AWG, kcmil and mm² are three languages for the same copper. Convert between them and get
the DC resistance per kilometre — computed from geometry and resistivity, corrected to your
actual conductor temperature, not just the 20 °C table value.
Runs entirely in your browser.
How the conversion works
AWG sizes are defined geometrically: for size n, the solid-wire diameter in mils is
5 × 92^((36−n)/39), and the area in kcmil is the diameter squared divided by 1000.
One kcmil is 0.5067 mm². So conversion is pure geometry — no rounding, no “approximately”.
A[kcmil] = (5 × 92^((36−n)/39))² / 1000 → A[mm²] = A[kcmil] × 0.5067075
d[mm] = √(A[kcmil] × 1000) × 0.0254
R(T) = ρ₂₀ / A × (1 + α·(T − 20)) per metre; × 1000 → Ω/km
ρ₂₀: copper 0.017241 Ω·mm²/m · aluminium 0.028264 Ω·mm²/m α₂₀: Cu 0.00393 /K · Al 0.00403 /K
Worked example
AWG 10 copper: area 10.38 kcmil = 5.26 mm², solid diameter 2.59 mm.
At 70 °C operating temperature, R = 0.017241 / 5.26 × (1 + 0.00393 × 50) × 1000 ≈
3.92 Ω/km — that is 20 percent above the 20 °C table figure of 3.28 Ω/km.
A voltage-drop calc done at 20 °C on a loaded, warm cable will flatter the cable by exactly
that kind of margin.
Limitations
- The resistance model is DC. For AC add skin and proximity effects — minor below about 25 mm² at power frequency, material above.
- Metric sizes are the nominal cross-section; actual compacted stranding varies a few percent by manufacturer.
- No ampacity here — that depends on insulation and installation method and belongs to the code tables.
Is AWG 10 the same as 6 mm²?
No — AWG 10 is 5.26 mm², 6 mm² is about 14 % more copper. Metric cables substitute upward from AWG, and the reverse needs care.
Why set operating temperature?
Copper resistance rises about 0.39 % per °C. At 90 °C it is 27 % above the 20 °C value — table-value voltage-drop calcs understate real losses on loaded cables.
Does it give ampacity?
No. Ampacity depends on insulation, installation and your code. This tool covers geometry, resistance, and feeds the voltage-drop and loss calculators.
What is a kcmil?
One thousand circular mils; 1 kcmil = 0.5067 mm². For solid wire, diameter in mils squared equals area in circular mils — no π needed.