Trades Math Basics: Fitting Allowances and Make-Up (Face-to-Center, End-to-Center, Take-Off)

Trades Math – Fitting Allowances and Make?Up (Face?to?Center, End?to?Center, Take?Off)

Study Guide for Apprentices & Journeymen

What This Is

Fitting allowances and make-up are the "hidden lengths" you must account for when cutting pipes, conduit, or framing members so they fit exactly between two points after bends, joints, or connections. Miss this, and your pipe run will be too short, your stud wall won’t fit the header, or your conduit won’t thread into the box. Real-world example: You’re running ½" EMT conduit from a junction box to a panel 10' away, with a 90° bend in the middle. If you just measure 10' and cut, the conduit won’t reach—you must subtract the "take-off" (the length the conduit loses inside the fittings) to get the correct cut length.

Key Terms & Formulas

• Face-to-Center (F-to-C): The distance from the face of a fitting (where the pipe/conduit enters) to the center of the bend or branch. Example: A ½" 90° EMT elbow has a face-to-center of 3¾"—this is how much length the fitting "eats" from your run.
• End-to-Center (E-to-C): The distance from the end of a fitting (where it threads into a box or another fitting) to the center of the bend. Example: A ¾" copper 90° street elbow has an end-to-center of 1½"—use this when threading into a valve or tee.
• Take-Off (TO): The total length lost inside fittings when assembling a run. Formula: TO = (F-to-C of first fitting) + (F-to-C of second fitting) – (any overlap, like a coupling). Example: Two ½" EMT 90° elbows (3¾" F-to-C each) with a coupling in between: TO = 3¾" + 3¾" – ¾" (coupling overlap) = 6¾".
• Make-Up: The extra length you add to a pipe or conduit to account for threads, glue, or compression fittings. Example: For threaded steel pipe, add ½" per joint for make-up (the part that threads into the fitting).
• Rolling Offset: A pipe/conduit run that changes direction in two planes (e.g., up and over a beam). Formula: Travel = ?(Rise² + Run² + Offset²). Example: A pipe rises 6", runs 8", and offsets 4": Travel = ?(6² + 8² + 4²) = ?(36 + 64 + 16) = ?116-10.77".
• Center-to-Center (C-to-C): The distance between the centers of two fittings or bends. Example: If two 90° elbows are 5' apart center-to-center, the cut length of the pipe between them is 5' – (F-to-C of first elbow) – (F-to-C of second elbow).
• Stub-Up: The vertical length of pipe/conduit from a fitting to a box or fixture. Formula: Stub-Up = Desired height – (F-to-C of fitting). Example: For a 48" stub-up with a ½" EMT 90° (3¾" F-to-C): 48" – 3¾" = 44¼" cut length.
• Coupling Overlap: The length a coupling or union adds to a run (usually ¾" for EMT, ½" for PVC). Subtract this from total take-off if using a coupling between fittings.
• Thread Engagement: The length of pipe that threads into a fitting (typically ½" for ½"–2" steel pipe). Add this to your cut length if threading into a fitting.
• Glue Make-Up (PVC): For solvent-welded PVC, add ½" per joint for glue make-up (the pipe slides into the fitting this far).

Step-by-Step / Process Flow

1. Measure the Total Run (Center-to-Center)

• Use a tape measure or laser to find the center-to-center (C-to-C) distance between the two points where the pipe/conduit must connect (e.g., between two boxes, a valve and a tee, or a stud wall’s top and bottom plates).
• Example: You’re running ½" EMT from a junction box to a panel 120" apart, with a 90° bend at each end.

2. Identify Fittings and Their F-to-C/E-to-C

• Look up the face-to-center (F-to-C) or end-to-center (E-to-C) for each fitting in the run. Manufacturers provide these in catalogs or on fitting boxes.
• ½" EMT 90° elbow: F-to-C = 3¾"
• ½" EMT coupling: Overlap = ¾"
• ¾" copper 90° street elbow: E-to-C = 1½"

3. Calculate Take-Off (TO)

• For a straight run with two fittings (e.g., two 90° elbows): TO = (F-to-C of first fitting) + (F-to-C of second fitting) – (coupling overlap, if any)

• Example: Two ½" EMT 90° elbows (3¾" each) with a coupling (¾" overlap): TO = 3¾" + 3¾" – ¾" = 6¾"

