Calculate wire resistance from AWG gauge or diameter, length, material, and temperature. Supports 7 conductor materials, temperature correction, cmil conversion. Includes temp effect visual and AWG...
The electrical resistance of a wire is determined by four factors: the conductor material's resistivity (ρ), the wire length (L), the cross-sectional area (A), and the temperature. The fundamental formula R = ρL/A tells the whole story: longer wires have more resistance, thicker wires have less, and materials with lower resistivity conduct better.
Temperature significantly affects resistance in metals. Copper's resistance increases about 0.393% per degree Celsius above 20°C. A copper wire that measures 1.000 Ω at 20°C will measure 1.314 Ω at 100°C. This temperature coefficient is critical for precision measurements, heating element design, and temperature sensing (RTDs use this effect intentionally).
This calculator computes wire resistance from AWG gauge or custom diameter, with 7 conductor materials including copper, aluminum, silver, gold, nichrome, and steel. Temperature correction is applied automatically. Results include resistance per meter, per foot, and per kilometer, plus cross-sectional area in mm² and circular mils. A visual shows how resistance changes from -40°C to 100°C.
Wire resistance affects voltage drop, power loss, heating, and signal integrity. Incorrectly estimated resistance can cause excessive voltage drop, wasted energy, or even fire. This calculator handles all the physics — material resistivity, area from AWG or diameter, temperature correction — so you get an accurate resistance value for real operating conditions.
R = ρ × L / A Temperature correction: ρ(T) = ρ₂₀ × (1 + α × (T − 20)) Where: ρ = resistivity (Ω·m) L = length (m) A = cross-sectional area (m²) α = temperature coefficient (1/°C) AWG to area: A(mm²) = π/4 × d² d(mm) = 0.127 × 92^((36−AWG)/39)
Result: R = 0.1565 Ω (5.217 mΩ/m)
AWG 12 copper: area = 3.31 mm², ρ = 1.724 × 10⁻⁸ Ω·m at 20°C. R = 1.724e-8 × 30 / 3.31e-6 = 0.1565 Ω. Per meter: 5.217 mΩ/m. Per 1000 ft: 1.59 Ω. This is a typical 20 A branch circuit wire.
Resistivity varies enormously: silver (1.59 × 10⁻⁸ Ω·m) is the best metallic conductor, followed by copper (1.72), gold (2.44), and aluminum (2.65). Nichrome (1.10 × 10⁻⁶) is 64× more resistive than copper — intentionally chosen for heating elements. Carbon steel (1.43 × 10⁻⁷) is 8× more resistive than copper but much stronger, used in structural cables. Superconductors reach exactly zero resistance below their critical temperature.
AWG (American Wire Gauge) is standard in North America. Europe uses mm² cross-sectional area directly (e.g., 2.5 mm², 4 mm², 6 mm²). SWG (Standard Wire Gauge, British) uses a different numbering. IEC 60228 defines standard conductor sizes in mm² used internationally. Converting between systems requires a reference table because the relationships are not linear.
Power lost in a wire equals I²R. A 100 m run of AWG 10 copper carrying 30 A: R = 2 × 100 × 3.277 mΩ/m = 0.655 Ω, power loss = 30² × 0.655 = 589 W. At $0.12/kWh, that is $619/year if running 24/7. This is why utilities use high voltage (reducing current for the same power) and why voltage drop calculations matter.
Metals have a positive temperature coefficient — resistance increases with temperature. For copper: α = 0.00393/°C. At 100°C, resistance is about 31% higher than at 20°C. This is why wires run hotter under heavy load: more current → more I²R heating → higher resistance → even more heating.
Circular mil (cmil) is an area unit used in North American wire sizing. It equals the area of a circle 1 mil (0.001 inch) in diameter. 1 cmil = 5.067 × 10⁻⁴ mm². AWG 10 = 10,380 cmil. The formula: cmil = d(mils)². It simplifies wire calculations because you skip π/4.
Aluminum is lighter (2.7 vs 8.9 g/cm³) and cheaper than copper. For the same weight, aluminum actually has lower resistance. Overhead power lines use aluminum (with steel core for strength). For building wiring, aluminum requires special connectors to prevent oxidation issues.
American Wire Gauge defines wire sizes by a number: larger numbers = thinner wire. AWG 0000 (4/0) is the thickest at 11.7 mm diameter. AWG 40 is the thinnest common size at 0.08 mm. Each step changes diameter by a factor of 92^(1/39) ≈ 1.123, and area by ~1.261.
V_drop = I × R_total (round-trip). For a 15 A load on 30 m of AWG 12 copper: R = 2 × 0.1565 = 0.313 Ω, V_drop = 15 × 0.313 = 4.7 V (3.9% on 120 V). This exceeds the NEC's 3% recommendation — consider AWG 10.
At high frequencies, current concentrates near the wire surface (skin effect). The effective cross-section decreases, increasing AC resistance. In copper at 60 Hz, skin depth is ~8.5 mm (insignificant for building wire). At 1 MHz, it is 66 μm (significant — only the outer shell conducts). At 1 GHz, it is 2.1 μm.