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Forty percent is the ceiling. Know it before you pull.
The NEC caps conduit fill at 53% for one conductor, 31% for two, and 40% for three or more — because pulling tension and heat dissipation, not empty space, are the real limits. This calculator cross-references THHN conductor areas against EMT and Schedule 40 PVC internal areas, reports your exact fill percentage, and tells you the maximum conductors of one size that fit each conduit size — the same numbers as NEC Chapter 9 Table C.
Electricians use it at the design table and again in the field when the pull plan changes — the 40% limit is the difference between a clean pull and stripped insulation. The grounds-count rule is the violation nobody sees coming; this counts them automatically.
Pulling geometry. A single cable slides cleanly; multiple conductors wedge against each other and the conduit wall, multiplying friction and trapping heat. The 40% limit for three or more conductors is what keeps long pulls possible and insulation intact.
Yes — every conductor in the conduit counts toward fill, including grounds. The classic violation: nine 12 AWG THHN conductors fit in ½-inch EMT, and adding a tenth as a ground pushes it over 40%. Count the ground in the calculator above and it will catch it.
Same trade size, different internal area — ¾-inch EMT has 0.533 sq in inside while Schedule 40 PVC has 0.508 sq in, because the plastic wall is thicker. Conduit fill tables are specific to raceway type; this calculator carries both.