🧰 Conduit Fill Calculator
Check how many wires fit in a conduit — get the NEC fill percentage, the minimum conduit size, and the maximum number of conductors. EMT, PVC, and RMC. Free, no sign-up.
A conduit fill calculator finds the percentage of a conduit's interior taken up by the wires inside it, and whether that stays within code. It adds the cross-sectional area of each conductor (NEC Chapter 9, Table 5), divides by the conduit's internal area (Table 4), and checks the result against the NEC fill limit. The core of it:
How to calculate conduit fill in 3 steps:
- Pick the conduit type and trade size (EMT, PVC Sch 40/80, RMC).
- Pick the conductor insulation and size, and how many you are running.
- Click Calculate for the fill %, the minimum conduit size, and the maximum wires.
For example, nine 12 AWG THHN conductors in ½-inch EMT fill it to 39.4% — just under the 40% limit, which is the maximum for that combination.
Conduit Fill Calculator
Pick a conduit and your conductors — see the NEC fill % and the minimum size.
Know If Your Wires Fit –
Before You Pull the Conduit
Cram too many wires into a conduit and you cannot pull them without nicking the insulation, and the heat has nowhere to go. This free calculator turns your conduit and conductors into an exact NEC fill percentage, the minimum conduit size, and the maximum number of wires that fit.
⚡ Try the Calculator NowConduit Fill Is About Heat and Pulling Tension
Conduit fill rules exist for two reasons. Too many conductors trap heat, which derates how much current each wire can safely carry. And a packed conduit is hard to pull, so the wires scrape and the insulation can tear. The NEC fill limits keep both problems in check.
The math itself is simple. Every conductor has a cross-sectional area, listed in NEC Chapter 9, Table 5 by insulation type and size. Every conduit has an internal area, listed in Table 4 by type and trade size. Conduit fill is just the total conductor area divided by the conduit area, expressed as a percentage.
This Conduit Fill Calculator does that lookup and the division for you. Pick the conduit and the conductors, and it returns the fill percentage, checks it against the NEC Table 1 limit (53% for one wire, 31% for two, 40% for three or more), and tells you the smallest conduit that works and how many more wires you could add.
How the Conduit Fill Calculator Works
Four picks and a count. Here is what each field does and what comes back.
Choose the Conduit Type
EMT, PVC Schedule 40, PVC Schedule 80, or RMC. Each has a different internal area for the same trade size, so the type matters as much as the size.
🧱 PVC and thicker-wall conduit hold fewer wires.Pick the Trade Size
From ½″ up to 4″. This is the conduit's nominal size, not its exact inside diameter — the calculator uses the precise NEC internal area behind the scenes.
Set the Wire Insulation
THHN/THWN-2 is the common building wire and the default. XHHW-2 and THW are slightly larger for the same gauge, so they take more room.
📏 Insulation type changes the conductor's area.Pick the Conductor Size
The AWG or kcmil size of the wires going in the conduit, from 14 AWG to 500 kcmil. For now the calculator handles one size at a time.
Enter the Number of Conductors
Count every conductor sharing the conduit, including the equipment ground. The count also sets the fill limit: 53% for one, 31% for two, 40% for three or more.
🔢 Include grounds and neutrals in the count.Calculate — Read the Fill
You get the fill percentage with a pass/fail against the limit, the minimum conduit size for your wires, the maximum number that fit, and the spare capacity left.
✅ Green means it fits; red means size up.How Conduit Fill Is Calculated
Two areas and a percentage, straight from NEC Chapter 9.
Step 1 — conductor area. Each conductor's cross-sectional area, including its insulation, is fixed in NEC Chapter 9, Table 5. A 12 AWG THHN wire is 0.0133 square inches; a 12 AWG XHHW-2 is 0.0181. Multiply the area of one conductor by how many of that size you are running.
Step 2 — conduit area. Each conduit's total internal area is fixed in Table 4 by type and trade size. A ¾″ EMT is 0.533 square inches inside; a ¾″ PVC Schedule 40 is 0.508. Schedule 80 PVC has thicker walls, so it has less room than Schedule 40 of the same trade size.
Step 3 — the percentage and the limit. Divide total conductor area by conduit area for the fill percentage. NEC Chapter 9, Table 1 caps it at 40% for three or more conductors, 31% for exactly two, and 53% for a single conductor. The cap leaves room for heat and for pulling.
