Measuring Center to Center and Fitting Make-Up
Why this matters
A piece of pipe cut 1/2 in short is scrap, and on copper or threaded pipe it is also a second trip to the truck and a joint made in a hurry. Cutting to length is the first thing an apprentice is asked to do on their own and the place most of their early waste comes from, almost always for the same reason: they measured between the wrong two points, or they remembered a fitting allowance from a different fitting. This card is the one method that works for every joint type, the three dimensions it depends on, and how to read them off the fitting in your hand.
The three dimensions
The drawing uses the figures from the worked example below.
CENTER A CENTER B
|<---------------------------------->|
C-C, 24 in
| |
+--+-----+ +-----+--+
| | [==============================] | |
| | | PIPE, cut 23 in | | |
+--+-----+ +-----+--+
ELL A face face ELL B
| | |<---------------------->|
| | gap, face to face 21 1/2 in
|<>| take-off 1/2 in (center to pipe end)
|<>| socket depth 3/4 in
center to face 1 1/4 in = take-off + socket
(not to scale)
- Center to center (C-C). The distance between the centerlines of the two fittings. Layout, rough-in dimensions and drawings are almost always given this way, because centerlines are what line up across a wall.
- Center to face. From the centerline of the fitting's other opening to the face of the socket (or the end of the female thread) the pipe enters.
- Socket depth. How far the pipe goes into the fitting before it hits the stop. For threaded fittings this is the make-up, the length of thread that engages when the joint is wrench-tight.
The useful combined number is the take-off: center to face minus socket depth, which is the distance from the fitting's centerline to the end of the pipe once it is seated.
The method
Cut length = C-C - (center to face, fitting A) - (center to face, fitting B) + (socket depth A) + (socket depth B)
Or, using take-offs: cut length = C-C - take-off A - take-off B.
Both forms say the same thing. The pipe has to span the face-to-face gap and then disappear into each fitting by its socket depth. Subtracting the fittings and adding back the sockets is the form to use when you are learning, because it makes you look at each dimension separately.
Read the dimensions off the fittings in your hand. Center to face differs between short and long-radius fittings, between wrought and cast, between pressure and DWV patterns, between makers, and between a 90 and a 45. A take-off remembered from last month's job is a guess. To measure one:
- Seat a short scrap of pipe fully into each opening of the fitting.
- Lay it on a flat surface. Along the first opening's axis, measure from the face of that socket to the far outside edge of the scrap in the other opening.
- Subtract half the pipe's outside diameter. That is center to face for the first opening; repeat from the other socket, since on a reducing or street fitting the two differ.
- Mark the scrap at the socket face, pull it, and measure from the mark to the end. That is socket depth as actually seated, which is what matters.
Write the numbers on the box or in a phone note for the fittings you stock. Two minutes once saves the measurement on every job after it.
Hook the tape at a centerline, not an edge. On a roughed-in wall, mark the centerline of each stub or fitting and measure between marks. Measuring outside to outside, or inside to inside, is off by one pipe diameter.
Worked example: a 3/4 in copper return between two 90s
A 3/4 in copper line has to drop 24 in center to center between two wrought 90 degree ells, the vertical leg of a short offset around a beam.
Dimensions read off the ells on the truck (illustrative, measured with the steps above, not a published value): center to face 1 1/4 in on each ell; socket depth as seated 3/4 in. So the take-off on each ell is 1 1/4 - 3/4 = 1/2 in.
Long form. 24 - 1 1/4 - 1 1/4 + 3/4 + 3/4. 24 - 2 1/2 = 21 1/2 in, which is the face-to-face gap. 21 1/2 + 1 1/2 = 23 in cut length.
Short form, the other direction. 24 - 1/2 - 1/2 = 23 in. The two methods agree.
Sanity check. The pipe has to be longer than the face-to-face gap (21 1/2 in) and shorter than C-C (24 in). 23 in is between them. If your answer ever falls outside that window, you added where you should have subtracted.
The common wrong answers, and what they produce.
- Cut to C-C, 24 in. 1 in too long. The pipe bottoms in both sockets and the ells sit 1 in further apart than the layout, so the run above no longer lines up.
- Subtracted center to face and forgot the sockets, 21 1/2 in. 1 1/2 in short. The pipe would only reach the faces and never enter either socket.
- Added one socket only, 22 1/4 in. 3/4 in short. The joint at one end has no engagement and fails on the first pressure test, or worse, holds for a while on a thin fillet of solder.