• For a rolling offset:

• Measure rise, run, and offset (the three dimensions of the "box" the pipe travels through).
• Use the rolling offset formula: Travel = ?(Rise² + Run² + Offset²)
• Subtract the F-to-C of the two fittings (usually 45° elbows for offsets).
  • Example: Rise = 6", Run = 8", Offset = 4", using ½" EMT 45° elbows (F-to-C = 2½"): Travel = ?(6² + 8² + 4²) = ?116-10.77" Cut length = 10.77" – 2½" – 2½" = 5.77"

4. Add Make-Up (If Needed)

• For threaded pipe: Add ½" per joint for thread engagement.
• Example: A 10' steel pipe run with two threaded 90° elbows: 10' + ½" + ½" = 10' 1" total cut length.
• For PVC: Add ½" per joint for glue make-up.
• For EMT: No make-up needed (compression fittings don’t add length).

5. Cut and Assemble

• Double-check: Measure twice, cut once. Dry-fit before gluing/threading.
• Mark your cuts: Use a permanent marker or scribe to note where to cut.
• Deburr: Always ream the inside of pipes/conduit to avoid damaging wires or creating turbulence in fluid flows.

Common Mistakes

Trade-Specific Insights

1. EMT vs. Rigid Conduit:
2. EMT fittings (compression or set-screw) have no thread engagement, so no make-up is needed.

3. Rigid conduit (threaded) requires ½" make-up per joint—factor this into your cut length.

4. PVC Glue Make-Up:

5. PVC fittings have a socket depth (usually ½" for ½"–2" pipe). Always add this to your cut length.

6. Pro tip: Mark the pipe at ½" from the end before gluing to ensure full insertion.

7. Copper Sweat Fittings:

8. Copper 90° elbows have no make-up (they’re soldered), but street elbows (male threads) require E-to-C measurements.

9. Field trick: Use a copper stub-out calculator (many apps exist) to avoid manual calculations.

10. Code Considerations:

11. NEC 358.28 (EMT): Conduit must be reamed to remove sharp edges that could damage wires.
12. IPC 605.3 (Plumbing): Pipe runs must allow for thermal expansion (e.g., add ¼" per 10' for copper in hot water systems).

Quick Check Questions

1. You’re running ½" EMT from a junction box to a panel 8' away, with a 90° bend at each end. The F-to-C of each 90° elbow is 3¾". What’s the cut length of the conduit?
2. Answer: 8' – 3¾" – 3¾" = 7' 4½".

3. Why? Subtract the F-to-C of both fittings from the total run.

4. A ¾" copper pipe run has two threaded 90° street elbows (E-to-C = 1½" each) and a coupling (overlap = ½"). The C-to-C distance is 6'. What’s the cut length?

5. Answer: 6' – 1½" – 1½" + ½" (make-up per joint) + ½" (make-up per joint) – ½" (coupling overlap) = 5' 11½".

6. Why? Subtract E-to-C, add thread make-up, and subtract coupling overlap.

7. You’re installing a rolling offset with a rise of 8", run of 12", and offset of 6". Using ½" EMT 45° elbows (F-to-C = 2½"), what’s the cut length of the conduit?

8. Answer: ?(8² + 12² + 6²) – 2½" – 2½" = ?(64 + 144 + 36) – 5" = ?244 – 5"-15.62" – 5" = 10.62".
9. Why? Calculate travel distance, then subtract F-to-C of both fittings.

Last-Minute Cram Sheet

1. EMT 90° elbow F-to-C: ½" = 3¾", ¾" = 4½", 1" = 5¾".
2. PVC 90° elbow F-to-C: ½" = 2", ¾" = 2½", 1" = 3".
3. Copper 90° street elbow E-to-C: ½" = 1¼", ¾" = 1½", 1" = 1¾".
4. Thread make-up (steel pipe): +½" per joint.
5. PVC glue make-up: +½" per joint.
6. Coupling overlap: EMT = ¾", PVC = ½".
7. Rolling offset formula: Travel = ?(Rise² + Run² + Offset²).
8. Stub-up formula: Cut length = Desired height – F-to-C.
9. For threaded fittings, use E-to-C (not F-to-C)!
10. Always ream conduit/pipe to avoid wire damage (NEC 358.28).