A worked example. Nine 12 AWG THHN conductors take up 9 × 0.0133 = 0.1197 square inches. In ½″ EMT (0.304 in² internal), that is 0.1197 ÷ 0.304 = 39.4% — just under the 40% limit. So nine is the maximum number of 12 AWG THHN that fit in ½″ EMT, which matches NEC Annex C.
Conduit Fill Charts: EMT & PVC (THHN)
The maximum number of THHN/THWN-2 conductors that fit at the NEC 40% limit (or 53%/31% for one or two). These match NEC Annex C and the calculator above.
EMT Conduit Fill Chart — Max THHN Conductors
| Wire (THHN) | ½″ | ¾″ | 1″ | 1¼″ | 1½″ | 2″ |
|---|---|---|---|---|---|---|
| 14 AWG | 12 | 22 | 35 | 61 | 84 | 138 |
| 12 AWG | 9 | 16 | 26 | 45 | 61 | 101 |
| 10 AWG | 5 | 10 | 16 | 28 | 38 | 63 |
| 8 AWG | 3 | 6 | 9 | 16 | 22 | 36 |
| 6 AWG | 1 | 4 | 7 | 12 | 16 | 26 |
| 4 AWG | 1 | 2 | 4 | 7 | 10 | 16 |
| 2 AWG | 1 | 1 | 3 | 5 | 7 | 11 |
| 1 AWG | 1 | 1 | 1 | 4 | 5 | 8 |
| 1/0 AWG | 0 | 1 | 1 | 3 | 4 | 7 |
| 2/0 AWG | 0 | 1 | 1 | 2 | 3 | 6 |
| 3/0 AWG | 0 | 1 | 1 | 1 | 3 | 5 |
| 4/0 AWG | 0 | 0 | 1 | 1 | 1 | 4 |
Read down to your wire size and across to the conduit size; the cell is the most THHN conductors of that gauge the conduit holds. For example, ¾″ EMT takes sixteen 12 AWG, and 1″ EMT takes twenty-six. Mixing sizes uses the 40% area instead, which the calculator handles. For the full chart across every conduit type, see the complete conduit fill chart.
PVC Schedule 40 Conduit Fill Chart — Max THHN Conductors
| Wire (THHN) | ½″ | ¾″ | 1″ | 1¼″ | 1½″ | 2″ |
|---|---|---|---|---|---|---|
| 14 AWG | 11 | 21 | 34 | 60 | 82 | 135 |
| 12 AWG | 8 | 15 | 25 | 43 | 59 | 99 |
| 10 AWG | 5 | 9 | 15 | 27 | 37 | 62 |
| 8 AWG | 3 | 5 | 9 | 16 | 21 | 36 |
| 6 AWG | 1 | 4 | 6 | 11 | 15 | 26 |
| 4 AWG | 1 | 1 | 4 | 7 | 9 | 16 |
| 2 AWG | 1 | 1 | 3 | 5 | 7 | 11 |
| 1 AWG | 0 | 1 | 1 | 3 | 5 | 8 |
| 1/0 AWG | 0 | 1 | 1 | 3 | 4 | 7 |
| 2/0 AWG | 0 | 1 | 1 | 2 | 3 | 6 |
| 3/0 AWG | 0 | 1 | 1 | 1 | 3 | 5 |
| 4/0 AWG | 0 | 0 | 1 | 1 | 1 | 4 |
PVC Schedule 40 holds slightly fewer conductors than EMT of the same trade size, because its internal area is a little smaller. Schedule 80 PVC, with its thicker walls, holds fewer still — switch the conduit type in the calculator to see the exact count.
NEC Conduit Fill Limits (Chapter 9, Table 1)
| Number of Conductors | Maximum Fill | Why |
|---|---|---|
| 1 conductor | 53% | Pulls straight, easy to cool |
| 2 conductors | 31% | Round wires leave a binding gap |
| 3 or more conductors | 40% | Pack efficiently; the usual case |
These limits apply to conduit and tubing 24 inches or longer. A short nipple — a length of 24 inches or less between two boxes — is allowed up to 60% fill under NEC 314.16, because pulling and heat are not concerns over such a short run.
What Affects Conduit Fill
Six things decide whether your wires fit. The first three set how much area the conductors need; the rest set how much room the conduit gives.
