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Conduit Fill Calculator

Check how full a run of EMT or PVC conduit is, from the size and number of THHN wires inside it, against the 53/31/40% fill limits.

Level: Grade 7

  • Common Core standard 7.G.B.4Know the formulas for the area and circumference of a circle and use them to solve problems; give an informal derivation of the relationship between the circumference and area of a circle.
  • Common Core standard 7.RP.A.3Use proportional relationships to solve multistep ratio and percent problems. Examples: simple interest, tax, markups and markdowns, gratuities and commissions, fees, percent increase and decrease, percent error.
Printable worksheet

Conduit Fill Calculator

Inputs
Conduit type

Count every conductor, including neutrals and equipment grounding wires.

Leave at 0 if every wire is the same size.

Leave at 0 if every wire is the same size.

A short nipple between enclosures may be filled to 60%.

Result

Conduit fill

18.29%

4 wires fill 18.29% of a 1/2 in EMT, within the 40% limit. Local code and the inspector have the final say.

Check
Within 40% limit
Fill limit used
40%
Total wire area
0.0556 in²
Conduit inside area
0.3039 in²
Area allowed for wires
0.1215 in²
Area left before the limit
0.066 in²
Most 12 AWG THHN wires alone
8
Number of wires
4

See the step-by-step solution ↓

Step-by-step solution

  1. Find the open space inside the 1/2 in EMT. Treat it as a circle with inside diameter d = 0.622 in, so its area A = π ÷ 4 × d²

    A=π4×0.6222=0.303858 in²

  2. Find the space one 12 AWG THHN takes up (0.133 in across, including insulation), then multiply by 4 wires

    4×π4×0.1332=0.055572 in²

  3. Find the percent fill: the wires' area divided by the conduit's open area, times 100

    0.0555720.303858×100=18.2887%

  4. Look up the fill limit for more than two conductors (40%) in NEC Chapter 9, Table 1, then find how much of the area the wires may use

    0.303858×40100=0.121543 in² allowed

  5. Work out how many 12 AWG THHN wires would fit on their own (each count is checked against its own limit)

    largest n with n×0.013893 ≤ allowed area: n = 8

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out whether your wires fit in a conduit without breaking the code's fill limit. Wires pulled into a conduit need room. Packed too tightly, they're hard to pull, their insulation can get scraped on the way in, and they can't shed heat. So the US National Electrical Code (NEC, also called NFPA 70) limits how much of a conduit's inside area the wires may take up. NEC Chapter 9, Table 1 sets the limits at 53% of the area for one conductor, 31% for two, and 40% for three or more.

This calculator does the area math for you. Choose EMT or Schedule 40 PVC and a trade size, or type the inside diameter of any conduit. Then enter up to three groups of THHN/THWN-2 copper wires by size and count. You'll see the percent fill, the limit that applies, and the area left. It also shows how many wires of the first size would fit on their own. The diameters come from published manufacturer data, not the code's own tables. You can replace any of them with the figures for the exact products you're using.

Where it's used

Why this math matters outside the classroom:

  • Pulling new circuits. An electrician adding two circuits to an existing 3/4 in EMT run checks that the new total stays under 40% before pulling.
  • Choosing a conduit size. Four 12 AWG wires fill 18.3% of 1/2 in EMT, so the smaller trade size is enough for that run.
  • Apprentice exams. Apprentices practise finding a circle's area from its diameter, adding the wire areas and dividing by the conduit area.
  • Solar and EV feeders. Installers check that large conductors such as 6 AWG feeders, with their ground, fit the conduit they plan to run outdoors.
  • Short nipples. A 12 in nipple between a panel and a gutter box may be filled to 60%, so more wires pass through a smaller size.

Grade level and standards: Grade 7; Common Core State Standards for Mathematics 7.G.B.4, 7.RP.A.3. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose the conduit type and trade size. Or choose Other and enter its inside diameter.
  2. Pick the wire size and enter how many of those wires run in the conduit. Count the neutrals and grounding wires too.
  3. If some wires are a different size, add them as group 2 or group 3. For wire types other than THHN, choose Other and enter the wire's outside diameter.
  4. Tick the nipple box only for a short section of 24 in or less between enclosures. Then read the fill and the check against the limit.

What the inputs mean

Trade size
Trade sizes are names, not measurements. For example, 1/2 in EMT has an inside width of about 0.622 in. The calculator uses the published inside diameter for each size.
Conduit inside diameter
From the conduit maker's specification sheet.
Number of these wires
Every conductor counts, including the neutral and any insulated or bare equipment grounding wire. A bare ground isn't THHN. To get it exact, enter it as its own group with its actual diameter.
Wire outside diameter
Use the overall diameter, measured over the insulation. Wire makers list it as the approximate OD (outside diameter) on their data sheets.
Number of group 2 wires
Leave at 0 if every wire is the same size.
Group 2 wire outside diameter
Overall diameter including insulation, from the wire maker's data sheet.
Number of group 3 wires
Leave at 0 if every wire is the same size.
Group 3 wire outside diameter
Overall diameter including insulation, from the wire maker's data sheet.
This is a nipple 24 in (600 mm) long or shorter
A nipple is a short length of conduit joining two boxes or enclosures. The code allows a higher fill in nipples no longer than 24 in.

