The Direct Answer: What Are Capacitors Measured In?
Capacitors are measured in Farads (F), named after Michael Faraday. One Farad represents the ability to store one coulomb of charge per volt of potential difference. Because a single Farad is an enormous amount of capacitance for standard electronics, we practically measure them in fractional sub-units: microfarads (µF), nanofarads (nF), and picofarads (pF).
When you are bench-testing a component, you aren't just looking at its nominal capacitance. You are evaluating its physical health by measuring its actual capacitance against its rated tolerance, and in the case of electrolytics, checking for Equivalent Series Resistance (ESR). Here is the standard unit breakdown you will see on schematics and multimeter displays:
| Unit | Symbol | Multiplier (Farads) | Common Applications |
|---|---|---|---|
| Millifarad | mF | 10^-3 (0.001 F) | Large power supply filtering, supercapacitors (rarely used on schematics; usually written as 1000µF) |
| Microfarad | µF or uF | 10^-6 (0.000001 F) | Power supply decoupling, audio coupling, motor run/start circuits, SMPS output filtering |
| Nanofarad | nF | 10^-9 | Snubber circuits, EMI filtering, mid-range audio crossovers, timing circuits |
| Picofarad | pF | 10^-12 | RF oscillators, high-frequency bypass, ceramic disc capacitors, tuned circuits |
Meter Setup and Safety: CAT Ratings and Discharging
Before you touch a probe to a capacitor, you must address the stored energy. A charged 400V, 100µF capacitor in a switching power supply can deliver a lethal shock or instantly vaporize the internal foil connections if shorted with a screwdriver.
If you are measuring capacitors on the primary (mains) side of a power supply, or testing HVAC run capacitors at 240V/480V, your multimeter must be rated CAT III 600V or CAT IV 600V minimum. Never use a cheap, un-rated hobby meter for mains-adjacent measurements. Always de-energize the circuit, lock out the breaker, and verify the circuit is dead with a non-contact voltage tester and your meter's voltage function before switching to capacitance mode.
The Proper Discharge Procedure
Never short a capacitor with a metal screwdriver. The massive dI/dt spike degrades the oxide dielectric layer inside electrolytics. Instead, discharge it through a power resistor. A 5-watt, 100-ohm to 220-ohm resistor safely bleeds off the charge in a few seconds without damaging the component. Clip a resistor across the terminals, wait 10 seconds, then verify with your DMM in DC voltage mode that the reading is < 0.1V.
DMM Setup Block
- Lead Jacks: Black lead to COM. Red lead to the V/Ω/Hz jack (do not use the Amps jack).
- Dial Position: Turn the dial to the Capacitance setting. On many meters (like the Fluke 87V or Brymen BM235), this shares a dial position with Ohms, Continuity, or Diode. You must press the SELECT or FUNC button until the display shows the capacitance symbol (often
--|(--or justnF/µF). - Range: Use Auto-ranging if available. If your meter is manual-ranging, select a range at least one step higher than the capacitor's printed value (e.g., set to 200µF to test a 47µF cap).
Probe Placement and Expected Readings
Capacitors must be measured out of circuit for accurate capacitance values. Parallel components (like resistors or inductors) on a PCB will skew the meter's AC test signal, giving you a wildly inaccurate reading. If you cannot desolder the component, at least lift one leg off the pad.
- Isolate: Desolder and remove the capacitor from the board.
- Zero the Meter: Touch the probes together and press the REL (Relative) button to zero out the stray capacitance of your test leads (usually 0.1nF to 0.5nF).
- Probe Placement: Touch the red probe to one leg and the black probe to the other. For non-polarized capacitors (ceramic, film), polarity does not matter. For polarized electrolytics, the DMM's red probe (positive test voltage) should ideally touch the anode (+) leg, though modern DMMs will measure capacitance correctly either way.
- Wait for Stabilization: Large electrolytics (>1000µF) take several seconds for the meter to charge the dielectric and calculate the value. Wait until the reading locks.
