About the Conduit Fill Calculator
This conduit fill calculator checks whether a group of THHN/THWN-2 building wires fits legally inside a conduit. Choose the conduit type and trade size, the wire gauge and how many current-carrying and grounding conductors you plan to pull, and it returns the fill percentage, the limit that applies, and the maximum number of that wire the conduit can hold.
It is meant for electricians, apprentices, estimators and DIYers planning a run of EMT or Schedule 40 PVC. The chart compares how many wires of the selected gauge fit in every trade size, so you can quickly step up a size when a run is too full or when you want spare room for future circuits.
Areas come from the National Electrical Code (NEC) Chapter 9 Table 4 (conduit) and Table 5 (THHN/THWN-2 conductors). The fill limits are 53% for one wire, 31% for two and 40% for three or more, or 60% for a nipple 24 inches or shorter. Always confirm with the code edition your jurisdiction has adopted — derating for more than three current-carrying conductors is a separate check.
With the default inputs, the conduit fill is 22.5%. Change any value above to recalculate instantly.
How to use the conduit fill calculator
- 1Pick the conduit type — EMT or Schedule 40 PVC.
- 2Choose the trade size you plan to install.
- 3Select the THHN/THWN-2 wire gauge.
- 4Enter the total number of conductors, including neutrals and grounds.
- 5Read the fill percentage and pass/fail result; use the chart to pick a larger size if needed.
Formula and method
Conduit fill compares the total cross-sectional area of the conductors, including insulation, with the internal area of the conduit. Each THHN/THWN-2 wire area comes from NEC Chapter 9 Table 5 and each conduit area from Table 4. The fill limit is 53% for a single conductor, 31% for two and 40% for three or more (Table 1); nipples up to 24 inches may be filled to 60%.
For the maximum number of identical wires, the usable area (40% or 60%) is divided by one wire area. Per Chapter 9 Note 7, if the result has a decimal of 0.8 or more it is rounded up to the next whole wire; otherwise it is rounded down. This matches the Annex C tables.
- N
- Number of conductors
- A_wire
- Area of one conductor with insulation (in²)
- A_conduit
- Total internal area of the conduit (in²)
- limit
- Allowed fill fraction (0.53, 0.31, 0.40 or 0.60)
Worked examples
Nine 12 AWG wires in 3/4" EMT
Nine 12 AWG THHN wires take 9 × 0.0133 = 0.1197 in², which is about 22.5% of the 0.533 in² inside 3/4" EMT — well under the 40% limit. 40% of 0.533 is 0.2132 in², enough for 16 wires of this size.
Four 6 AWG wires in 1" PVC
Four 6 AWG THHN wires total 4 × 0.0507 = 0.2028 in². 1" Schedule 40 PVC has 0.832 in² inside, so the fill is 24.4%. 40% of 0.832 is 0.3328 in², which divided by 0.0507 gives 6.56 — rounded down to 6 wires.
Twenty 10 AWG wires in 1" EMT (over fill)
Twenty 10 AWG wires need 20 × 0.0211 = 0.422 in², or 48.8% of 1" EMT. The limit is 40%, which allows only 16 wires (0.3456 ÷ 0.0211 = 16.4), so the run needs 1-1/4" EMT.
Frequently asked questions
What is the maximum conduit fill allowed by the NEC?+
NEC Chapter 9 Table 1 allows 53% fill for one conductor, 31% for two conductors and 40% for three or more. Nipples 24 inches or shorter may be filled to 60%.
How many 12 AWG wires can go in 3/4" EMT?+
Up to 16 THHN/THWN-2 12 AWG conductors fit in 3/4" EMT at 40% fill. Remember that more than three current-carrying conductors require ampacity derating, which often limits you before fill does.
Do ground wires count toward conduit fill?+
Yes. Every conductor in the raceway counts toward fill, including equipment grounding conductors and neutrals. Grounds do not count as current-carrying for derating, but they do take up space.
Why is two-wire fill only 31%?+
Two round conductors side by side can jam across the conduit diameter when pulled around bends, so the code uses a lower 31% limit for exactly two conductors to reduce pulling damage.
Does this work for other insulation types like XHHW or bare wire?+
This calculator uses THHN/THWN-2 areas from Table 5. XHHW, RHW and bare conductors have different areas, so check the table for your insulation type before relying on the result.