EMT vs PVC vs RMC
Conduit type sets the wall thickness and the internal area. Here is how much room each gives at the same ¾″ trade size, and where each is used.
| Type | ¾″ Internal Area | Best For |
|---|---|---|
| EMT | 0.533 in² | Indoor and exposed runs; the default for commercial |
| PVC Schedule 40 | 0.508 in² | Underground, wet and corrosive locations |
| PVC Schedule 80 | 0.409 in² | Exposed PVC where physical damage is a risk |
| RMC | 0.549 in² | Maximum protection, risers, hazardous areas |
How to Size a Conduit: Step by Step
Whether you size by hand or with the calculator, the procedure is the same four steps.
Write down every conductor sharing the raceway — hots, neutrals, and the equipment grounding conductor. Note each one's size and insulation type, since both set its area.
Look up each conductor's area in NEC Chapter 9, Table 5 and add them together. With one wire size, that is area-per-wire times the count; with mixed sizes, sum each group.
Find the conduit's internal area in Table 4 and multiply by the limit — 40% for three or more conductors. That product is the most conductor area the conduit may hold.
If the total conductor area is at or below the 40% figure, the conduit works. If it is over, move up one trade size and check again. The calculator does this and reports the smallest size that fits.
Why Overfilling a Conduit Costs You
The fill limits are not red tape. Cross them and you pay in heat, in damaged wire, and in failed inspections.
Conductors give off heat under load, and a packed conduit traps it. That is why the NEC derates ampacity once more than three current-carrying conductors share a raceway: four to six are limited to 80% of their rated ampacity, seven to nine to 70%, and ten to twenty to 50%. Overfill a conduit and the wires inside may no longer be large enough for their load.
There is a mechanical cost too. Pulling wire through an overfilled conduit takes more force, and that force drags insulation across the other conductors and the conduit edges. A nicked conductor can short or fail months later, in a wall where it is expensive to reach. And an over-fill conduit is a clear code violation that an inspector will flag, meaning rework before the job can pass.
Real Conduit Fill Examples
Three runs worked through with the same NEC math the calculator uses, so you can check your own.
Conduit Fill Best Practices
A few habits keep conduit fill correct and the pull easy. These are the ones that matter most.
The first is counting everything. The equipment grounding conductor counts toward fill even though it carries no current in normal use, and so do neutrals. Leaving them out is the most common reason a conduit that "should" fit comes up short on inspection.
The second is sizing for what the run might carry later, not just today. Conduit is cheap relative to labor, so a run that may gain circuits is worth a size up. The 40% limit is a not-to-exceed, not a target — finishing a pull at 35% leaves room and makes the wire easier to draw.
Six Habits for Getting Fill Right
When This Calculator Is the Wrong Tool
The fill math here is exact and matches NEC Annex C, but a few cases need more than a fill percentage. Here is where to look further.
1. It handles one conductor size at a time. Real conduits often mix sizes — for example three 12 AWG hots and a 10 AWG neutral. For mixed runs, calculate each size's total area, add them, and compare the sum to the conduit's 40% area. Different sizes always use the 40% column.
2. It is fill, not ampacity derating. Conduit fill and conductor ampacity are separate checks. Once more than three current-carrying conductors share a raceway, the NEC derates their ampacity (80%, 70%, 50% and so on). A conduit can pass the fill test and still force you to upsize the wire for heat.
3. Nipples follow a different limit. A length of conduit 24 inches or less between two boxes is a nipple and may be filled to 60%, not 40%. This calculator uses the standard 40/31/53% limits for runs longer than 24 inches.
4. Other raceways have their own rules. Wireways, auxiliary gutters and cable trays are not sized by the Chapter 9 conduit tables — wireways, for instance, use a 20% fill. Use the rules specific to the raceway type.
Where to go instead: for code-critical work, a licensed electrician and the full NEC are the authority — including Chapter 9 Tables 1, 4 and 5, the Annex C fill tables, and the ampacity adjustment factors of 310.15. This calculator is a fast, accurate first pass, not a substitute for a code-compliant design.
Conduit Terms You'll See On This Page
Quick reference for the terms used throughout this conduit fill calculator.
- Conduit Fill
- The percentage of a conduit's internal cross-section taken up by the conductors inside it.
- Raceway
- Any enclosed channel for holding wires — conduit, tubing, wireway or similar.
