You measure microfarads using a digital multimeter with a capacitance mode. Most modern multimeters include this function, marked by a symbol that looks like two parallel lines with a gap between them, sometimes labeled “Cap” or “C.” The process takes under a minute once you know the steps, but skipping the safety precautions can result in a painful shock from a charged capacitor.
What You Need Before You Start
A digital multimeter with a capacitance setting is the standard tool. Not every budget multimeter includes one, so check the dial for the two-parallel-lines symbol before purchasing. You’ll also need a resistor for safely discharging the capacitor before you touch it. A 10,000-ohm (10 kΩ), 5-watt ceramic resistor is a versatile choice that works for most situations you’ll encounter in home electronics and appliance repair.
Capacitors store electrical charge even after the power is turned off. Larger ones, like the run capacitor in an air conditioner, can hold enough energy to cause serious injury. Before handling any capacitor, turn off all power to the circuit, verify with your multimeter that the power is actually off, then discharge the capacitor by holding the resistor across its two terminals for several seconds. You can confirm it’s fully discharged by switching your multimeter to voltage mode and checking for a zero reading.
The discharge time depends on the capacitor’s size. The rule of thumb is five “time constants,” where one time constant equals the resistance multiplied by the capacitance. For a 100 microfarad capacitor discharged through a 10 kΩ resistor, that’s about 5 seconds total. Larger capacitors or higher voltages call for longer waits and higher-rated resistors.
Step-by-Step Measurement
Once the capacitor is safely discharged, remove it from the circuit. Measuring a capacitor while it’s still connected to other components will give you inaccurate readings because the multimeter picks up stray capacitance from surrounding parts.
Turn your multimeter’s dial to the capacitance setting (the parallel-lines symbol). On some meters, capacitance shares a dial position with another function, and you’ll need to press a secondary button to activate it. Check your meter’s manual if the setting isn’t obvious.
Connect the red and black test leads to the capacitor’s terminals. Hold them in place for a few seconds. The multimeter will automatically select the appropriate range and display the capacitance value in microfarads (µF), nanofarads (nF), or picofarads (pF) depending on the size of the component.
Polarized vs. Non-Polarized Capacitors
Non-polarized capacitors, like the small ceramic discs on a circuit board, can be connected to your test leads in either direction. It doesn’t matter which probe touches which terminal.
Polarized capacitors are different. Electrolytic and tantalum types are designed for current flow in one direction, and polarity matters during measurement. The positive terminal is typically the longer lead. The negative terminal is marked with a stripe or band on the capacitor’s body. When measuring, connect the red (positive) probe to the positive terminal and the black probe to the negative terminal. Reversing the polarity can produce a lower reading or even show zero, making a perfectly good capacitor look faulty.
Reading and Interpreting the Display
Your multimeter will show a number followed by a unit prefix. If you’re testing a motor run capacitor rated at 35 µF, you should see a value somewhere near 35. It won’t be exact, and that’s normal. Capacitors are manufactured with a tolerance rating, printed on the body as a percentage.
A capacitor with ±5% tolerance is a precision component. One rated at ±10% is standard for most general electronics. And ±20% tolerance is common in power supply filtering and other applications where exactness doesn’t matter much. So a 100 µF capacitor with ±20% tolerance is perfectly fine anywhere between 80 µF and 120 µF.
If the display reads “OL” (over limit), it means one of two things: either the capacitance is higher than your multimeter can measure on its current range, or the capacitor is faulty. Try a higher range first. If it still reads OL, the capacitor has likely failed open and needs replacing. A reading of zero on a component that should have significant capacitance also points to a dead part.
Converting Between Units
Multimeters may display results in microfarads, nanofarads, or picofarads, and capacitor labels use all three. The conversions are straightforward:
- 1 microfarad (µF) = 1,000 nanofarads (nF) = 1,000,000 picofarads (pF)
- 1 nanofarad (nF) = 0.001 µF = 1,000 pF
- 1 picofarad (pF) = 0.001 nF = 0.000001 µF
So if your multimeter reads 4,700 nF and you need the value in microfarads, divide by 1,000 to get 4.7 µF. If a tiny ceramic capacitor reads 220 pF and you want nanofarads, divide by 1,000 for 0.22 nF. These conversions come up constantly when comparing a meter’s readout to the value printed on a component.
Improving Accuracy on Small Values
When measuring very small capacitors (in the low picofarad range), the test leads themselves introduce a small amount of stray capacitance that can skew results. Most quality multimeters have a “Relative” or “REL” mode that zeroes out this error. To use it, set the meter to capacitance mode with nothing connected to the leads, press the REL button to store the baseline reading, then connect to your capacitor. The meter subtracts the lead capacitance automatically, giving you a cleaner measurement.
When Capacitance Alone Isn’t Enough
A capacitor can measure close to its rated microfarad value and still be failing. That’s because capacitance is only part of the picture. The other critical health indicator is something called equivalent series resistance (ESR), which represents internal resistance that builds up as a capacitor ages. A capacitor with rising ESR can’t deliver current as efficiently, causing circuits to malfunction even though the capacitance reading looks fine.
Aluminum electrolytic capacitors are the most common culprits. Industry standards consider one end-of-life when capacitance drops by 20% or ESR doubles from its original value. A standard multimeter’s capacitance mode won’t measure ESR. You need a dedicated ESR meter or an LCR meter for that. If you’re troubleshooting a power supply, amplifier, or motherboard where capacitors are the prime suspects, checking ESR alongside capacitance gives a much more complete diagnosis. Visually bulging or leaking capacitors should be replaced regardless of what the meter says.