Educational · Dimension-based · Comparison only
Conduit Fill Calculator
Sum cable cross-sections from overall insulated OD, divide by conduit internal area, and compare to general one-/two-/three-or-more fill bands. Enter measured diameters — not trade size or fake AWG presets.
Dimension fill planner
Enter measured conduit internal diameter and overall insulated cable OD. No trade-size or AWG lookup tables.
Comparison only — not a code-compliant or safe-to-install result. Local edition, authority, and exceptions (including nipples) are not applied automatically.
Cable / conductor groups
1/24 groups · assemblies use overall OD and count as one each
17.0%
Comparison band for 3 cables/conductors: 40% · within general band
- Conduit ID
- 1.049"
- Conduit area
- 0.8643 in²
- Cable / conductor area
- 0.1473 in²
- Capacity at 40% band
- 0.3457 in²
| Qty | OD | Area |
|---|---|---|
| 3 | 0.25" | 0.1473 in² |
General bands for comparison: 1 → 53%, 2 → 31%, ≥3 → 40%. Ampacity, derating, pull tension, and jamming are not calculated.
Results are estimates for reference only. Always verify measurements against your bender and local electrical code before cutting or bending conduit.
How to use this conduit fill calculator
1. Measure conduit internal diameter
Use the actual internal diameter (ID) of the raceway, not the trade size stamped on the stick. Enter ID in inches or millimetres.
2. Add cable or conductor groups
For each group, enter how many identical cables or conductors you have and each one's overall insulated outer diameter, including jackets. A cable assembly counts as one using its overall OD.
3. Read fill percent and comparison band
The planner sums quantity × π × OD² / 4, divides by conduit area, and compares to general bands: 53% for one, 31% for two, and 40% for three or more. Results are comparison only.
4. Review intermediate areas
Check conduit area, total cable area, and capacity at the comparison band. Invalid, zero, negative, fractional quantity, or OD greater than ID inputs suppress the result.
5. Copy or print for the gang box
Use Sample to load a worked example, Reset to clear, Copy result for notes, or Print for a paper card. Always verify against your authority and manufacturer data.
Worked example
Suppose a conduit measures 1.049" internal diameter and you plan three conductors at 0.25" overall OD each (including insulation). Area of one circle is π·d²/4.
- One conductor area = π·(0.25)²/4 ≈ 0.0491 in²
- Three conductors = 3 × that ≈ 0.1473 in²
- Conduit area = π·(1.049)²/4 ≈ 0.8643 in²
- Fill% ≈ 17.0% against the ≥3 comparison band of 40%
Load the Sample button in the planner to reproduce these inputs. Switch units to millimetres — physical fill% stays the same.
Formula and comparison bands
Cable area = Σ (quantity × π × OD² / 4). Conduit area = π × ID² / 4. Fill% = cable area ÷ conduit area × 100. General comparison bands used by manufacturer education pages: 1 → 53%, 2 → 31%, ≥3 → 40%. A multi-conductor cable assembly counts as one using its overall OD.
This page does not reproduce NEC Chapter 9 tables, trade-size IDs, or AWG diameters. Confirm measured dimensions and the edition your authority adopts. Nipples and other exceptions are not applied automatically.
Sources
- Service Wire — Conduit Fill (Customer Q&A)Primary manufacturer education page checked 2026-10-01.
- Southwire — Conduit fill calculatorManufacturer calculator reference checked 2026-10-01.
Bend
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Offsets, 90° stub-ups, and saddles with shrink and take-up.
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Bending cheat sheet
Multipliers, shrink constants, and typical EMT take-up.
Conduit fill calculator FAQ
Is this conduit fill calculator code compliant?
No. It is an educational dimension planner for comparison only. It does not apply your local code edition, authority exceptions, nipples, ampacity, derating, pull tension, or jamming rules, and it does not certify a safe-to-install fill.
Why do I enter internal diameter instead of trade size?
Trade size is a nominal label, not the free inside diameter. This tool uses measured ID and overall insulated OD so the arithmetic stays honest without shipping unlicensed NEC trade-size or conductor tables.
What fill percentages does WattsCalc compare against?
General manufacturer-education comparison bands: 53% for one cable or conductor, 31% for two, and 40% for three or more. Multi-conductor assemblies count as one when you enter the overall jacketed OD.
Can I mix different cable diameters in one conduit?
Yes. Add a group for each distinct overall OD, set the quantity in that group, and the planner sums the cross-sections. Quantity must be a whole number; fractional counts are rejected.
Does switching between inches and millimetres change the math?
No. Inches and millimetres are physically equivalent. The same ID and OD convert 1 in = 25.4 mm, and fill percent stays the same within rounding.