Wire Resistance Calculator

Calculate the resistance of a wire based on its material, length, gauge and temperature. Optionally enter a current to see estimated voltage drop and power dissipation.

Wire material

Resistivity Ω·m (× 10⁻⁸) Enter resistivity at 20 °C

Temp. coefficient per °C (× 10⁻³) Optional — leave blank to skip temperature adjustment

Wire length

Unit

Wire gauge / size

AWG gauge

Wire diameter

Unit

Temperature °C Resistivity is adjusted relative to 20 °C standard values

Wire resistance —

Resistance per metre —

Wire diameter —

Cross-sectional area —

Nearest AWG —

Voltage drop —

Power dissipation —

For educational and reference use only. Always verify results before use in real-world designs or safety-critical applications. For more information, see Calculation Assumptions and Disclaimer.

How to Use This Calculator

Select a wire material from the dropdown — the calculator uses standard resistivity and temperature coefficient values automatically. If you need a non-standard alloy, choose Custom resistivity and enter the values manually.

Enter the wire length and choose the appropriate unit. For gauge, use the AWG selector for standard wire sizes, or switch to Manual diameter to enter a metric or imperial diameter directly.

Adjust the temperature if your wire will operate outside of the standard 20 °C reference. The calculator applies the material's temperature coefficient to adjust resistivity accordingly.

Expand Voltage drop & power and enter a current in amperes if you want to see estimated voltage drop and power dissipation for that wire run.

Results update instantly as you type. Use Show calculation steps to see the full working, and Copy results to copy a plain-text summary to your clipboard.

Formulas

Wire resistance — calculated from the material's resistivity, the wire's length, and its cross-sectional area:

R = ρ × (L / A)

Temperature-adjusted resistivity — resistivity rises with temperature for most metals:

ρ_T = ρ₂₀ × [1 + α(T − 20)]

Voltage drop — the voltage lost across the wire when current flows:

V_drop = I × R

Power dissipation — the heat generated in the wire:

P = I² × R

Where: R = resistance (Ω), ρ = resistivity (Ω·m), L = length (m), A = cross-sectional area (m²), α = temperature coefficient (per °C), T = temperature (°C), I = current (A).

Example Calculation

Given: Copper wire, 10 m long, 20 AWG, at 20 °C, carrying 2 A.

Copper resistivity: ρ = 1.724 × 10⁻⁸ Ω·m
20 AWG cross-sectional area: A = 0.5176 mm² = 5.176 × 10⁻⁷ m²

Step 1 — Wire resistance:
R = 1.724 × 10⁻⁸ × (10 / 5.176 × 10⁻⁷) ≈ 0.333 Ω

Step 2 — Voltage drop:
V_drop = 2 × 0.333 ≈ 0.666 V

Step 3 — Power dissipation:
P = 2² × 0.333 ≈ 1.33 W

Wire resistance: 0.333 Ω | Voltage drop: 0.666 V | Power: 1.33 W

AWG, Diameter & Cross-Sectional Area Reference

The table below lists common AWG sizes with their exact diameter, cross-sectional area (CSA), and the nearest IEC 60228 standard cable size. Use it to cross-reference American wire gauges with metric specifications.

AWG	Diameter (mm)	Diameter (in)	CSA (mm²)	Nearest IEC 60228 (mm²)
0000	11.684	0.4600	107.22	95
000	10.405	0.4096	85.03	70
00	9.266	0.3648	67.43	70
0	8.251	0.3249	53.48	50
1	7.348	0.2893	42.41	35
2	6.544	0.2576	33.63	35
3	5.827	0.2294	26.67	25
4	5.189	0.2043	21.15	25
6	4.115	0.1620	13.30	16
8	3.264	0.1285	8.37	10
10	2.588	0.1019	5.26	6
12	2.053	0.0808	3.31	4
14	1.628	0.0641	2.08	2.5
16	1.291	0.0508	1.31	1.5
18	1.024	0.0403	0.823	1.0
20	0.8128	0.0320	0.519	0.5
22	0.6438	0.0253	0.326	0.35
24	0.5106	0.0201	0.205	0.2
26	0.4049	0.01594	0.129	0.12
28	0.3211	0.01264	0.0810	0.09
30	0.2546	0.01003	0.0509	0.05
32	0.2019	0.00795	0.0320	—
34	0.1601	0.00630	0.0201	—
36	0.1270	0.00500	0.0127	—
38	0.1007	0.00397	0.00795	—
40	0.07988	0.003145	0.00501	—

IEC 60228 cross-sections are nominal values. AWG sizes are not metrically exact — always verify with your cable's datasheet for rated current capacity.

Frequently Asked Questions

What is wire resistance?
Wire resistance is the opposition a conductor presents to the flow of electric current. It depends on the material's resistivity, the wire's length, and its cross-sectional area. Longer wires and thinner wires have higher resistance.

Why does temperature affect wire resistance?
Most metals increase in resistivity as temperature rises. Each material has a temperature coefficient (α) that describes this relationship. At higher temperatures, atoms vibrate more, impeding electron flow and increasing resistance.

What is AWG and how does it relate to wire diameter?
AWG stands for American Wire Gauge. It is a standardised system where a higher AWG number means a thinner wire. For example, 40 AWG is very thin (0.0799 mm diameter) while 0 AWG is thick (8.251 mm diameter). Thinner wires have smaller cross-sectional areas and therefore higher resistance per unit length.

Why is copper the most common wire material?
Copper has very low resistivity (1.724 × 10⁻⁸ Ω·m), second only to silver among common metals. It is also ductile, widely available, and relatively affordable, making it the default choice for electrical wiring.

What is a safe voltage drop for a wire run?
A common rule of thumb is to keep voltage drop below 3% of the supply voltage for power circuits, and below 5% for lighting circuits. Excessive voltage drop reduces device performance and can cause overheating.