- EMT
- Electrical Metallic Tubing — thin-wall steel conduit, the most common type for indoor and exposed work.
- RMC
- Rigid Metal Conduit — heavy threaded steel conduit used where maximum physical protection is needed.
- PVC (Sch 40 / 80)
- Rigid plastic conduit. Schedule 80 has thicker walls and less internal area than Schedule 40.
- THHN / THWN-2
- The most common building-wire insulation, and the most compact for a given gauge.
- XHHW-2
- A cross-linked polyethylene insulation, slightly larger in area than THHN for the same size.
- Fill Limit
- The maximum allowed fill from NEC Table 1: 53% for one conductor, 31% for two, 40% for three or more.
- Nipple
- A length of conduit 24 inches or less between two boxes, allowed up to 60% fill.
- Derating
- Reducing a conductor's rated ampacity when more than three current-carrying conductors share a raceway.
- Equipment Grounding Conductor
- The safety ground wire. It counts toward conduit fill even though it carries no current in normal operation.
- kcmil
- Thousand circular mils — the area unit for conductors larger than 4/0 AWG.
- Trade Size
- A conduit's nominal size designation, such as ¾″ — not its exact inside diameter.
- NEC Chapter 9
- The National Electrical Code chapter with the conduit and conductor area tables used for fill.
- Annex C
- The NEC appendix listing the maximum number of conductors of one size per conduit, derived from Chapter 9.
Frequently Asked Questions
The most common questions about conduit fill.
How do I calculate conduit fill?
Conduit fill is the share of a conduit's interior cross-section taken up by the conductors inside it. Add the cross-sectional area of every conductor from NEC Chapter 9 Table 5, then divide by the conduit's internal area from Table 4. NEC limits fill to 40% for three or more conductors, 31% for two, and 53% for a single conductor.
What is the 40% conduit fill rule?
NEC Chapter 9, Table 1 limits a conduit carrying three or more conductors to 40% fill. The limit is 53% for one conductor and 31% for exactly two. The 40% cap leaves room for heat to dissipate and for the wires to be pulled without damaging the insulation.
How many 12 AWG THHN wires fit in 3/4 inch EMT?
Sixteen 12 AWG THHN conductors fit in 3/4 inch EMT at the 40% fill limit. For 1/2 inch EMT the limit is nine, and for 1 inch EMT it is twenty-six. These counts come from NEC Annex C and match this calculator.
What size conduit do I need for my wires?
It depends on the size, type and number of conductors. Enter them above and the calculator returns the minimum EMT, PVC and RMC size. As a reference, three 8 AWG THHN conductors fit in 1/2 inch EMT, and four 6 AWG THHN fit in 3/4 inch EMT.
Is conduit fill different for EMT and PVC?
Yes. PVC has slightly less interior area than EMT of the same trade size, so it holds fewer conductors. PVC Schedule 80 has thicker walls and even less room than Schedule 40. The calculator uses each type's exact NEC Chapter 9 area.
Is this conduit fill calculator NEC compliant?
The conductor and conduit areas come from NEC Chapter 9 Tables 4 and 5, and the fill limits from Table 1. Results match the NEC Annex C fill tables. It is an estimating aid, not a substitute for the current adopted code or a licensed electrician.
Do I count the ground wire in conduit fill?
Yes. The equipment grounding conductor counts toward conduit fill, along with neutrals, even though the ground carries no current in normal operation. Count every conductor physically in the raceway.
What is the conduit fill limit for a nipple?
A nipple — a length of conduit 24 inches or less between two boxes — may be filled to 60% under NEC 314.16, instead of the usual 40%. Pulling tension and heat are not concerns over such a short run. This calculator uses the standard 40% limit for longer runs.
Can I mix different wire sizes in one conduit?
Yes. To check a mixed run, find each conductor's area from Table 5, add them all together, and compare the total to 40% of the conduit's internal area. Conductors of different sizes always use the 40% limit. This calculator currently handles one size at a time, so sum mixed runs by hand.
The Electrical Cluster & More
Wire sizing is one part of an electrical job. Start with its sibling calculator and the guides behind the numbers, then the rest of the build.
Check Your Conduit Fill
in 30 Seconds
Pick your conduit and conductors — get the NEC fill percentage, the minimum conduit size, and the maximum number of wires that fit. All free.
Check My Conduit Fill Takes 30 seconds · No account needed · EMT, PVC & RMC