Conduit Fill Calculator — EMT and PVC, NEC Chapter 9

How to use the conduit fill calculator

Pick the conduit type and trade size. Enter the number of wires and their size; if the run has more than one size, use the second and third groups. Count every wire in the conduit, the ground included. The calculator uses THHN/THWN-2 dimensions, the usual building wire in conduit; other insulation types have different areas.

This is a planning check. Your local code edition and the inspector have the final say.

The formula (NEC Chapter 9)

Wire area = number of wires × each wire’s area (Table 5)
Fill = wire area ÷ the conduit’s inside area (Table 4) × 100
Limit (Table 1): 53% for 1 wire, 31% for 2, 40% for 3 or more
Note 7: when all the wires are the same size, a count ending in 0.8 or more rounds up to the next whole wire

Worked example: two 20 A circuits and a 30 A circuit in 3/4 in EMT

Five 12 AWG wires (2 hots, 2 neutrals, ground): 5 × 0.0133 = 0.0665 sq in
Three 10 AWG wires (2 hots, ground): 3 × 0.0211 = 0.0633 sq in
Wire area 0.1298 sq in; 8 wires, so the limit is 40%
3/4 in EMT has 0.533 sq in inside: 0.1298 ÷ 0.533 = 24.4% fill, which fits
1/2 in EMT (0.304 sq in) would be 42.7%, too full, so 3/4 in is the smallest EMT that fits

How many wires fit

Most THHN/THWN-2 wires of one size, ground included. These match the NEC’s Annex C tables.

EMT	1/2 in	3/4 in	1 in
14 AWG	12	22	35
12 AWG	9	16	26
10 AWG	5	10	16
8 AWG	3	6	9
6 AWG	2	4	7

PVC Schedule 40	1/2 in	3/4 in	1 in
14 AWG	11	21	34
12 AWG	8	15	25
10 AWG	5	9	15
8 AWG	3	5	9
6 AWG	1	4	6

PVC Schedule 80	1/2 in	3/4 in	1 in
14 AWG	9	17	28
12 AWG	6	12	20
10 AWG	4	7	13
8 AWG	2	4	7
6 AWG	1	3	5

Common mistakes

Leaving out the ground. It counts toward fill like any other wire.
Using the 40% limit for two wires. Two wires may fill only 31%, less than three.
Assuming Schedule 80 holds as much as Schedule 40. Its thicker walls leave less room in the same trade size.
Sizing by fill alone. More than three current-carrying wires in one conduit also means derating their ampacity.

Frequently asked questions

How many 12 AWG wires can go in 3/4 inch EMT?

16 THHN/THWN-2 wires, counting the ground. 1/2 in EMT holds 9 and 1 in EMT holds 26. In 3/4 in PVC Schedule 40 it's 15, and in Schedule 80 it's 12.

What is the maximum conduit fill?

40% of the conduit's inside area for three or more wires, 31% for two and 53% for a single wire (NEC Chapter 9, Table 1). A conduit nipple 24 in or shorter between enclosures may be filled to 60%.

Does the ground wire count in conduit fill?

Yes. Every equipment ground or bonding wire in the conduit counts at its actual size, insulated or bare. The calculator counts it as THHN; a bare ground is smaller, so that errs on the safe side.

Why does the calculator say a conduit fits when the fill is over the limit?

Note 7 to the Chapter 9 tables: when all the wires are the same size and the count works out to a whole number plus 0.8 or more, you may round up. Two 6 AWG wires in 1/2 in EMT fill 33.4%, over the 31% limit, but the count comes to 1.86 wires, which rounds up to 2.

Is PVC Schedule 80 smaller inside than Schedule 40?

Yes. The walls are thicker, so the same trade size has less room. A 1 in conduit has 0.832 sq in inside in Schedule 40 and 0.688 sq in in Schedule 80, so it holds 25 wires of 12 AWG instead of 20.

Do I need to derate the wires too?

Fill only checks that the wires fit. With more than three current-carrying wires in one conduit, their ampacity also has to be reduced under NEC 310.15(C)(1), which can call for bigger wire.

Can I use this for my permit?

Use it to choose the conduit size before you buy. Your local code edition and the inspector have the final say, and long pulls with several bends may need a bigger conduit than the minimum.

Sources

Formulas and defaults last reviewed October 30, 2026. How we check our math.