Expected Reading Table: Good vs. Bad
A 'good' reading isn't always the exact number printed on the can. According to All About Circuits and standard IEC 60384 specifications, different dielectrics have vastly different manufacturing tolerances.
| Capacitor Type | Printed Value | Standard Tolerance | Good Reading Range | Bad Reading (Replace) |
|---|---|---|---|---|
| Ceramic (MLCC) | 100 nF (0.1µF) | ±10% (X7R) to -20%/+80% (Y5V) | 90 nF – 110 nF (for X7R) | < 80 nF or Shorted (OL/0) |
| Film (Polyester/Polypropylene) | 1 µF | ±5% or ±10% | 0.95 µF – 1.05 µF | < 0.90 µF or Open (OL) |
| Aluminum Electrolytic | 470 µF | -20% / +80% | 376 µF – 846 µF | < 350 µF (Dried out) |
OL (Over Limit) on a known-good large electrolytic, your meter's internal test current might be too weak to charge it within the timeout window. This is a common limitation of sub-$50 multimeters when testing caps over 2,200µF.
Decision Tree: Troubleshooting Capacitor Measurements
Use this decision path to determine your next move based on what your multimeter displays. Do not guess; let the measurement dictate the action.
| Meter Display | Physical Condition | Diagnosis | Concrete Action / Part Pick |
|---|---|---|---|
| Within Tolerance Range | No bulging, no leaked electrolyte | Component is healthy. | Reinstall or keep as spare. No action required. |
| > 20% Below Nominal (e.g., 470µF reads 210µF) | May have a slightly domed top or crusty vent | Electrolyte has boiled off due to heat/age. Capacitance lost. | Bin it. Replace with a Panasonic FM/FK series or Rubycon ZL series low-ESR electrolytic rated for 105°C. |
| 0.000 or Near Zero (with continuity beep) | May look perfect externally | Internal short circuit. Dielectric has failed. | Bin it. Before installing the new cap, check the surrounding MOSFETs and diodes; a shorted cap often takes out the driving regulator. |
| OL (Open Loop) | Often physically intact | Internal open circuit (common in small ceramics cracked by board flex) or meter timeout. | Verify with a known-good cap. If still OL, bin it and replace with a C0G/NP0 ceramic for stability. |
Common Mistakes That Give Misleading Readings
If your readings are jumping around or seem physically impossible, you are likely falling victim to one of these bench errors:
- Measuring In-Circuit: This is the most common mistake. If you measure a 100nF decoupling capacitor while it's still soldered to a board populated with other 100nF caps and low-ohm resistors, the meter measures the parallel impedance of the entire local circuit. You will get a reading that makes no sense. Fix: Desolder at least one leg.
- Finger Capacitance: The human body acts as a dielectric. If you hold a small picofarad capacitor by its leads with your bare fingers while probing it, your body adds 50pF to 150pF of stray capacitance in parallel. A 22pF RF capacitor will falsely read as 100pF. Fix: Use insulated alligator clips or a PCB breakout jig for sub-nF measurements.
- Skipping the REL (Relative) Button: Test leads have inherent capacitance. If you are measuring a 2.2pF capacitor and your leads add 1.5pF, your reading will be off by nearly 70%. Fix: Short the probes and press REL before measuring small values.
- Ignoring ESR: A multimeter's capacitance function applies a low-frequency test signal. A failing electrolytic might still read 470µF perfectly, but its internal ESR (Equivalent Series Resistance) has spiked from 0.05Ω to 15Ω, making it useless for high-frequency SMPS filtering. A standard DMM cannot measure ESR. (See the next section for the solution).
When to Upgrade from a DMM to an LCR Meter
For general HVAC troubleshooting, basic power supply repair, and audio coupling checks, the capacitance function on a high-quality True-RMS DMM (like the Fluke 87V) is entirely sufficient. However, a DMM hits a wall when you need to measure ESR, dielectric loss (D factor), or test at specific frequencies (1kHz vs 100kHz).
If you are designing switching power supplies, repairing high-end audio amplifiers, or troubleshooting RF oscillators, you need a dedicated LCR meter. An LCR meter applies an AC test signal at selectable frequencies (typically 100Hz, 1kHz, and 100kHz) and calculates both the primary value (C) and the secondary loss parameter (ESR or D).
The Default Recommendation: For 90% of bench and field troubleshooting, rely on your DMM's capacitance mode, keep a 5W 100-ohm bleeder resistor in your pouch, and always isolate the component. If the µF reading is more than 20% below nominal on an electrolytic, bin it and solder in a 105°C low-ESR replacement. But the moment you suspect a capacitor is failing despite showing the correct µF value, stop using the DMM and pick up a dedicated LCR meter like the DER EE DE-5000 or the Uni-T UT612 to check the ESR.