Formula and method

Conduit area   A = π ÷ 4 × d(conduit)²
Wire area      a = π ÷ 4 × d(wire)²   for each wire
Fill (%)       = (sum of wire areas) ÷ A × 100
Limit          = 53% (1 wire), 31% (2 wires), 40% (3 or more), 60% (nipple ≤ 24 in)
Allowed area   = A × limit ÷ 100

In the formulas, A is the open area inside the conduit and a is the area one wire takes up. d is a diameter: the conduit's inside diameter, or the wire's outside diameter. π is pi, the familiar circle number.

Both the inside of the conduit and each wire are treated as circles, so their areas come from their diameters. The fill is the share of the inside the wires cover. The limit depends only on how many conductors there are. That's why two wires get a lower limit than three. Two round wires that are each nearly half the conduit's width can wedge side by side.

The inside diameters come from manufacturers' specifications. For EMT, it's the outside diameter minus twice the standard wall thickness, from an EMT maker's submittal data. For PVC, it's the published minimum average inside diameter. THHN diameters are wire makers' approximate overall diameters. The code's own tables use slightly different figures, so a count near the limit can differ by one wire.

Worked example

Four 12 AWG THHN wires (two circuits with their neutrals) run in 1/2 in EMT.

  1. Conduit area: π ÷ 4 × 0.622² = 0.303858 in².
  2. Each 12 AWG THHN wire, 0.133 in across: π ÷ 4 × 0.133² = 0.013893 in², so four take 0.055572 in².
  3. Fill: 0.055572 ÷ 0.303858 × 100 = 18.29%.
  4. With more than two conductors, the limit is 40%: 0.40 × 0.303858 = 0.121543 in² allowed, leaving 0.065972 in².
  5. On their own, 8 wires of this size fit (0.40 × 0.303858 ÷ 0.013893 = 8.75, rounded down).

Result: The conduit is 18.29% full, comfortably within the 40% limit.

Understanding the result

A fill at or under the limit means the wires take no more of the inside than NEC Chapter 9 allows for that number of conductors. Many electricians leave extra room below the limit on long runs or runs with several bends. Pulling gets much harder as the fill rises.

The maximum count is the most wires of the first size that would fit alone. In some cases the code lets you round a fractional count up. Chapter 9, Note 7 allows a result of 0.8 or more to round up when every conductor is the same size. Check the rule in the code edition your area uses before relying on the extra wire.

Fill is only one check. The wires also need enough ampacity (safe current-carrying capacity) once it's reduced for the number of current-carrying wires in the conduit. And the runs must stay within the number of bends the code allows between pull points.

Assumptions and limitations

  • Wires and conduit are round, and the published diameters are used as given.
  • The built-in wire sizes are copper THHN/THWN-2. Other insulation types are entered by diameter.
  • The limits are those of NEC Chapter 9, Table 1 and its Note 4 for nipples.
  • Manufacturers' diameters vary slightly between brands and between versions of their data sheets. Check results near the limit with the figures for the products actually installed. Or use the tables in the code edition your area has adopted.
  • It doesn't check jamming (three wires lining up across the inside), ampacity derating, cable assemblies, or bends.
  • Local codes, the inspector and the equipment instructions have the final say. This is a learning and planning aid, not electrical advice. Have a licensed electrician design and install the work.

Frequently asked questions

Why is the limit for two wires lower than for three?

Two round wires sit side by side across the conduit. If their combined width is close to the conduit's inside diameter, they can jam during the pull. The 31% limit keeps them small enough compared with the conduit. Three or more wires nest together, so 40% is allowed.

Do grounding wires count toward conduit fill?

Yes. Equipment grounding wires, insulated or bare, take up space and count in the fill. Enter them with their own size. For a bare wire, choose Other and enter its diameter.

Why does my answer differ slightly from a code table?

The code's tables use their own standard dimensions, while this calculator uses figures published by conduit and wire makers. The difference is usually a few hundredths of a square inch. Near the limit, that can change the maximum count by one wire. You can enter any diameter to match the source you need.

How many 12 AWG wires fit in 3/4 in EMT?

With a 0.824 in inside diameter and 0.133 in wires, 40% of the area allows 15 wires (0.40 × 0.533 ÷ 0.0139 = 15.3). In practice, the rules for many current-carrying wires in one conduit often limit the number well before fill does. Those rules lower each wire's allowed current (ampacity derating).

Sources

How this calculator is checked

Method: Circle areas from published diameters; fill = wire area ÷ conduit area. Limits per NEC (NFPA 70) Chapter 9, Table 1 (53%, 31%, 40%) and Note 4 (60% for nipples ≤ 24 in), cited by section only. THHN ODs from Southwire SPEC 10000 and an AutomationDirect THHN data sheet; EMT bores from Atkore/Allied EMT OD and wall; PVC bores from Atkore ATCE-SPEC-6709-2402. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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