About the Handiwork AWG Wire Size Calculator
The AWG Wire Size Calculator converts any American Wire Gauge size — from 4/0 down to 40 AWG — into diameter (mm and inches), cross-sectional area (mm² and kcmil), and DC resistance per length for copper or aluminum conductors, all computed from the standard AWG geometric formula rather than a static lookup table. For common building-wire gauges (14 AWG through 4/0), it also shows copper ampacity at 60°C, 75°C, and 90°C insulation ratings from NEC Table 310.15(B)(16).
How to use the Handiwork AWG Wire Size Calculator
- Choose an AWG size from the dropdown, from 4/0 (largest) down to 40 (smallest).
- Choose copper or aluminum — resistance updates for the selected conductor material.
- Read the diameter, cross-sectional area, resistance per length, and (for copper, in the building-wire range) ampacity.
How AWG sizing works
American Wire Gauge is a logarithmic scale: each step of 6 gauges roughly doubles the cross-sectional area, and each step of 3 gauges roughly doubles the resistance per length. The diameter in millimeters for gauge n is d = 0.127 × 92^((36−n)/39), which this calculator evaluates directly — so it works for every standard gauge, not just the handful usually printed on a chart. Sizes above 0 AWG continue as 00 (2/0), 000 (3/0), and 0000 (4/0), each one step larger than 0 AWG.
Resistance and why material matters
DC resistance per unit length is resistivity divided by cross-sectional area (R = ρ ÷ A). Copper has a resistivity of about 1.72 × 10⁻⁸ Ω·m at 20°C (100% IACS conductivity); aluminum is roughly 61% as conductive, so an aluminum conductor needs a larger cross-section than copper to carry the same current with the same voltage drop. That is why aluminum branch-circuit wiring is typically sized one or two gauges larger than the copper equivalent.
Reading the ampacity figures
The 60°C, 75°C, and 90°C columns are the conductor's insulation temperature rating, not the wire material — a higher-rated insulation permits more current before the conductor itself reaches an unsafe temperature. These values come from NEC Table 310.15(B)(16) for not more than three current-carrying conductors in a raceway or cable, at a 30°C ambient temperature. The actual breaker or fuse size for a circuit is normally set by the equipment and termination rating (commonly the 60°C or 75°C column), not the conductor's maximum column.
When to size wire differently
This table applies to typical enclosed circuits with up to three current-carrying conductors. Ampacity drops with more bundled current-carrying conductors, higher ambient temperature, or longer conduit runs in poor airflow, and it can be higher for a single conductor in free air. Voltage drop over distance is a separate consideration from ampacity — a gauge that is safely rated for a load's current can still drop too much voltage over a long run. Use the voltage drop calculator to check that separately.
Continue this workflow
Use the adjacent tool when the next step calls for a different input, output, or method.
- Voltage Drop CalculatorAmpacity tells you a gauge is safe to carry the current — the voltage drop calculator tells you whether that gauge is also thick enough over the actual length of the run.
- Ohm's Law CalculatorCombine this wire's resistance per length with Ohm's law to estimate the voltage drop and power loss across a specific run length.
Assumptions and limitations
- Ampacity is shown for copper only, for AWG 14 through 4/0, using NEC Table 310.15(B)(16) at 30°C ambient with no more than three current-carrying conductors — real installations may need ambient, bundling, or conduit-fill adjustments.
- Aluminum ampacity is not included; consult NEC Table 310.15(B)(16) or a licensed electrician, since aluminum conductors are rated lower than copper at the same gauge.
- Resistance figures are DC values at 20°C; AC resistance is higher at larger gauges and higher frequencies due to skin effect, which this calculator does not model.
- This tool is an engineering reference, not an electrical code compliance check — always verify against the electrical code that applies in your location before wiring a real circuit.
Sources and standards
These authoritative references were used to verify the method and guidance on this page.
Frequently asked questions
What does a lower AWG number mean?
A lower AWG number means a thicker wire. The scale runs backwards from what you might expect: 0000 (4/0) is the thickest gauge this calculator covers, and 40 AWG is hair-thin. Each 6-gauge step roughly doubles the cross-sectional area.
What gauge wire do I need for a given amperage?
Pick the smallest gauge whose ampacity (at the insulation temperature rating your termination supports, usually 60°C or 75°C) meets or exceeds your circuit’s load. For example, a 20 A circuit needs at least 12 AWG copper at 75°C. Also check voltage drop separately for long runs.
Why is aluminum wire rated for less current than copper?
Aluminum is only about 61% as electrically conductive as copper, so it has higher resistance — and generates more heat for the same current — at the same gauge. That is why aluminum wiring is typically sized larger than the copper equivalent for the same ampacity.
What is kcmil, and how does it relate to AWG?
Kcmil (thousand circular mils) is the unit used for conductors larger than 4/0 AWG, where the AWG sequence stops. A circular mil is the area of a circle one mil (0.001 in) in diameter; kcmil is that area divided by 1,000. This calculator shows the kcmil equivalent for every AWG size so you can cross-reference both systems.
Does this account for AC skin effect?
No — the resistance values are DC resistance at 20°C. Skin effect raises the effective AC resistance of larger conductors at higher frequencies, which matters for heavy power cables and high-frequency signal wiring but is negligible for small-gauge wire at 50/60 Hz.