What Is a Farad?
The complete guide to the farad, the SI unit of electrical capacitance.
Last updated: 2026-05-21
Definition
A farad (symbol: F) is the SI unit of electrical capacitance, the measure of how much electric charge a component can store at a given voltage. It is defined as one coulomb of charge per volt of potential difference:
1 F = 1 C/V
In words, a capacitor has a capacitance of one farad when storing one coulomb of charge raises the voltage across its terminals by one volt. Expressed in SI base units, the farad works out to kilograms, meters, seconds, and amperes as 1 F = 1 A²·s⁴ ÷ (kg·m²).
The farad is a very large unit. One coulomb is the charge carried by roughly 6.24 × 1018 electrons, so storing a full coulomb for each volt demands an enormous amount of physical material. A capacitor of one farad built with traditional construction would be impractically big, which is why everyday components are rated in tiny fractions of a farad.
Practical Scale
Because the farad is so large, real-world capacitors are almost always specified using metric submultiples. Three appear constantly in electronics:
- Microfarad (µF) — one millionth of a farad (10−6 F). The working range for electrolytic capacitors in power supplies, audio circuits, and motor starters.
- Nanofarad (nF) — one billionth of a farad (10−9 F). Common for film and ceramic capacitors used in filtering and timing.
- Picofarad (pF) — one trillionth of a farad (10−12 F). Used for the small ceramic capacitors in radio-frequency, oscillator, and high-speed digital circuits.
At the other extreme, supercapacitors (also called ultracapacitors) finally reach whole farads and beyond, with single devices rated from about 1 F up to several thousand farads. They bridge the gap between conventional capacitors and batteries, storing far more charge than ordinary components while still charging and discharging quickly.
Everyday Examples
| Component | Typical Capacitance | Where It Is Used |
|---|---|---|
| Small ceramic capacitor | 1–100 pF | RF tuning, oscillators |
| Film capacitor | 1 nF–10 µF | Filtering, signal coupling |
| Electrolytic capacitor | 1–10,000 µF | Power supply smoothing |
| Motor-start capacitor | 25–1,400 µF | AC motors, compressors |
| Supercapacitor | 1–3,000 F | Backup power, energy storage |
Common Submultiples
| Unit | Symbol | Value in Farads | Factor |
|---|---|---|---|
| Farad | F | 1 F | 1 |
| Millifarad | mF | 0.001 F | 10−3 |
| Microfarad | µF | 0.000001 F | 10−6 |
| Nanofarad | nF | 0.000000001 F | 10−9 |
| Picofarad | pF | 0.000000000001 F | 10−12 |
To move up the scale: 1 µF = 1,000 nF = 1,000,000 pF. A common conversion mistake is mixing nanofarads and microfarads, since a factor of 1,000 separates them. When a datasheet lists a value like 0.1 µF, that is the same as 100 nF or 100,000 pF.
History
The farad is named after Michael Faraday (1791–1867), the English scientist whose experiments on electromagnetic induction and electrochemistry shaped the foundations of electrical engineering. The name was proposed in the 1860s, and the unit was formally adopted into the international system of electrical units that grew into the modern SI.
Faraday himself worked largely without advanced mathematics, relying instead on careful experiment and physical intuition. Honoring him with the unit of capacitance recognizes his discovery of how electric and magnetic fields store and transfer energy — the very principle a capacitor relies on.
Need to switch between farads, microfarads, nanofarads, and picofarads? Use our capacitance converter for fast, precise results.