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Conduit Fill Calculator

This conduit fill calculator works out what percentage of a conduit your cables actually take up, and compares it against the NEC Chapter 9 fill limit for that number of conductors. Pick your conduit type and trade size, add a row for each cable you're pulling — Cat6, coax, fiber, THHN building wire, or a custom diameter in inches or millimetres — and the fill percentage and a pass/fail badge update as you go. It replaces flipping between a fill chart and a calculator, handles mixed pulls that charts can't, and runs entirely in your browser with no signup.

Pick your conduit type and trade size at the top, then add a row per cable with its quantity. The bar at the bottom shows the fill percentage and a pass/fail badge against the NEC limit — tap it to see the exact square-inch areas and, if you're over, the smallest conduit size that would fit.

Know your conduit and what's going in it? Get the fill percentage in a few taps.

Use the free Conduit Fill Calculator →

What Is Conduit Fill Percentage?

Conduit fill percentage is the share of a conduit's internal cross-section taken up by the cables inside it. Add up the area of every cable, divide by the area inside the pipe, and you have the fill ratio. The limit exists for two practical reasons: conductors bunched together trap heat, and an overstuffed conduit is punishing to pull — jackets scrape, tension climbs and cables can jam or get damaged on the way through.

Because area goes up with the square of diameter, fill is far less forgiving than it feels. A Cat6A jacket is only about a third wider than Cat6, but it eats roughly 1.9 times the area — so a conduit that comfortably held six Cat6 runs will not hold six Cat6A runs. That non-linearity is exactly what makes eyeballing a pull unreliable.

How to Use the Conduit Fill Calculator

Everything is reactive — there's no submit button, and the bar at the bottom updates on every tap:

Open capsuletools.app/conduit-fill-calculator/
Pick the conduit type — EMT, IMC, RMC, PVC Schedule 40 or 80, ENT, FMC or LFMC, or Custom to type an inside diameter yourself
Pick the trade size — the small number under each size is that conduit's real inside diameter in inches
Add your cables — one row per cable type, with a quantity. Use Add cable for a second or third type in the same run
Read the bar — fill percentage, the limit that applies, and PASS or FAIL. Tap it to expand the exact areas

The expanded panel shows the total cable area, the conduit's full internal area, the area actually allowed at your limit, and how much room is left (or how far over you are). If the run fails, it also names the smallest size of the same conduit type that would fit — so a FAIL comes with the answer attached.

Conduit: 3/4″ EMT (0.824″ inside → 0.5333 in²)
Cables: 4 × Cat6 (0.22″ OD → 0.0380 in² each)
Cable area = 4 × 0.0380 = 0.1521 in²
Fill = 0.1521 / 0.5333 = 28.5% · limit 40% → PASS

NEC Conduit Fill Limits: 53%, 31%, and 40% Explained

NEC Chapter 9, Table 1 gives three limits, and which one applies depends only on how many conductors or cables are in the raceway:

Conductors in the conduit	Maximum fill
1	53%
2	31%
3 or more	40%

The dip at two conductors surprises people, but it's geometry: two circles in a tube pack together poorly and leave awkward, unusable gaps, so the code allows less. This is also why adding one cable can make a passing run fail — going from one conductor to two drops your ceiling from 53% to 31%. The calculator counts the total quantity across every row and switches limits for you, so you see that jump happen rather than having to remember it.

Conduit Trade Size Chart and Supported Types

Inside diameters come from NEC Chapter 9, Table 4 — the same table an inspector uses. Every type carries its full published trade-size range:

EMT — electrical metallic tubing, 1/2″ to 4″
IMC — intermediate metal conduit, 1/2″ to 4″
RMC — rigid metal conduit, 1/2″ to 6″
PVC Schedule 40 and Schedule 80 — 1/2″ to 6″
ENT — electrical nonmetallic tubing, 1/2″ to 2″
FMC and LFMC — flexible and liquidtight flexible, from 3/8″

Here's the EMT trade size chart — nominal size against real inside diameter:

Trade size	Inside diameter
1/2″	0.622″
3/4″	0.824″
1″	1.049″
1-1/4″	1.380″
1-1/2″	1.610″
2″	2.067″

Note that trade size is not inside diameter. A 3/4″ EMT is 0.824″ inside; 3/4″ Schedule 80 PVC is only 0.722″, because the thicker wall steals room. Swapping type at the same trade size genuinely changes your answer, which is why the picker shows the real inside diameter under every size.

Cable and Wire Presets — Data, Coax, Fiber, and THHN Building Wire

Presets are grouped so a low-voltage run and a power run are equally quick:

Network — Cat5e, Cat6, Cat6A, shielded Cat6A, Cat7
Coax — RG59, RG6, RG6 quad-shield, RG11
Fiber — 2, 4, 6 and 12-strand
Low-voltage — thermostat, speaker and alarm cable
THHN / THWN-2 — 14 AWG through 500 kcmil, from NEC Chapter 9, Table 5

There's an important difference between those groups. THHN/THWN-2 diameters are published code-table values, so results for building wire match the NEC's own Annex C conductor counts exactly. Data, coax, fiber and low-voltage diameters are nominal — no code table standardises a Cat6 jacket, and real cable varies by manufacturer and between plenum and riser ratings. The tool says so when a nominal preset is in play. For a tight pull, take the OD off your cable's spec sheet.

