick length, or nowhere.
Fix: shrink on the cut length. More in common mistakes.
Questions
- Where do I hook the tape to measure conduit for a bend?
- On the cut end of the pipe that the layout measurement is taken from: the free end for a stub-up, the end nearest the offset or saddle for those. Measure every mark on that stick from the same end.
- Why should the mark go all the way round the pipe?
- Because a tick on top is hidden under the shoe when you line the arrow up from the side, so you end up guessing, and a guessed mark is 1/8" (3.2 mm) or 1/4" (6.4 mm) out, a different amount every time.
- How accurate do conduit marks need to be?
- Read to the nearest sixteenth in imperial, or to the whole millimetre in metric. Closer than that is lost in the pencil line. What matters more is that every mark comes from the same end.
- Do I add the shrink to the mark or to the length?
- On an offset, to the length of the stick. Mark 1 sits on the layout measurement; the bends pull the far end back, so the stick has to start longer by the shrink. The one mark that does move is a three-point saddle centre.