How to Measure the Capacitance of a Capacitor with a Multimeter

To accurately measure the capacitance of a capacitor, set your digital multimeter (DMM) to the capacitance mode (usually marked with a || or T symbol), completely discharge the component, and place the red lead on the anode (+) and the black lead on the cathode (-). A good reading will fall within ±5% to ±20% of the rated microfarad (µF) or picofarad (pF) value printed on the component casing. If the meter reads 'OL' (Over Limit) or zero, the capacitor is either open or shorted and must be replaced.

While the concept is straightforward, real-world bench and jobsite conditions introduce variables like parasitic in-circuit paths, dielectric absorption, and lethal stored energy. Below is the exact procedure for getting reliable, repeatable measurements without blowing your meter's internal fuse or risking a shock.

Meter Setup and Safety Category (CAT) Requirements

Before touching any probes to a component, you must configure your meter correctly and verify its safety rating for the environment you are working in.

Meter Setup Block

• Dial Position: Rotate the selector to the capacitance function. On auto-ranging meters like the Fluke 87V or Brymen BM235, this is a dedicated position. On manual meters, you may need to press a secondary 'Hz/µF' button to toggle the display into capacitance mode.
• Lead Jacks: Insert the black lead into the COM (Common) jack. Insert the red lead into the V/Ω/Cap jack. Never leave the red lead in the Amps/mA jack, as this creates a near-dead short across the capacitor when you apply the probes.
• Range Selection: If using a manual-ranging meter, start at the highest capacitance range (e.g., 2000µF) and step down until you get a stable reading with the most significant digits. Auto-ranging meters will handle this, though they may take 3 to 5 seconds to lock onto high-value electrolytics.

Safe Discharge and Probe Placement Procedure

Capacitors store electrical energy, sometimes at lethal voltages. A microwave oven capacitor can hold 4,000V DC, and a 400V HVAC run capacitor can deliver a painful, muscle-locking shock. You must discharge the capacitor before measuring.

1. Discharge the Component: Use a dedicated bleeder resistor (a 20kΩ, 5-watt wirewound resistor is ideal for most bench work) attached to insulated alligator clips. Bridge the resistor across the capacitor terminals for 10 to 15 seconds. For large motor-start capacitors, follow up by shorting the terminals with an insulated flathead screwdriver to ensure zero residual voltage.
2. Isolate if Necessary: For the most accurate reading, desolder and remove at least one leg of the capacitor from the PCB. Measuring in-circuit often yields false highs due to parallel impedance paths.
3. Probe Placement (Polarized/Electrolytic): Identify the polarity stripe on the capacitor casing (indicating the negative/cathode lead). Place the red probe on the long leg (anode/+) and the black probe on the short leg (cathode/-). While modern DMMs measure capacitance regardless of polarity, matching the polarity ensures the internal measurement voltage doesn't stress the dielectric layer.
4. Probe Placement (Non-Polarized/Ceramic/Film): Polarity does not matter. Place one probe on each lead. Keep your fingers off the metal probe tips; the human body introduces roughly 50pF to 100pF of stray capacitance, which will completely ruin readings for small ceramic capacitors.
5. Wait for Stabilization: High-value capacitors (above 1000µF) require the meter to charge the dielectric to measure it. Hold the probes steady for up to 10 seconds until the reading stops climbing.

Expected Capacitance Readings: Good vs. Failing Components

A capacitor's health isn't just about whether it reads a number; it's about whether that number falls within the manufacturer's specified tolerance. Most electrolytics have a ±20% tolerance, while HVAC run capacitors are typically held to a tighter ±5% or ±6% spec. Consult the All About Circuits capacitor guide for deeper theory on dielectric tolerances.

Note: If a 40µF HVAC run capacitor reads 35µF, the compressor will struggle to start, draw excessive amperage, and eventually trip the thermal overload. Replace it immediately, even if the meter registers a value.

