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Wire Gauge Calculator (AWG)

Convert between AWG and mm², calculate wire current capacity, resistance, and voltage drop

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Formula

d=0.127×92((36−AWG)/39)mm,A=π(d/2)2d = 0.127 × 92^((36-AWG)/39) mm, A = π(d/2)²

Reference: ASTM B258 - Standard Specification for Standard Nominal Diameters

d— Wire diameter (mm)
AWG— American Wire Gauge number
A— Cross-sectional area (mm²)
ρ— Resistivity (Ω·mm²/m)
L— Wire length (m)

How It Works

This calculator converts between AWG (American Wire Gauge), metric mm^2, and SWG (Standard Wire Gauge) for electrical engineers and international projects requiring wire size standardization. Per ASTM B258, AWG diameter follows d(inches) = 0.005 × 92^((36-AWG)/39), while IEC 60228 defines metric sizes in mm^2 cross-sectional area (0.5, 0.75, 1.0, 1.5, 2.5, 4, 6, 10, 16, 25, 35... mm^2). Key conversions: AWG 14 = 2.08 mm^2 (nearest IEC: 2.5 mm^2), AWG 12 = 3.31 mm^2 (nearest: 4 mm^2), AWG 10 = 5.26 mm^2 (nearest: 6 mm^2). Copper resistivity is 1.724 × 10^-8 ohm·m at 20 C per NIST; resistance R = rho × L / A. Temperature coefficient: +0.393%/C for copper, +0.403%/C for aluminum. Proper gauge selection prevents 15-25% efficiency loss from voltage drop in long runs.

Worked Example

Problem

Convert a European 4 mm^2 cable specification to AWG for US installation. Calculate resistance for 30 m length and verify compatibility with 25 A circuit breaker.

Solution
  1. IEC 4 mm^2: closest AWG by area is AWG 12 (3.31 mm^2) or AWG 11 (4.17 mm^2)
  2. Per ASTM B258: use AWG 11 for equivalent or better, AWG 12 if locally available
  3. Resistance of 4 mm^2 copper at 20 C: R = 1.724e-8 × 30 / (4e-6) = 0.129 ohm
  4. Resistance of AWG 12: R = 1.724e-8 × 30 / (3.31e-6) = 0.156 ohm (21% higher)
  5. Voltage drop at 25 A (AWG 12): V = 25 × 0.156 × 2 = 7.8 V (3.4% at 230 V - acceptable)
  6. NEC ampacity AWG 12 @ 75 C: 25 A (matches breaker at 100% - use AWG 10 for 80% rule)
  7. Recommendation: use AWG 10 (5.26 mm^2) for code compliance and lower voltage drop

Practical Tips

  • ✓AWG to IEC conversion per common practice: AWG 18 = 0.82 mm^2 (use 1.0), AWG 16 = 1.31 mm^2 (use 1.5), AWG 14 = 2.08 mm^2 (use 2.5), AWG 12 = 3.31 mm^2 (use 4.0), AWG 10 = 5.26 mm^2 (use 6.0). Always round UP to next IEC size for safety
  • ✓Voltage drop rule of thumb: keep < 3% for branch circuits, < 5% total per NEC. At 120 V/15 A, 3% = 3.6 V max drop = 0.24 ohm round-trip. Use 2 × length × resistance-per-foot calculation
  • ✓Aluminum requires 2 AWG upsize vs copper for equal ampacity (61% conductivity per ASTM). AWG 12 Cu = AWG 10 Al. Also requires antioxidant compound on connections per NEC 110.14

Common Mistakes

  • ✗Using wrong conversion - AWG 12 = 3.31 mm^2, NOT 12 mm^2. AWG is logarithmic; metric mm^2 is linear. Each 3 AWG steps = 2× area, so AWG 9 = 6.63 mm^2, AWG 6 = 13.3 mm^2, AWG 3 = 26.7 mm^2
  • ✗Ignoring temperature coefficient - copper resistance increases 21.5% from 20 C to 75 C operating temperature. A 0.156 ohm wire at 20 C becomes 0.190 ohm at 75 C. Use operating temperature for voltage drop calculations
  • ✗Confusing solid vs stranded conductor area - per ASTM B8, stranded wire has 2-3% larger OD for same cross-sectional area. AWG numbers apply to copper cross-section, not overall diameter

Frequently Asked Questions

AWG is a North American logarithmic scale per ASTM B258; lower number = larger wire. mm^2 is direct cross-sectional area per IEC 60228. Conversion is non-linear: AWG 12 = 3.31 mm^2, AWG 10 = 5.26 mm^2, AWG 8 = 8.37 mm^2. IEC sizes follow a preferred number series: 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50 mm^2.
Per NIST: copper resistance = R_20 × (1 + 0.00393 × (T - 20)). At 75 C: 1.22× resistance vs 20 C. At -40 C: 0.76× resistance. Ampacity tables (NEC 310.16) account for insulation temperature rating and ambient. High ambient reduces ampacity; cold environments allow higher current.
Yes, with proper derating per NEC. Aluminum conductivity = 61% of copper (ASTM B8). Use 2 AWG sizes larger: AWG 12 Cu = AWG 10 Al for same ampacity. Require AL/CU rated terminals and antioxidant compound. Aluminum is common for > AWG 8 due to cost savings (30-50% cheaper than copper per lb).
Per NEC 310.15: cross-sectional area (larger = more current), insulation temperature rating (60/75/90 C), ambient temperature, number of conductors bundled, and installation method (conduit vs free air). Ampacity = safe current without exceeding insulation temperature limit. Voltage drop may further limit practical current.
Per NEC 210.19 guideline (< 3% drop): upsize wire, reduce circuit length, or increase voltage. Voltage drop = I × R × 2 (for round trip). For 100 ft at 15 A with AWG 14: V_drop = 15 × 0.00252 × 200 = 7.6 V (6.3% at 120 V - too high). Upsize to AWG 12: 4.8 V (4%), or AWG 10: 3.0 V (2.5% - compliant).

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