Sources and scope for this page: International Annealed Copper Standard (IACS) (reference definition fixed since 1913; 100% IACS = 1.7241 × 10⁻⁸ Ω·m at 20 °C), covering the %IACS column and the conductivity reference; and IEC 60028:1925 (withdrawn but still the source of the 1.7241 value), covering the copper resistivities (one more source is tabulated below). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.
Page last changed 2026-11-02 · based on n/a — reference definition fixed since 1913; 100% IACS = 1.7241 × 10⁻⁸ Ω·m at 20 °C
Quick Answer
Electrical Resistivity of Materials
| Material[3] | Class[3] | Resistivity Ω·m[3] | Resistivity Ω·mm²/m[2] | Conductivity % IACS[1] | Temp. coefficient /°C[3] | Note[3] |
|---|---|---|---|---|---|---|
| Silver # | Pure metal | 1.59e-08 | 0.0159 | 108.4 | 0.0038 | The best conductor of any metal; too costly for bulk wiring |
| Copper (annealed, 100% IACS) # | Pure metal | 1.724e-08 | 0.01724 | 100 | 0.00393 | The reference point of the IACS scale |
| Copper (hard-drawn) # | Pure metal | 1.77e-08 | 0.0177 | 97.41 | 0.00393 | About 3% more resistive than annealed, from work hardening |
| Gold # | Pure metal | 2.44e-08 | 0.0244 | 70.66 | 0.0034 | Used for contacts because it does not oxidise |
| Aluminium # | Pure metal | 2.65e-08 | 0.0265 | 65.06 | 0.00429 | 61% of copper's conductivity, but 30% of its density |
| Aluminium alloy 6061-T6 # | Alloy | 3.7e-08 | 0.037 | 46.6 | 0.0039 | Alloying costs about 40% of pure aluminium's conductivity |
| Tungsten # | Pure metal | 5.6e-08 | 0.056 | 30.79 | 0.0045 | Used for lamp filaments because of its melting point, not its conductivity |
| Zinc # | Pure metal | 5.9e-08 | 0.059 | 29.22 | 0.0037 | Galvanising coating |
| Brass (70/30) # | Alloy | 7e-08 | 0.07 | 24.63 | 0.0015 | Alloying cuts conductivity sharply |
| Nickel # | Pure metal | 6.99e-08 | 0.0699 | 24.67 | 0.006 | Basis of resistance alloys |
| Iron (pure) # | Pure metal | 9.71e-08 | 0.0971 | 17.76 | 0.0065 | Much more resistive than copper |
| Carbon steel # | Alloy | 1.43e-07 | 0.143 | 12.06 | 0.006 | About 8× copper — why steel is not used for conductors |
| Stainless steel 304 # | Alloy | 7.2e-07 | 0.72 | 2.395 | 0.00094 | 42× copper; used for heating elements and for its corrosion resistance |
| Nichrome (Ni-Cr 80/20) # | Alloy | 1.1e-06 | 1.1 | 1.567 | 0.0004 | 64× copper and heat resistant — the standard heating element alloy |
| Kanthal A-1 (Fe-Cr-Al) # | Alloy | 1.45e-06 | 1.45 | 1.189 | 5e-05 | Very low temperature coefficient; furnace elements |
| Mercury # | Liquid metal | 9.58e-07 | 0.958 | 1.8 | 0.0009 | 56× copper; the basis of older thermometers and relays |
| Graphite (bulk) # | Non-metal | 7e-06 | 7 | 0.2463 | -0.0005 | Negative temperature coefficient — resistance falls as it heats |
| Silicon (pure, 20 °C) # | Semiconductor | 2300 | 2.3e+09 | 7.496e-10 | -0.07 | Semiconductor; resistance falls steeply with temperature and rises with doping |
| Germanium (pure) # | Semiconductor | 0.46 | 4.6e+05 | 3.748e-06 | -0.048 | Semiconductor |
| Sea water # | Electrolyte | 0.2 | 2e+05 | 8.62e-06 | — | Conductive because of dissolved salts |
| Drinking water # | Electrolyte | 20 | 2e+07 | 8.62e-08 | — | Varies by orders of magnitude with mineral content |
| Deionised water # | Electrolyte | 1.8e+05 | 1.8e+11 | 9.578e-12 | — | Thousands of times less conductive than drinking water |
| Glass (soda-lime) # | Insulator | 1e+11 | 1e+17 | 1.724e-17 | — | Insulator; drops sharply when hot |
| Rubber (hard) # | Insulator | 1e+13 | 1e+19 | 1.724e-19 | — | Classic insulator |
| PTFE # | Insulator | 1e+22 | 1e+28 | 1.724e-28 | — | One of the best solid insulators available |
| Air (dry, 20 °C) # | Insulator | 1.6e+13 | 1.6e+19 | 1.078e-19 | — | Insulating until the field exceeds about 3 kV/mm, when it ionises |
Values are for 20 °C unless the row says otherwise. Resistivity is strongly temperature-dependent — for pure metals it rises with temperature (positive coefficient), while semiconductors and graphite fall. %IACS (International Annealed Copper Standard) is conductivity as a percentage of annealed copper, where 100% IACS = 1.7241 × 10⁻⁸ Ω·m = 58 MS/m. The Ω·mm²/m column is numerically equal to µΩ·m and is the unit European cable and heating-element datasheets usually use.
Resistivity, Resistance and Conductivity
Resistivity is a property of the material; resistance is a property of the object. The relationship is R = ρL/A — resistance is resistivity times length divided by cross-sectional area. That is why a long thin wire has more resistance than a short fat one of the same metal, and why the two must not be used interchangeably.