Mistakes That Cause Misleading Capacitance Readings

When a reading doesn't make sense, it is rarely the meter's fault. Look for these common bench and jobsite errors:

• Measuring In-Circuit: Capacitors in a PCB are surrounded by parallel resistors, inductors, and semiconductor junctions. Your meter sends a small AC test signal to measure capacitance; parallel paths will divert this signal, usually resulting in a reading that is wildly higher than the actual component value. Always lift at least one leg.
• Ignoring Dielectric Absorption: If you discharge a capacitor, measure it, remove the probes, and then short it again, you may see a small spark. This is dielectric absorption—the insulating material slowly releasing trapped charge. If you don't fully discharge the cap before testing, the residual voltage will confuse the meter's internal ADC, resulting in erratic, drifting numbers.
• Using the Wrong Test Leads: Standard 3-foot test leads have their own parasitic capacitance (roughly 10pF to 15pF per foot). If you are trying to measure a 22pF ceramic capacitor in an RF circuit, the leads alone will double your reading. Use short, rigid SMD tweezers or a dedicated LCR meter for sub-nanofarad values.
• Confusing ESR with Capacitance: A capacitor can read perfectly at 470µF on your DMM but still fail in a high-frequency switching power supply because its Equivalent Series Resistance (ESR) has spiked due to dried electrolyte. A standard DMM capacitance test will not catch high ESR; you need an ESR meter for that.

Frequently Asked Questions

How to measure the capacitance without a dedicated multimeter?

If your DMM lacks a capacitance function, you have two options. First, use a dedicated LCR meter (like the DER EE DE-5000L), which applies an AC signal at 1kHz or 120Hz to calculate both capacitance and ESR with high precision. Second, you can calculate it manually using the RC time constant method: charge the capacitor through a known resistor (e.g., 10kΩ) using a DC power supply, and use an oscilloscope to measure the time it takes for the voltage to reach 63.2% of the supply voltage. Using the formula t = R × C, you can solve for C. For practical capacitor theory and formulas, standard textbooks cover this RC transient behavior in depth.

How to measure the capacitance when the reading keeps drifting?

A drifting reading on a digital display usually indicates one of three things. First, the capacitor may not be fully discharged; residual voltage is fighting the meter's internal test voltage. Second, the capacitor has high dielectric absorption (common in older film and electrolytic types), meaning it is slowly absorbing and releasing charge during the test cycle. Third, if you are measuring very small values (under 100pF), your body's proximity to the probes is introducing stray capacitance. Step back, use shorter leads, and let the meter auto-zero before connecting the component.

How to measure the capacitance of high-voltage motor-start or microwave capacitors safely?

High-voltage capacitors (like the 1µF to 2µF, 2100V AC capacitors in microwave ovens) can store lethal charges even after the appliance is unplugged. Do not attempt to measure exact capacitance on a microwave capacitor unless you are trained and using a CAT IV rated setup. For diagnostic purposes on these specific high-voltage components, technicians typically skip the capacitance measurement and instead use a high-voltage megohmmeter or a standard DMM in resistance mode to check for internal shorts. If a microwave capacitor reads 0 ohms (shorted) or shows physical bulging/oil leakage, it is bad. If it reads 'OL' (infinite resistance), it is generally considered functional for basic troubleshooting, though a dedicated high-voltage LCR test is the only way to verify exact µF degradation.

What is the difference between measuring capacitance and measuring ESR?

Capacitance measures the component's ability to store an electrical charge (the 'size of the water tank'). ESR (Equivalent Series Resistance) measures the internal resistive losses caused by the leads, plates, and electrolyte (the 'friction in the pipes'). A 1000µF power supply filter capacitor might measure exactly 1000µF on your multimeter, but if the electrolyte has boiled off due to heat, its ESR might spike from a normal 0.05Ω to 5Ω. In a high-frequency circuit, that 5Ω resistance will cause massive voltage ripple and heat generation, destroying the circuit. Always measure both capacitance and ESR when diagnosing switching power supplies or motherboard failures.