Wire gauge calculator: AWG, mm², ampacity and voltage drop
This wire gauge calculator converts between American Wire Gauge, cross-section in mm², diameter in mm or inches and kcmil, lists typical ampacity for each size, and calculates the voltage drop over a cable run for DC, single-phase or three-phase circuits in copper or aluminium. A third mode turns the question around: give it the current, distance and the largest drop you accept, and it names the smallest gauge that satisfies both the drop limit and the ampacity table.
It is useful for solar and battery wiring, automotive and boat 12 V circuits, LED strips and extension leads: anywhere a long run or a heavy current makes the wire part of the circuit. Resistance uses the IEC 60228 resistivities for annealed copper (0.017241 Ω·mm²/m) and aluminium (0.028264 Ω·mm²/m) at 20 °C, with a temperature correction for warm conductors.
Everything runs in your browser. Ampacity figures follow the NEC Table 310.16 pattern and are typical values for guidance; insulation type, bundling, ambient temperature and local codes change what is allowed, so confirm with the applicable standard before installation.
How to use the Wire Gauge Calculator
- Pick Convert & table, Voltage drop or Recommend gauge.
- Enter the wire size as AWG (including 0000 or 4/0), mm², or a diameter in mm or inches; odd sizes are matched to the closest AWG.
- For voltage drop, add the supply voltage, current, one-way length in metres or feet, conductor material, circuit type and conductor temperature.
- For a recommendation, set the maximum drop in percent; the calculator checks every gauge up to 4/0 against both the drop and the ampacity table.
- Read the result, the breakdown and the highlighted row in the AWG table; Copy table pastes the reference data into a spreadsheet.
Wire Gauge Calculator features
Full AWG table
Sizes 4/0 to 30 AWG with diameter, area, kcmil, Ω/km for copper and aluminium, and NEC-style ampacity at 60, 75 and 90 °C.
Any input unit
AWG, mm², mm or inch diameter — the nearest standard gauge is shown when the size is not an exact AWG.
Voltage drop with the working
See the one-way resistance, the round-trip multiplier (2 or √3), the drop in volts and percent, the voltage at the load and the heat in the wire.
Temperature correction
Resistance is scaled with the copper (0.00393/°C) or aluminium (0.00403/°C) coefficient for the conductor temperature you enter.
Smallest safe gauge
The recommendation never suggests a wire whose typical ampacity is below your current, even if voltage drop alone would allow it.
How AWG, mm² and voltage drop are calculated
American Wire Gauge is logarithmic: every six gauge numbers halve the diameter and every three halve the area. The diameter in millimetres is d = 0.127 × 92^((36 − n) / 39), where n is the gauge (use 0, −1, −2 and −3 for 1/0 to 4/0). Area is π d² / 4, so AWG 10 is 2.588 mm across and 5.26 mm². Metric cable is sold by area in mm²; 2.5 mm² sits between AWG 13 and 14, and 4 mm² is close to AWG 11.
Resistance of a conductor is R = ρ × L ÷ A. Current must travel to the load and back, so DC and single-phase circuits use Vdrop = 2 × I × Rone-way; a balanced three-phase circuit uses √3 instead of 2. The drop is divided by the supply voltage to give a percentage, which is why 12 V systems need far thicker cable than 230 V mains for the same power.
| Case | Numbers | Drop |
|---|---|---|
| 12 V, 10 A, 5 m, AWG 10 Cu | R = 0.017241 × 5 ÷ 5.26 = 16.4 mΩ | 2 × 10 × 0.0164 = 0.33 V (2.7 %) |
| 12 V, 10 A, 5 m, AWG 14 Cu | R = 41.5 mΩ | 0.83 V (6.9 %) — too much |
| 230 V, 16 A, 30 m, 2.5 mm² Cu | R = 0.207 Ω | 6.6 V (2.9 %) |
Reading the ampacity columns
The three copper columns correspond to the 60 °C, 75 °C and 90 °C insulation ratings in the NEC Table 310.16 pattern, for up to three current-carrying conductors in a raceway or cable at 30 °C ambient. Most terminations are rated 75 °C, so the calculator uses that column for its checks. The table does not cover 16 AWG and smaller, which are governed by other rules (flexible cords, chassis wiring), so those cells are blank rather than guessed.
- In the NEC, 14, 12 and 10 AWG copper are also limited by overcurrent protection to 15, 20 and 30 A respectively.
- Ambient temperatures above 30 °C and more than three bundled conductors require derating.
- Aluminium needs about 1.6 times the copper area for the same resistance and needs terminations rated for it.
- Voltage drop is a design recommendation (3 % branch, 5 % total is the common guidance); ampacity is the safety limit.
- Only conductor resistance is modelled, which is accurate for DC and for AC up to about 2/0 AWG; larger AC conductors should include reactance from the cable data sheet.
Related calculators
Use the battery runtime calculator to find the current a battery system draws, the Ohm's law calculator for volts, amps and resistance, and the watt calculator when you know watts and need amps.
Wire Gauge Calculator FAQ
What gauge wire do I need for 30 amps?
In the NEC 310.16 pattern, 10 AWG copper is rated 30 A at 60 °C and 35 A at 75 °C, so 10 AWG is the usual minimum for a 30 A circuit. Use Recommend gauge with your length and voltage to see whether voltage drop pushes you to 8 AWG.
How do I convert AWG to mm²?
Compute the diameter with d = 0.127 × 92^((36 − n)/39) mm and the area with π d²/4. The wire gauge calculator does this for every size and also matches a mm² value to the closest AWG.
Should I enter the one-way or the round-trip length?
One-way. The calculator multiplies by 2 for DC and single-phase circuits and by √3 for three-phase, which accounts for the return path.
Why is the drop so high on a 12 V system?
The percentage is the drop divided by the supply voltage. A 0.5 V drop is 4 % of 12 V but only 0.2 % of 230 V, so low-voltage systems need much thicker wire for the same current and distance.
Are the ampacity numbers safe to use directly?
They are typical values from the NEC Table 310.16 pattern for standard conditions. Insulation, bundling, ambient temperature, termination ratings and local regulations change the allowed current. Treat them as guidance and check the applicable code.