Wire Gauge Calculator

Wire AWG from current capacity and voltage drop.

About this calculator

This calculator picks the smallest (cheapest, most flexible) AWG wire gauge that satisfies two independent constraints simultaneously: the wire's rated current-carrying capacity and your maximum acceptable voltage drop over the run. It walks a built-in AWG table (30 gauge down to 2 gauge, each with a diameter and a conservative chassis-wiring current rating) from smallest to largest, and for each candidate computes the round-trip resistance using copper's resistivity (0.01724 Ω·mm²/m at 20°C) and the wire's actual cross-sectional area from its diameter, then the resulting voltage drop at your specified current over the full out-and-back run. Critically, the calculator doubles your entered one-way length to account for both the supply and return conductors, a detail that's easy to forget when measuring a physical wire run.

It stops at the first gauge in the table that both carries your current safely and keeps the voltage drop under your percentage limit, then reports that gauge's actual resistance, power lost as heat (I²R), and the voltage actually delivered at the load. Two caveats worth knowing: the current ratings in the table are for open chassis wiring in free air, not wire bundled tightly in a harness or run through conduit, both of which require derating for heat buildup; and the resistivity value assumes room-temperature copper — wire running hot, near a motor or engine, will have measurably higher resistance and voltage drop than this calculator predicts.

Inputs

Current (A)

System Voltage (V)

Wire Length (m, one way)

Max Voltage Drop (%)

Tab to move between fields

Results

Recommended AWG

Voltage Drop (V)

0.25

Wire Diameter (mm)1.63

Voltage Drop (%)2.07

Voltage at Load (V)11.75

Total Wire Resistance (Ω)0.05

Power Loss (W)1.24

Wire Max Current (A)32

Required Area Mm21.44

How to Use This Calculator

Enter the circuit current (A), system voltage (V), and one-way wire length (m).
Set the maximum allowable voltage drop (%) — typically 2–3% for power circuits.
Review the recommended AWG gauge, wire diameter (mm), and actual voltage drop.
Verify the voltage at the load meets your minimum operating requirement.

How the result changes with Current (A)

Current (A)	Recommended AWG	Voltage Drop (V)
2.5	18	0.31
3.75	16	0.3
7.5	12	0.23
13	10	0.26

What each input means

Current (A): Maximum current the wire must carry in amps.
System Voltage (V): Supply voltage of your circuit (e.g. 5V, 12V, 24V).
Wire Length (m, one way): One-way wire length in meters. Total run (supply + return) is 2x this.
Max Voltage Drop (%): Maximum acceptable voltage drop as a percentage (3% is standard, 5% for non-critical).

What each result means

Recommended AWG: Smallest wire gauge (largest number) meeting both current and voltage drop requirements.
Wire Diameter (mm): Conductor diameter of recommended gauge.
Voltage Drop (V): Actual voltage drop across the full wire run (both directions).
Voltage Drop (%): Voltage drop as percentage of supply voltage.
Voltage at Load (V): Actual voltage delivered to the load after wire losses.
Total Wire Resistance (Ω): Round-trip wire resistance (both conductors).
Power Loss (W): Power dissipated as heat in the wire (I²R).
Wire Max Current (A): Maximum safe current for the recommended gauge (chassis wiring).

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Current (A) = 5, System Voltage (V) = 12, Wire Length (m, one way) = 3, Max Voltage Drop (%) = 3 = 4 input(s) provided
Calculate Recommended AWG
Recommended AWG = 2
14 = 14
Calculate Voltage Drop
Voltage Drop = recTotalRes * currentAmps
0.2485 = 0.2485
Calculate Wire Diameter
Wire Diameter = awgTable[awgTable.length - 1].diamMm
1.628 = 1.628
Calculate Voltage Drop
Voltage Drop = (actualVoltageDrop / voltage) * 100
2.07 = 2.07

Engine last updated September 1, 2026. Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why does the calculator double my entered wire length?

The 'Wire Length' input asks for the one-way distance, but current has to travel out to the load and back to complete the circuit, so the engine doubles that length before computing round-trip resistance and voltage drop. This is a common point of confusion — if you measure a 3-meter run to your load and enter 3, the calculator correctly treats it as 6 meters of total conductor, which is the physically accurate way to size for voltage drop.

Why might the recommended gauge be thicker than the wire's current rating alone would suggest?

The calculator walks the AWG table and picks the first gauge that satisfies both constraints together — safe current capacity and your maximum voltage drop percentage — not just current alone. A long, low-voltage run (like 12V over many meters) often needs a much thicker wire to keep voltage drop under your limit than the current rating by itself would require, since resistance and thus voltage drop increase with length regardless of how much current the wire could physically handle.

Are the current ratings in the AWG table safe for wire bundled in a harness or run through conduit?

No — the table's maxAmps figures are for open chassis wiring in free air, a conservative baseline. Wire bundled tightly with other conductors or routed through conduit can't dissipate heat as effectively, so it needs to be derated below these table values for the same safe current-carrying capacity. If your installation involves bundling, size up a gauge or two from what this calculator recommends.

Will my actual voltage drop be different if the wire gets hot, like near an engine or motor?

Yes — the resistivity value used here (0.01724 Ω·mm²/m) is copper's resistivity at 20°C room temperature, but copper's resistance increases with temperature. Wire running near a hot engine, motor, or in direct sun will have measurably higher resistance in service than this calculation assumes, which means slightly more voltage drop and power loss than the reported numbers.

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