As a rough guide at the 40% limit, with a typical 0.22″ Cat6:

EMT trade size	Cat6 (0.22″)	12 AWG THHN
1/2″	3	9
3/4″	5	16
1″	9	26
1-1/4″	15	45
1-1/2″	21	61

Custom Cable Diameters — Inches or Millimeters

Two escape hatches cover anything the presets don't. Choose Custom OD… at the bottom of a cable row's list to type an exact outside diameter — for specialty fiber, armoured cable, an odd multi-conductor, or a jacket you've measured with calipers. And choose the Custom conduit option to type an inside diameter directly.

Both accept inches or millimetres with a tap, which is what makes this usable outside the US. Metric conduit — 20 mm, 25 mm, 32 mm, 40 mm, 50 mm — is named by its outside diameter, so it never appears in NEC trade-size tables. Enter its real bore instead: 25 mm conduit is roughly 20 mm inside, though wall thickness (and therefore bore) varies by duty class, so measure or check the manufacturer's figure rather than assuming.

This Tool Is a Calculation Aid, Not a Code Compliance Certification

A PASS here means one specific thing: the cable area you entered fits within the NEC Chapter 9, Table 1 percentage for that conductor count. That's a single requirement among many, and area fill is usually the easiest one to satisfy.

It does not check ampacity or conductor derating for bundled conductors, the number of bends between pull points, pull tension or jam ratio, insulation temperature ratings, box fill, burial depth, or the nipple allowance for short conduit runs. It also has no idea which NEC edition or local amendments your jurisdiction has adopted — and adopted editions differ widely, while other countries work to the CEC in Canada, IEC-based rules, or their own national wiring codes entirely.

Use it the way you'd use a fill chart in your pocket: to size a run quickly and sanity-check a design. Then confirm the finished work against your current adopted code and your local inspector or AHJ. Electrical work should be done by someone qualified to do it.

Frequently asked questions

How do I calculate conduit fill percentage?

Add up the cross-sectional area of every cable going in, then divide by the conduit's internal area and multiply by 100. Each cable's area is pi times half its outside diameter squared, so a 0.22-inch Cat6 is 0.0380 square inches. Six of them is 0.2281 square inches, and 3/4-inch EMT has 0.5333 square inches inside, giving 42.8 percent fill. This calculator does all of that from the conduit and cable you pick, so you never touch the formula or the NEC tables.

What are the NEC conduit fill limits for 1, 2, and 3 or more cables?

NEC Chapter 9, Table 1 sets three limits based on how many conductors or cables are in the raceway: 53 percent for a single conductor, 31 percent for exactly two, and 40 percent for three or more. The two-conductor limit is the strictest because two circles pack together badly and leave little room to pull. The calculator counts your total quantity across all rows and applies the right limit automatically, so adding a second cable can tighten the target from 53 percent to 31 percent.

How many Cat6 cables fit in a conduit?

Using a typical 0.22-inch Cat6 jacket and the 40 percent limit that applies to three or more cables: 3 in 1/2-inch EMT, 5 in 3/4-inch, 9 in 1-inch, 15 in 1-1/4-inch and 21 in 1-1/2-inch. Cat6A is noticeably fatter at around 0.30 inches, so counts drop by roughly half. Jacket diameters vary between manufacturers and between plenum and riser versions, so check your spec sheet and enter the real OD if the pull is tight.

Does this calculator work for metric (mm) conduit sizes?

Yes. Choose the Custom conduit option and enter the conduit's inside diameter in millimetres — useful for 20 mm, 25 mm, 32 mm, 40 mm and 50 mm conduit, which is sized by outside diameter and so is not in the NEC trade-size tables. Cable rows accept a custom OD in millimetres too. Enter the actual internal diameter, not the nominal size: 25 mm conduit is roughly 20 mm inside depending on wall thickness, and the wall thickness varies by duty class.

Can I mix different wire sizes and cable types in one conduit fill calculation?

Yes, and that is the main reason to use a calculator rather than a lookup chart. Add one row per cable type with its own quantity — for example four Cat6, two RG6 coax and a 12 AWG THHN pair in the same run. The tool sums each row's area separately and totals the conductor count for the fill limit. Published fill charts almost always assume every conductor is identical, which is why they cannot answer a mixed pull.

Does a passing result mean my conduit run is code-compliant?

No. A PASS means only that the cross-sectional area you entered is within the NEC Chapter 9, Table 1 percentage for that conductor count. It is one requirement out of many. It does not check ampacity or conductor derating, the number of bends between pull points, pull tension, jam ratio, temperature rating, box fill or burial depth, and it does not know which NEC edition or local amendments your jurisdiction has adopted. Treat it as a fast sizing check and confirm the finished design against your current adopted code and your local inspector or AHJ.