Conduit Size Calculator

What size conduit do I need?

This conduit size calculator adds up the cross-sectional area of your wires, divides by 0.40, and picks the smallest EMT trade size whose inside area is at least that big. The formula is required area = number of conductors × area of one conductor ÷ 0.40. Six 12 AWG THHN wires need 6 × 0.0133 ÷ 0.40 = 0.1995 in², so 1/2-inch EMT works.

Enter the number of current-carrying, neutral, and ground conductors, and their wire gauge (14, 12, 10, 8, or 6 AWG). The calculator returns the minimum EMT trade size from 1/2 inch up to 2 inches.

The 40% rule in plain words

Picture a straw packed with spaghetti. Fill it too full and you cannot push another strand through without scraping the others. Packed wires also trap heat. The National Electrical Code handles both problems by limiting how much of a conduit’s inside area the wires may fill. With three or more conductors, the limit is 40%.

The calculator works backward from that limit:

1. Wire area: count × THHN area for the gauge
2. Required conduit area: wire area ÷ 0.40
3. Pick a size: the first EMT size whose internal area meets or beats it

Worked examples

1. Two 20-amp circuits sharing a run: six 12 AWG THHN

• Wire area: 6 × 0.0133 = 0.0798 in²
• Required area: 0.0798 ÷ 0.40 = 0.1995 in²
• 1/2-inch EMT has 0.304 in², so the answer is 1/2 inch

2. Ten 12 AWG THHN

• Wire area: 10 × 0.0133 = 0.133 in²
• Required area: 0.133 ÷ 0.40 = 0.3325 in²
• That is more than 1/2 inch EMT holds (0.304 in²) but less than 3/4 inch (0.533 in²). The answer is 3/4 inch.

3. Four 6 AWG THHN for a subpanel feeder

• Required area: 4 × 0.0507 ÷ 0.40 = 0.507 in², giving 3/4 inch

Maximum THHN conductors per EMT size (40% fill)

EMT trade size	Internal area	#14	#12	#10	#8	#6
1/2 in	0.304 in²	12	9	5	3	2
3/4 in	0.533 in²	21	16	10	5	4
1 in	0.864 in²	35	25	16	9	6
1-1/4 in	1.496 in²	61	44	28	16	11
1-1/2 in	2.036 in²	83	61	38	22	16
2 in	3.356 in²	138	100	63	36	26

These counts are for EMT only and all-same-size THHN wires. PVC, rigid, and flexible conduit have different inside areas at the same trade size.

How to count your conductors

• Count every wire in the run. That includes hots, neutrals, switch legs, travelers, and the equipment ground.
• Check the gauge on the insulation. It is printed every couple of feet along the jacket, along with the insulation type.
• Mixed gauges: add the area of each wire separately, divide by 0.40, then compare with the internal areas in the table above. The calculator handles one gauge at a time.
• Plan for later. Upsizing one trade size now costs little and leaves room for a future circuit.

Common conduit sizing mistakes

• Sizing for fill and forgetting derating. More than three current-carrying conductors in one raceway reduces their allowable ampacity. A run can pass the fill check and still need bigger wire. Check that with the wire size calculator.
• Too many bends. NEC limits a run between pull points to 360 degrees of bends. A tight fill with several 90s is hard to pull. The conduit bending calculator helps lay out offsets.
• Using the 40% figure for one or two wires. Two conductors are limited to 31% fill, which is stricter. For two wires, check the result with the conduit fill calculator.
• Larger gauges. This tool covers 14 to 6 AWG. For 4 AWG and up, use the NEC Chapter 9 tables directly.

Box sizing is a separate rule. See the electrical box fill calculator.

Electrical work is code-governed and safety-critical. Have a licensed electrician confirm conductor sizing, conduit fill, and grounding against the code edition your area has adopted.