Solder the joint with the pipe fully seated; the sweating steps are owned by How to Sweat a Copper Water Joint That Passes First Time.
Threaded pipe: make-up is not socket depth
A tapered pipe thread (NPT) does not bottom out; it wedges. How far the pipe goes in depends on size, how clean the threads are, the dope or tape, and how hard you pull on the wrench. That length is the make-up.
Measure it, do not assume it. Thread a nipple of the size you are using into one of the fittings, pull it wrench-tight the way you will on the job, and mark the nipple at the face. The length from the mark to the end is your make-up for that pipe size and fitting. For small residential sizes it commonly runs around 1/2 in on 1/2 in pipe and a little more on 3/4 in, but threads cut by different dies and fittings from different foundries vary by a thread or two, and a thread or two on a gas line is the difference between a tight joint and a weep.
Example, black iron gas. Two 3/4 in malleable 90s, 30 in C-C. Measured on the fittings in hand (illustrative, not a published value): center to face 1 1/8 in, make-up 9/16 in. Take-off = 1 1/8 - 9/16 = 9/16 in. Cut length = 30 - 9/16 - 9/16 = 28 7/8 in, threaded both ends. Check: face-to-face is 30 - 2 1/4 = 27 3/4 in; plus 9/16 + 9/16 = 1 1/8 in gives 28 7/8 in. The threading and assembly steps are owned by How to Cut, Thread and Assemble Black Iron Gas Pipe.
A threaded run is also the one place where the last fitting in a run sets its own orientation. A 90 that has to point straight up may need a quarter turn more or less than the make-up you measured. Leave the cut where your measurement put it and accept the wrench position, or use a union at the end of the run rather than over-tightening to point a fitting.
Runs with several fittings: tolerances stack
Every measurement carries some error, call it 1/16 in at the tape and at the cut. Across a run of six fittings and five pieces of pipe, those errors add, and the last fitting lands off its mark by more than any single cut was off.
Three habits contain the stack:
- Measure each piece from the fitting already installed, not from the drawing. You cut to what is there, so the errors do not carry forward.
- Dry-fit before you commit, on copper and PVC especially, and mark across the joint so it goes back together in the same rotation.
- Close the run with a piece measured last. Leave the final piece until everything else is in, then measure it to the actual gap.
DWV: sweeps, hubs and the slope you are cutting into
Drainage fittings make the remembered-number habit worse, because one size of pipe has several patterns of 90: a short-turn quarter bend, a long sweep and a closet bend each carry a very different center to face, and the long sweep's can be several times the short one's. A 2 in hub's socket depth is also noticeably deeper than a 3/4 in copper cup, so a copper habit carried onto PVC cuts every piece short. Measure the fittings you are using, the same way.
On a horizontal drain, the C-C you measure along a sloped run is along the pipe, not level. At 1/4 in per foot the difference over a few feet is a fraction of a sixteenth and can be ignored; on a long run between fixed points, measure along the pipe line, not along the floor. Slope itself is owned by Drain Pipe Slope + Venting Quick Reference.
PEX, press and push fittings
The method is the same and only the socket depth changes meaning. On PEX expansion and crimp fittings, the pipe goes over a barb or insert to a shoulder; on press fittings, the depth is marked on the pipe before pressing and the fitting maker states it; on push-fit fittings, the insertion depth is set by the maker and usually shown with a gauge or a printed mark. Read each from the fitting or its instructions and mark the pipe so you can see it went all the way home; see Reading PEX Pipe and Fitting Markings Before You Join for what the markings on the pipe tell you.
Offsets and angled runs
When two fittings are not on a straight line, for example a 45 degree offset around a joist, the C-C along the angled piece is not the distance you can measure on the wall. You compute it from the offset with a constant for the fitting angle (for a 45, the travel is the offset times about 1.414), and then apply this card's take-off method to the travel. Those constants are owned by Pipe Offset Constants for Every Fitting Angle; this card starts where you have the C-C along the pipe.
References
- See related: Pipe Offset Constants for Every Fitting Angle
- See related: How to Sweat a Copper Water Joint That Passes First Time
- See related: How to Cut, Thread and Assemble Black Iron Gas Pipe
- See related: Reading PEX Pipe and Fitting Markings Before You Join
- Fitting manufacturers' dimension sheets, for the published center-to-face of the fitting you stock