The standard micro farad symbol is µF (using the Greek letter mu). However, on physical components, schematics, and multimeters, you will frequently encounter uF, MFD, or 3-digit EIA codes like 104. Below is the complete reference for identifying, converting, and testing these values on the bench.
The Complete Micro Farad Symbol & Capacitance Reference Table
Capacitance is measured in Farads, but practical electronics rarely use whole Farads. Use this table to translate between the strict IEC symbols, ASCII schematic shortcuts, and the physical 3-digit codes printed on ceramic and film capacitors.
| Unit Name | Standard Symbol (IEC) | Schematic / ASCII Alt | EIA 3-Digit Code Example | Practical Application |
|---|---|---|---|---|
| Picofarad | pF | pF, mmF (vintage) | 101 (100 pF) | RF tuning, high-frequency decoupling, oscillator timing |
| Nanofarad | nF | nF | 103 (10 nF) | Audio filtering, snubber circuits, EMI suppression |
| Microfarad | µF | uF, MFD, mkF | 104 (0.1 µF), 105 (1 µF) | Power supply decoupling, motor run/start, bulk filtering |
| Millifarad | mF | mF (often misused for µF) | N/A (marked directly) | Supercapacitors, large audio crossover networks |
| Farad | F | F | N/A (marked directly) | EDLC supercapacitors, memory backup, regenerative braking |
Regional Standards, Vintage Markings, and the 'mF' Trap
While wire color codes are strictly governed by regional electrical codes (like NEC in the US or IEC 60446 in Europe), capacitor symbols fall under component marking standards. The global authority is IEC 60062, which defines the alphanumeric marking codes for capacitors and resistors. In the US, schematic symbols follow ANSI/IEEE 315.
However, regional and historical variants still show up on workbenches today:
- US / Vintage (MFD): Stands for 'MicroFarad'. You will still see this on HVAC motor run capacitors and vintage amplifier schematics. It is identical to µF.
- Soviet / Eastern Bloc (мкФ or mkF): The Cyrillic 'мк' translates to 'micro'. If you are repairing vintage Eastern European gear, 'мкФ' is your micro farad symbol.
- Japanese (JIS): Often uses the 'R' decimal system for small values (e.g., R47 means 0.47 µF) on older tantalum and electrolytic capacitors.
Rows People Get Wrong (The Bench Nightmares)
When reading the table above, hobbyists and even seasoned techs frequently trip over three specific translations:
- The '104' EIA Code Misread: A ceramic capacitor stamped '104' does not mean 104 µF. The EIA code is a base-10 multiplier in picofarads. '10' is the base, '4' is the number of zeros. 100,000 pF = 100 nF = 0.1 µF. This is the most common decoupling capacitor in electronics; memorize this specific translation.
- The 'mF' Multimeter Trap: In strict IEC terminology, 'm' means milli (10^-3) and 'µ' means micro (10^-6). Therefore, 1 mF = 1000 µF. However, many budget digital multimeters (DMMs) incorrectly label their microfarad testing range as 'mF'. If your meter reads '10 mF' on a standard 10µF electrolytic, the meter manufacturer has lazily used 'm' instead of 'µ'. Always verify your DMM's manual to confirm what the dial position actually measures.
- Nanofarad Decimal Shifts: Confusing nF and µF leads to ordering parts that are 1000x off. 47 nF is 0.047 µF. If a schematic calls for 47n and you install a 47µF electrolytic, your audio filter will pass sub-sonic frequencies and likely oscillate.
Safe Interpretation When Markings Are Faded or Missing
Electrolytic capacitors operate under high thermal stress. Over a decade, the electrolyte bakes, the plastic sleeve shrinks, and the micro farad symbol and value rating completely fade off the barrel. Guessing the value based on physical size is a fast track to a failed repair. Here is the exact bench procedure for identifying and validating an unmarked capacitor:
- Discharge and Isolate: Safely discharge the capacitor using a high-wattage bleed resistor. Desolder at least one leg of the capacitor from the PCB to prevent parallel circuit impedance from skewing your readings.
- Measure Capacitance (100 Hz): Connect the component to an LCR meter set to 100 Hz (the standard testing frequency for aluminum electrolytics). If the reading is 42 µF, the original part was almost certainly a 47 µF capacitor that has dried out and lost 10% of its capacity.
- Measure ESR (100 kHz): Switch the LCR meter to 100 kHz to read Equivalent Series Resistance. A healthy 47 µF cap should have an ESR well under 1.0 Ω. If your capacitance reads close to normal but ESR is >5 Ω, the dielectric is degraded and the part must be replaced.
- Deduce from Circuit Topology: If the capacitor is completely dead (open circuit) and unreadable, look at its location.
- Across a 5V/12V DC rail near an IC: Likely 10 µF to 100 µF bulk filtering.
- Directly across an IC's VCC/GND pins: Likely 0.1 µF (104 code) ceramic decoupling.
- On the output of a linear regulator: Check the regulator's datasheet; many require a specific 1 µF to 10 µF tantalum or low-ESR ceramic for loop stability.
When replacing an unmarked electrolytic, always upgrade to a 105°C rated, low-ESR series (such as Panasonic FR, Rubycon ZL, or Nichicon PW). A 105°C part will outlast an 85°C replacement by a factor of four in a hot power supply environment.
Micro Farad Symbol FAQ
Is uF the exact same as the micro farad symbol µF?
Yes. 'uF' is simply the ASCII typewriter and early-computer substitute for 'µF'. Because the Greek letter mu (µ) was not available on standard English keyboards or early CAD software, engineers adopted the lowercase 'u' as a visual and functional equivalent. In modern EDA tools like KiCad or Altium, 'uF' and 'µF' are treated as identical units.
What does MFD mean on an old motor run capacitor?
MFD stands for 'MicroFarad'. It is a legacy terminology standard that originated in the US electrical industry and is still heavily used today in HVAC (Heating, Ventilation, and Air Conditioning) and well-pump motor applications. If your AC compressor requires a '45+5 MFD' dual run capacitor, you need a 45 µF + 5 µF dual capacitor. Do not confuse MFD with 'milli-Farad'.
How do I convert a 3-digit EIA code to the micro farad symbol?
Take the first two digits as your base number, and use the third digit as the number of zeros to append. The base unit is always picofarads (pF).
Example: 473.
Base: 47. Multiplier: 3 zeros (000).
Result: 47,000 pF.
Convert to microfarads by dividing by 1,000,000: 47,000 / 1,000,000 = 0.047 µF.
Why does my multimeter say 'mF' when testing a 10µF capacitor?
This is a widespread labeling error on budget digital multimeters. The manufacturer has incorrectly used the milli prefix ('m') instead of the micro prefix ('µ') on the dial and LCD screen. If you test a known-good 10 µF capacitor and the screen reads '10.0 mF', your meter is actually measuring in microfarads. Always cross-reference your meter's user manual to confirm the actual measurement scale of that specific dial position.