Resistivity is quoted in Ω·m in SI, and in Ω·mm²/m in most European cable and heating-element datasheets. The two differ by exactly a factor of 10⁶, so Ω·mm²/m is numerically identical to µΩ·m — which is why copper's 1.7241 × 10⁻⁸ Ω·m is written as 0.017241 Ω·mm²/m, or 17.241 nΩ·m. Mixing the two units without converting introduces a million-fold error.
Conductivity is simply the reciprocal of resistivity, in S/m. The %IACS scale is a convenient normalisation for metals: it expresses conductivity as a percentage of annealed copper, so pure silver is 108% IACS, aluminium is 61%, and stainless steel 304 is around 2.4%. Anything above 100% IACS conducts better than the copper it is measured against.
Temperature Coefficient and Why It Matters
The temperature coefficient in the table is the fractional change in resistivity per degree Celsius at 20 °C, so a coefficient of 0.00393 means resistivity rises by 0.393% for each degree. Over a 55 °C rise from 20 °C to a 75 °C conductor rating, copper's resistance rises about 22% — which is why cable ampacity tables are published against a conductor temperature, not an ambient one.
Two practical consequences follow. First, voltage drop calculated at 20 °C understates the drop in a loaded conductor; use the resistance at the expected operating temperature. Second, the positive coefficient is what makes a conductor fail safely: as current rises the conductor heats, its resistance rises, and the current is limited — the opposite of thermal runaway.
The exceptions are instructive. Graphite and semiconductors have negative coefficients — their resistance falls as they heat, which is why a graphite brush or a semiconductor device can run away thermally if it is not current-limited. Nichrome and Kanthal have very small coefficients — around 0.0004 and 0.00005 per °C — which is exactly why they are used for heating elements: the element's resistance barely changes as it heats, so the power output is predictable.
Frequently Asked Questions
- Why is copper used for wiring rather than silver?
- Silver is 8% more conductive, but it costs roughly 70 times as much and tarnishes. Copper is the practical optimum: it is the second-best metallic conductor, it is abundant and workable, and its oxide is conductive enough not to ruin a connection. Silver is used only where the extra conductivity or the oxidation resistance justifies the cost — high-frequency contacts and switchgear.
- Is aluminium a worse conductor than copper?
- Per unit of cross-section, yes — aluminium is 61% as conductive, so it needs about 1.6 times the area. Per unit of weight aluminium wins: it is 30% of copper's density, so the same mass of aluminium carries roughly twice the current of the same mass of copper. That is why overhead transmission lines are aluminium, often with a steel core for strength, while building wiring is copper.
- What is 100% IACS?
- The International Annealed Copper Standard defines 100% IACS as a conductivity of 5.800 × 10⁷ S/m, which corresponds to a resistivity of 1.7241 × 10⁻⁸ Ω·m at 20 °C. It is a fixed reference value, not a rating of any particular copper product, and conductivity better than 100% is normal — pure silver reaches about 108% IACS, and oxygen-free high-conductivity copper slightly exceeds 100%.
- Does bending or working a conductor change its resistance?
- Yes, slightly. Cold working a metal increases its resistivity by disrupting the crystal lattice — hard-drawn copper is about 3% more resistive than annealed copper, which is why the two appear as separate rows in the table. Annealing restores the conductivity. The effect is small compared with temperature, but it matters in precision work and in sizing flexible conductors made of many fine strands.
- Why does stainless steel make a poor conductor?
- Its resistivity is about 42 times copper's, because chromium and nickel alloying elements scatter electrons heavily and the alloy's crystal structure is far from a pure metal's. That is exactly what makes it useful as a heating element or a resistance wire — high resistance converts current into heat — and exactly why it is never used for power distribution.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | Conductivity expressed as % IACS | derived | % IACS = (1.7241 × 10⁻⁸ Ω·m ÷ resistivity) × 100, using the IACS reference resistivity. Values above 100% are valid and expected — silver exceeds it. |
| [2] | Resistivity converted to Ω·mm²/m | derived | Ω·mm²/m = Ω·m × 10⁶, exact. Numerically equal to µΩ·m. Checked at build time for every row. |
| [3] | Published electrical resistivity values for conductors, alloys and insulators | reference | compilations as published 2024–2026 |
Data Sources
| Standard | Revision | What it covers on this page |
|---|---|---|
| International Annealed Copper Standard (IACS) | n/a — reference definition fixed since 1913; 100% IACS = 1.7241 × 10⁻⁸ Ω·m at 20 °C | the %IACS column and the conductivity reference |
| IEC 60028 — International standard of resistance for copper | IEC 60028:1925 (withdrawn but still the source of the 1.7241 value) | the copper resistivities |
| Published resistivity tables for metals, alloys and insulators | compilations as published 2024–2026 | every value in the table |
Verification scope
No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.
Derived values — the following values on this page are calculated, not taken directly from the standard:
| Value | How it is derived |
|---|---|
| Resistivity in Ω·mm²/m | Ω·m × 10⁶, exact. Rechecked at build time for every row to within 1%. |
| Conductivity as % IACS | 1.7241 × 10⁻⁸ ÷ resistivity × 100. Copper is held as a known-value check at exactly 100% to confirm the reference is being applied correctly. |
Values are for 20 °C and for the purest form of each material. Alloying, cold work, impurities and temperature all move resistivity substantially — hard-drawn copper is 3% more resistive than annealed, and a 55 °C temperature rise adds about 22%. Insulator resistivity varies enormously with moisture, temperature and grade, so treat those rows as order-of-magnitude figures only.
Cite This Page
Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page:
Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.