The picofarad (pF) represents $10^{-12}$ Farads and is the standard base unit for high-frequency filtering, RF tuning, and oscillator timing. On schematics, the picofarad symbol is written as pF (US/IEEE standard) or simply p (IEC/European standard). On physical ceramic components, the symbol is rarely printed; instead, manufacturers use the 3-digit EIA code (where the implicit base unit is picofarads) or alphanumeric stamps (like 4p7). If you are reading a schematic or trying to identify a scavenged component, use the reference table below to translate the marking into an exact capacitance value.
The Master Picofarad Symbol & Code Reference Table
Read this table by matching the context (schematic vs. physical part) to the format. The "Actual Value" column is your ground truth.
| Context | Symbol / Code Format | Example | Actual Picofarad Value | Practical Notes |
|---|---|---|---|---|
| Schematic (IEEE 315) | Explicit Unit | 10 pF | 10 pF | Standard US practice. Space between number and unit is common. |
| Schematic (IEC 60617) | Unit as Decimal | 4p7 | 4.7 pF | The 'p' replaces the decimal point to prevent misreading. |
| Physical (3-Digit EIA) | XY × 10^Z pF | 104 | 100,000 pF (100 nF) | First two digits are significant, third is multiplier. Base is always pF. |
| Physical (2-Digit) | Direct Value | 22 | 22 pF | Used for values under 100 pF. No multiplier digit. |
| Physical (Alphanumeric) | Number + Letter + Number | 1n0 | 1,000 pF (1 nF) | Letter indicates the magnitude (p, n, µ) and acts as the decimal. |
| Old Soviet (GOST) | Number + Cyrillic 'П' | 51П | 51 pF | Often just stamped '51' on tiny mica/ceramic discs, assuming pF. |
Rows People Get Wrong (And How to Fix Them)
Misreading capacitor codes is the most common cause of dead-on-arrival RF circuits and failing oscillators. Here are the specific traps to avoid:
- The "104" Trap: Beginners frequently read "104" as 104 pF. It is actually $10 \times 10^4$ pF, which equals 100,000 pF (or 100 nF / 0.1 µF). This is the universal decoupling capacitor value. If you put a literal 104 pF cap on an IC VCC pin, the chip will brownout under load.
- The "221" Shift: This means $22 \times 10^1$ pF = 220 pF. Do not confuse it with 22 pF (which would be stamped simply as "22" or "220" where the zero is the multiplier $10^0$).
- Alphanumeric Decimals: A stamp reading 4p7 means 4.7 pF. A stamp reading p47 means 0.47 pF. The position of the letter dictates the decimal placement. See the All About Circuits capacitor marking guide for visual examples of these microscopic stamps.
Regional & Standard Variants: IEEE vs. IEC vs. GOST
Which standard applies to you depends on where your schematic was drafted or where the equipment was manufactured.
In the US, IEEE 315 (formerly ANSI Y32.2) governs schematic symbols. It typically uses parallel lines for non-polarized capacitors and explicitly writes "pF" or "µF" next to the value. Conversely, older Eastern Bloc equipment relies on GOST standards, where the picofarad symbol was sometimes denoted by the Cyrillic letter П (Pe), or omitted entirely on small ceramics because picofarads were the assumed default for any value under 1000.
Safe Interpretation When Markings Are Faded or Missing
Ceramic capacitors, especially the cheap monolithic MLCCs and epoxy-dipped discs, are notorious for losing their markings due to heat, flux cleaning, or simply being too small to print on. Never guess blindly; use this decision path to identify the part safely.
The Faded Marking Decision Path:
- If the cap is connected directly between an IC's VCC and GND pins: It is almost certainly a 100 nF (100,000 pF / code 104) decoupling capacitor. Replace with a standard 0805 or 0603 X7R MLCC.
- If the cap is connected to the pins of a quartz crystal oscillator: It is a load capacitor, typically ranging from 10 pF to 33 pF. It must be a C0G/NP0 dielectric. Replace with a 22 pF C0G if the exact value is unmeasurable (22 pF is the most common default for 16MHz/20MHz crystals).
- If the cap is in an RF matching network or antenna trace: Values are usually between 1 pF and 47 pF. You must desolder and measure.
Measurement Technique for Picofarads:
When measuring values under 100 pF, you must set your LCR meter's test frequency to 1 MHz, not the default 1 kHz. At 1 kHz, a 22 pF capacitor has an impedance of roughly 723 MΩ. At this impedance, the stray capacitance of your test leads and the moisture on your fingers will swamp the reading. At 1 MHz, the impedance drops to ~7.2 kΩ, yielding a stable, accurate measurement.
Decision Path: Picking the Exact Replacement Part
Once you know the picofarad value, you must select the correct dielectric material. Using the wrong dielectric will cause your circuit to drift with temperature or fail under voltage bias. Use the matrix below to terminate your search with a concrete part number.
| Application Scenario | Required Dielectric | Why It Matters | Concrete Part Pick (0805 Package) |
|---|---|---|---|
| Crystal Oscillator Load / RF Tuning / Precision Timing | C0G / NP0 (Class I) | Zero voltage coefficient; temperature drift is 0 ± 30 ppm/°C. Frequency stays locked. | KEMET C0805C220J5GACTU (22 pF, 50V, C0G) |
| General Decoupling / Bypass / Audio Coupling | X7R / X5R (Class II) | High volumetric efficiency. Capacitance can drop 20-40% under DC bias, which is acceptable for power rail noise filtering. | Murata GRM21BR71H104KA88L (100,000 pF / 100nF, 50V, X7R) |
| High-Voltage Snubber / Mains Filtering | C0G or X7T (Rated >1kV) | Requires physical distance between internal plates to prevent arc-over. Standard 50V parts will explode. | Vishay VJ2225Y102JXGAT (1000 pF, 2kV, C0G) |
For deep-dive specifications on dielectric behaviors and voltage derating curves, refer to the Vishay Capacitor Selection Guide. Always verify the physical footprint (e.g., 0805 vs 0603) against your PCB pads before ordering, as picofarad-range C0G capacitors are frequently only available in slightly larger case sizes than their X7R equivalents due to the thicker dielectric layers required.
