The microfarad symbol is most formally written as µF (using the Greek letter mu), representing one-millionth of a farad ($10^{-6}$ F). However, due to ASCII keyboard limitations and legacy industrial conventions, you will frequently see it typed as uF or labeled as MFD (microfarad) on motor run capacitors and vintage equipment. Misinterpreting these symbols—especially confusing MFD with millifarads (mF)—is a leading cause of blown components and motor failures on the workbench.
The Complete Microfarad & Capacitor Marking Reference Table
Capacitor markings vary wildly depending on whether the manufacturer follows the international IEC 60062 standard, the US-based EIA standard, or legacy conventions. Use this table to translate physical markings into actual microfarad values.
| Marking / Symbol | Standard Origin | Multiplier / Base Value | Practical Microfarad (µF) Equivalent |
|---|---|---|---|
| µF / uF | Universal / SI | Base ($10^{-6}$ F) | 1.0 µF |
| MFD | US Legacy / Industrial | Base ($10^{-6}$ F) | 1.0 µF |
| nF | IEC 60062 | $10^{-9}$ F | 0.001 µF |
| pF | IEC 60062 / EIA | $10^{-12}$ F | 0.000001 µF |
| 104 | EIA 3-Digit | $10 \times 10^4$ pF | 0.1 µF |
| 105 | EIA 3-Digit | $10 \times 10^5$ pF | 1.0 µF |
| 4R7 | IEC Alphanumeric | R = decimal point | 4.7 µF |
| 2n2 | IEC Alphanumeric | n = decimal point | 0.0022 µF |
| mF | SI Standard | $10^{-3}$ F (Millifarad) | 1000 µF (NOT micro!) |
Regional Standards and the 'Rows People Get Wrong'
When sourcing components globally, you must identify which regional standard the manufacturer used. The International Electrotechnical Commission (IEC) standard 60062 dominates European and modern Asian manufacturing, utilizing alphanumeric codes where the unit prefix replaces the decimal point (e.g., 4R7 for 4.7 µF). Conversely, the US-based Electronic Industries Alliance (EIA) popularized the 3-digit pF multiplier system (e.g., 104) for ceramic and film capacitors.
The Most Dangerous Misreads
Even experienced technicians fall into specific traps when reading capacitor codes under poor lighting or when dealing with mixed-standard bins.
- mF vs. MFD (The Motor Killer): This is the most critical distinction in the table. 'mF' strictly means millifarad ($10^{-3}$ F, or 1000 µF). 'MFD' is an archaic but still widely used abbreviation for microfarad. If a schematic calls for a 10 MFD motor run capacitor and you install a modern 10mF (10,000 µF) electrolytic, the motor winding will overheat and burn out due to excessive phase-shift current.
- The '104' Trap: Beginners often read an EIA '104' ceramic capacitor as 104 µF. It is actually 100,000 pF, which translates to 0.1 µF. The first two digits are significant figures (10), and the third digit is the multiplier (add four zeros) in picofarads.
- The 'R' Decimal Confusion: A marking of '4R7' means 4.7 µF. A marking of '47' without a letter on a small ceramic disc usually means 47 pF (0.000047 µF). The presence of the 'R' is mandatory to indicate the decimal place in IEC alphanumeric coding.
Safe Interpretation When Markings Are Faded or Missing
Heat, solvent exposure, and age routinely destroy capacitor silk-screening. When the microfarad symbol and value are illegible, you must rely on measurement and circuit context to select a safe replacement.
- Measure with an LCR Meter: A standard digital multimeter (DMM) capacitance function is often too slow or inaccurate for degraded parts. Use a dedicated LCR meter (like the DER EE DE-5000 or Keysight U1733C). Set the test frequency to 100Hz or 120Hz for electrolytics, and 1kHz for ceramics and films. Measuring an electrolytic at 1kHz will yield a falsely low microfarad reading due to the capacitor's internal equivalent series inductance (ESL).
- Check Equivalent Series Resistance (ESR): If the capacitance reads within 20% of the suspected original value but the ESR is above 1 ohm (for values >10µF), the electrolyte has dried out. Replace it regardless of the faded markings.
- Voltage Derating Rule: If the voltage rating is rubbed off, never guess based on physical size alone. Analyze the circuit's maximum rail voltage. For a 5V logic rail, use a minimum 10V or 16V rated replacement. For a 12V system, use a 25V or 35V part. Always uprate the voltage by at least 50% to account for ripple and transient spikes.
- Temperature and Tolerance: If replacing a faded ceramic capacitor in a timing or filter circuit, default to C0G/NP0 dielectric (which has near-zero temperature drift) rather than X7R or Y5V, which can lose up to 50% of their microfarad value when DC bias is applied.
Frequently Asked Questions
Is uF the same as the microfarad symbol µF?
Yes. Because the Greek letter mu (µ) was not available on early ASCII keyboards and is still missing from many basic schematic capture tools, 'u' became the universally accepted engineering substitute. In modern PCB silkscreens and datasheets, 'uF' and 'µF' are electrically and legally identical. Never interpret 'uF' as 'micron-farad' or any other unit.
What does MFD mean on an old motor run capacitor?
MFD stands for 'microfarad' in legacy US industrial nomenclature. It is an abbreviation of 'micro-farad' where the 'M' represents 'micro'. Do not confuse this with the modern SI prefix 'M' (Mega, $10^6$) or 'm' (milli, $10^{-3}$). A 40 MFD motor capacitor is exactly 40 µF. According to NIST SI prefix guidelines, 'micro' should be denoted by µ, but the MFD legacy persists heavily in HVAC and industrial motor repair.
How do I read a 3-digit capacitor code into microfarads?
Take the first two digits as your base number, and use the third digit as the number of zeros to append, giving you the value in picofarads (pF). To convert to microfarads (µF), divide that result by 1,000,000. For example, a code of '225' means 22 followed by five zeros = 2,200,000 pF. Divided by 1,000,000, this equals 2.2 µF. For a deeper dive into component math, reference standard All About Circuits textbooks on passive components.
Can I replace a capacitor if the microfarad symbol is rubbed off?
Yes, but only if you can verify the circuit's operating voltage and measure the existing component's actual capacitance and ESR. If the capacitor is completely shorted or open and cannot be measured, you must trace the circuit to determine the required value. Never install a replacement with a lower voltage rating than the original design, and always ensure the physical footprint and lead spacing (e.g., 5mm vs 7.5mm pitch) match the PCB pads to avoid mechanical stress on the leads.
