Conduit Fill Calculator
Check conduit fill and find minimum conduit size for EMT, IMC, RMC and PVC using 2026 NEC Chapter 9 data. Built for electricians, electrical contractors and estimators.
Conduit Fill & Sizing Calculator
Calculate conduit fill percentage or find the minimum conduit size for THHN / THWN / THWN-2 conductors using 2026 NEC Chapter 9 data.
Conductors
Add each conductor size used in the raceway.
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Electrical Estimating ServicesHow to use the conduit fill calculator
- Choose a mode. Find Minimum Conduit Size checks the available trade sizes. Check Selected Conduit evaluates the size you choose.
- Select the raceway. Choose EMT, IMC, RMC, PVC Schedule 40 or PVC Schedule 80.
- Enter the conductors. Add one row per wire size and enter its quantity. This version supports THHN, THWN and THWN-2. Include insulated grounding and bonding conductors of the supported type.
- Check the installation condition. Enable the short-nipple option only when the installation qualifies for that NEC provision.
- Calculate and review. Read the recommended size or selected-size status, allowable area, conductor breakdown and any Note 7 explanation. Recalculate after changing inputs.
2026 NEC conduit fill rules
The electrical conduit fill calculator uses Chapter 9 data for insulated conductor area and raceway allowances. The standard fill condition depends on the total number of individual conductors, not the number of input rows.
- One conductor: 53%.
- Two conductors: 31%.
- Three or more conductors: 40%.
- Qualifying short nipple: 60%. This applies to conduit or tubing nipples no longer than 24 inches, excluding connectors, installed between boxes, cabinets or similar enclosures.
Special notes can affect the result. Chapter 9 Note 7 allows specified rounding when determining a permitted count of same-size conductors; it also addresses a single conductor. The calculator identifies results that pass under this rule. Do not apply same-size count rounding to a mixed-size bundle.
Pass/fail uses the published Table 4 allowable area for the applicable condition. Displayed fill percentage uses the separately rounded total raceway area, so a near-boundary percentage can differ slightly from the nominal limit. Review the allowable area and explanation together.
Reference: NFPA 70, 2026 edition, Chapter 9. Verify the code edition adopted for your project and applicable installation requirements.
How minimum conduit size is determined
The tool multiplies each conductor's Table 5 area by its quantity and adds the row totals. It selects the applicable fill condition, checks the published raceway allowances and evaluates Note 7 where applicable. The minimum recommendation is the smallest listed trade size that passes these checks.
Fill sizing does not determine conductor ampacity, voltage drop, bend compliance, pulling difficulty or all installation requirements. Pull length, bends and conductor arrangement may justify a larger raceway.
Choose the correct conduit type
EMT conduit fill
Select EMT for electrical metallic tubing. Use its own Table 4 allowances when checking a branch-circuit bundle or finding a minimum trade size.
IMC conduit fill
Intermediate metal conduit has its own internal dimensions. An EMT result should not be carried over to IMC simply because the trade-size labels match.
RMC conduit fill
Rigid metal conduit uses the RMC table. Confirm the specified raceway before applying the calculated allowance to a feeder or equipment connection.
PVC Schedule 40 conduit fill
Use the PVC Schedule 40 selection for that raceway construction. Enter all supported insulated conductors sharing the raceway, including the grounding conductor where installed.
PVC Schedule 80 conduit fill
Schedule 80 has less internal space than Schedule 40 at the same trade size. Recalculate using Schedule 80 wherever that construction is specified.
Worked example: four #12 conductors in EMT
Enter 3 × #12 in one row and 1 × #12 in a second row, select EMT and leave the nipple option off. Each supported #12 conductor occupies 0.0133 in², giving 4 × 0.0133 = 0.0532 in².
Four conductors use the 40% condition. The published allowance for ½-inch EMT is 0.122 in², so the bundle passes and the minimum listed recommendation is ½-inch EMT. Its displayed fill is 17.5%. This illustrates fill only, not a complete circuit design.
Frequently asked questions
Is this conduit size calculator free?
Yes. You can calculate conduit fill and minimum size without creating an account.
Which insulation types are supported?
This version supports the Table 5 THHN, THWN and THWN-2 group, from #14 through 1000 kcmil. Other insulation types, bare conductors and cables need their applicable dimensions and are outside this version's inputs.
Can I mix conductor sizes?
Yes. Add a separate row for each size. The tool adds the occupied areas and uses the total conductor count to select the standard fill condition.
Do grounding conductors count toward conduit fill?
Yes, grounding and bonding conductors occupy space and must be included. This tool supports insulated conductors in the listed insulation group; do not use an insulated-wire entry as a substitute for a bare conductor.
Why can a result pass above the nominal fill percentage?
A same-size bundle may qualify under Note 7. Small differences can also arise from the separately rounded published allowance and total area. Read the status explanation and allowable area rather than judging only the displayed percentage.
Does a passing result mean the installation meets every NEC requirement?
No. This is a fill and sizing reference. Ampacity adjustment, conductor suitability, raceway use, bends and other project requirements require separate checks.
Can Build IT estimate the rest of my electrical scope?
Yes. Our team can prepare a quantity takeoff or a detailed electrical estimate from the drawings. Request an electrical estimate with your plans and bid deadline.