Conduit Fill Calculator

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Check whether your conductors fit a conduit, or find the smallest conduit size that holds them, using NEC Chapter 9 fill tables.Learn more ▾Show less ▴
This calculator sizes electrical raceways the way the National Electrical Code does: it adds the cross-sectional area of every conductor and compares the total to the conduit's permitted fill area. Choose a conduit type and size to verify fill, or let the tool recommend the minimum size. It supports EMT, IMC, RMC, PVC, ENT, and flexible conduit, with the THHN, XHHW, THW, RHW, and TW conductor areas published in NEC Tables 4 and 5.

Conductors

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Cables

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Conductor breakdown

Conductor Qty Area each (in²) Total (in²)
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About This Tool

The Conduit Fill Calculator checks how much of a conduit's interior is occupied by the wires inside it, following the National Electrical Code. It sums each conductor's area and compares the total to the code's permitted fill, so electricians, inspectors, and DIYers can confirm a raceway is legal and pullable before any wire goes in.

Sources: NEC (NFPA 70)

How to Use

  1. Choose your conduit type — and a trade size if you want to check a specific conduit, or switch to "find minimum size" to size it automatically.
  2. Add each conductor by insulation, size, and quantity. Include every wire in the raceway, neutrals and grounds included.
  3. Read the fill percentage, the permitted limit, and the pass/fail result — or the recommended minimum conduit size.

Methodology

Calculations follow NEC (NFPA 70) Chapter 9. Table 1 sets the permitted fill: 53% for one conductor, 31% for two, and 40% for three or more; a nipple of 24 inches or less may be filled to 60%. Conductor areas come from Table 5 and conduit internal areas from Table 4. The total conductor area is compared to the conduit area times the permitted percentage. Single-size, single-insulation results match the maximum counts tabulated in NEC Annex C.

Sources: NEC (NFPA 70) Chapter 9 · NEC Chapter 9 fill tables (conduit.site)

Understanding Your Results

The fill percentage is the conductor area divided by the conduit's total internal area. A result at or below the permitted limit (40% for three or more conductors) passes; above it, you need a larger conduit or fewer wires. Remember that fill is only the space rule. When more than three current-carrying conductors share a raceway, their ampacity must also be reduced under NEC 310.15(C)(1) — a separate check from physical fit.

Sources

Practical Examples

Example 1: Nine 12 AWG THHN conductors in 1/2-inch EMT. Each is 0.0133 in², totaling 0.1197 in². The 40% limit for 1/2-inch EMT is 0.122 in², so they just fit — matching the nine-conductor maximum in NEC Annex C. Example 2: Four 8 AWG THHN conductors total 0.1464 in². That exceeds 1/2-inch EMT (0.122 in² at 40%), so the calculator recommends 3/4-inch EMT, whose 40% area is 0.213 in².

Conduit fill tips

• Count every conductor, including the neutral and the equipment grounding conductor. They all take space. • THHN/THWN is the most compact common insulation — switching from THW or RHW to THHN can let you stay in a smaller conduit. • Leave room to grow. Many electricians target 40% even where a higher single- or two-conductor limit would be allowed, so future conductors can be added. • A short nipple (24 in or less) may be filled to 60%, but the conductors in it still count toward ampacity derating unless an exception applies. • If you exceed fill, stepping up one trade size usually more than doubles the available area.

All calculations are performed locally in your browser. No data is sent to any server.

Frequently Asked Questions
What is conduit fill and why does it matter?
Conduit fill is the percentage of a raceway's internal cross-sectional area that is taken up by the conductors inside it. The National Electrical Code limits fill so that wires can be pulled without damaging insulation and so heat can dissipate. For three or more conductors, the limit is 40% of the conduit's area; exceeding it is a code violation and can lead to overheating and failed inspections.
How many wires can I put in a conduit?
It depends on the conductor size, insulation type, and conduit type and size. This calculator adds up each conductor's cross-sectional area from NEC Chapter 9, Table 5, compares the total to the permitted fill area, and tells you whether it fits. Use "find minimum size" mode to get the smallest conduit that holds your wires. The results match the fill counts in NEC Annex C.
What is the 40% fill rule?
NEC Chapter 9, Table 1 sets the maximum fill by conductor count: one conductor may fill up to 53% of the conduit, two conductors up to 31%, and three or more conductors up to 40%. The lower limits for one and two conductors account for the way wires arrange themselves and the space needed to pull them.
Which conduit types does this calculator support?
It covers the most common raceways: EMT, IMC, and RMC (rigid metal), PVC Schedule 40 and Schedule 80, ENT (electrical nonmetallic tubing), and flexible types FMC, LFMC, and LFNC. Each has its own internal area from NEC Chapter 9, Table 4, because wall thickness differs between types even at the same trade size.
Does it work for THHN and THWN wire?
Yes. The calculator includes THHN/THWN/THWN-2, THW/THHW, XHHW/XHHW-2, RHH/RHW/RHW-2, and TW, each with the approximate area from NEC Chapter 9, Table 5. THHN/THWN is the most common building wire and has the smallest area for a given gauge, so it fits more conductors in a conduit than thicker insulations.
What is a nipple and the 60% fill rule?
A nipple is a short length of conduit, 24 inches (600 mm) or less, connecting two boxes, cabinets, or enclosures. Because a short run is easy to pull, NEC Chapter 9, Note 4 permits filling a nipple to 60% of its area regardless of conductor count. Check the nipple option to apply this allowance. Ampacity adjustment factors do not apply to nipples.
Do I count the ground wire in conduit fill?
Yes. Every conductor in the raceway counts toward fill, including the equipment grounding conductor and the neutral. Add each one to the calculator with its actual size and insulation. The only common exception is a bare grounding conductor, which still takes space — enter it using its conductor size so the fill total stays conservative.
Does a fuller conduit require derating the conductors?
It can. Conduit fill (the physical space rule) is separate from ampacity adjustment (the heat rule). When more than three current-carrying conductors share a raceway, NEC 310.15(C)(1) reduces their allowable ampacity — 80% for 4–6 conductors, 70% for 7–9, and so on. Use a voltage drop and wire size calculator to check ampacity after confirming the conductors physically fit.
Does this match NEC Annex C?
Yes. NEC Annex C lists the maximum number of same-size conductors for each conduit type, derived from the Chapter 9 Table 1 percentages and the Table 4 and 5 areas. This calculator computes from those same source tables, so a single-insulation, single-size entry reproduces the Annex C counts — for example, nine 12 AWG THHN conductors in 1/2-inch EMT.
Is my data private?
Completely. The calculator runs entirely in your browser. Your conduit and conductor entries are never uploaded, stored on a server, or shared. You can use it offline once the page has loaded, and sharing a result only encodes your inputs in the link if you choose to copy it.