Conduit Fill Calculator
EMT, IMC, RMC, PVC Schedule 40, and PVC Schedule 80 with THHN / THWN-2 conductors from 14 AWG through 750 kcmil. The calculator finds the smallest trade size that meets the NEC fill limit for your conductor mix and shows you the fill percentage at every common size.
Conduit type
NEC Ch. 9 Table 4Quick presets
Conductors (THHN / THWN-2)
NEC Ch. 9 Table 5Fill at every trade size
NEC Ch. 9 Table 1Frequently asked questions
What is the maximum conduit fill allowed by the NEC?
NEC Chapter 9, Table 1 sets the maximum cross-sectional area of conductors as a percentage of the conduit's internal area: 53% for a single conductor, 31% for two conductors, and 40% for three or more conductors. These limits apply to all standard raceway types — EMT, IMC, RMC, PVC, FMC, ENT, and others.
Where do these conductor and conduit dimensions come from?
Conductor cross-sectional areas come from NEC Chapter 9, Table 5 (this calculator uses THHN/THWN/THWN-2 values, the most common in residential and commercial branch circuits). Conduit total internal areas come from NEC Chapter 9, Table 4. Both tables list the dimensions used in this calculator without modification.
Why does the answer change with conductor count?
The fill rule is stricter for fewer conductors because of how heat dissipates inside the raceway and how easily conductors can be pulled. Three or more conductors get the standard 40% allowance. Two conductors are limited to 31% — pulling two stiff conductors through a tight conduit is harder than pulling many smaller ones. A single conductor gets 53% — the most generous allowance.
Does this calculator work for mixed conductor sizes?
Yes. Add as many rows as you need with different gauges. The calculator sums each conductor's individual cross-sectional area and compares the total to the conduit's allowable fill area. A typical 200A feeder might have three #2/0 ungrounded conductors and one #4 grounding conductor — add each separately.