What capacitance is and why you might need to check it

Capacitance is the ability of a component to store electrical charge. When you check capacitance, you are measuring how much charge a capacitor can hold, expressed in units called farads. A capacitor that has failed or degraded will either hold too little charge or too much, and checking it tells you whether the part is still working or needs replacement.

You check capacitance when a device stops working and you suspect a capacitor is the problem — common signs are a power supply that won't turn on, a motor that won't start, or a screen that flickers. A multimeter with a capacitance setting can tell you in seconds whether the capacitor is dead or still functional.

Most people check capacitance on capacitors that are already removed from the circuit board, because a capacitor still connected to other components can give a false reading. If you are not comfortable desoldering, you can often get a rough sense by visual inspection first — a bulging or leaking capacitor is definitely bad and does not need a meter to confirm.

Key Takeaways

  • A multimeter with a capacitance setting measures how much electrical charge a capacitor can store, shown in farads or microfarads.
  • Remove the capacitor from the circuit board before testing, because other components connected to it will interfere with the reading.
  • Set your multimeter to the capacitance setting (usually marked with a symbol that looks like two parallel lines), then touch the probes to the capacitor's leads.
  • Compare the reading to the value printed on the capacitor's body — if the meter shows zero or a wildly different number, the capacitor has failed.
  • Discharge the capacitor by touching a screwdriver across its leads before you touch it with your bare hands, because stored charge can cause a shock.

Preparing the capacitor and your multimeter

Before you measure anything, discharge the capacitor completely. Even a small capacitor can hold enough charge to give you a painful shock. Take an insulated screwdriver and touch both leads of the capacitor at the same time — you may see a small spark, which means the charge is gone. If the capacitor is still on the circuit board, you can skip this step for now, but discharge it as soon as you remove it.

Next, locate the capacitance setting on your multimeter. It is usually marked with a symbol that looks like two short parallel lines (the capacitor symbol), sometimes with the letter F for farads next to it. On analog multimeters, it may be on a separate dial position. On digital multimeters, it is often grouped with other measurement modes and may require you to press a button to cycle through options.

If your multimeter does not have a capacitance setting, you cannot measure capacitance with it — you will need a different meter. Many basic multimeters sold in hardware stores do not include this function, so check before you buy if capacitance testing is something you plan to do regularly.

Removing the capacitor from the circuit board

If the capacitor is soldered to a circuit board, you will need to remove it before testing. Use a soldering iron to heat the solder joints where the capacitor's leads connect to the board, then gently pull the capacitor away with tweezers or needle-nose pliers. If you have never soldered before, this is the step where most people get stuck — the solder needs to be hot enough to flow, but not so hot that you damage the board or the capacitor itself.

If you are not comfortable soldering, you have two other options. You can take the device to a repair shop and ask them to test the capacitor in place, which some shops can do with specialized equipment. Or you can test the capacitor while it is still on the board, understanding that the reading may not be perfectly accurate because other components are in parallel with it — but if the capacitor is completely dead, the meter will usually show zero or a very low number anyway.

Once the capacitor is free, clean off any excess solder with a damp sponge or brass wire cleaner. Do not use water on the circuit board itself if you plan to put it back together, but the capacitor itself can handle a little moisture.

Taking the capacitance reading

Set your multimeter to the capacitance setting. On most digital multimeters, you will see a dial with different ranges — typically marked 2000p, 20n, 200n, 2µ, 20µ, 200µ, or similar. These are different scales: p means picofarads, n means nanofarads, and µ means microfarads. Start with the highest range (usually 200µ) and work your way down to a lower range if the reading is too small.

Touch the red probe to the positive lead of the capacitor (usually the longer lead, or marked with a plus sign on the capacitor's body) and the black probe to the negative lead (the shorter lead, or marked with a minus sign). Hold the probes steady for a second or two — capacitors charge slowly, and the meter needs a moment to get an accurate reading. You should see a number appear on the display.

If the reading is zero or very close to zero, the capacitor is likely dead. If the reading is wildly different from the value printed on the capacitor (for example, the capacitor is marked 10µ but the meter shows 2µ), the capacitor has degraded and should be replaced. A reading within about 10 to 20 percent of the marked value is usually considered acceptable, though some people prefer a tighter tolerance.

Understanding the capacitor markings

Every capacitor has a value printed on its body, usually in microfarads (µF or µ) or nanofarads (nF or n). This is the capacitance you are trying to match with your meter. For example, a capacitor marked "10µ" should read close to 10 microfarads on your multimeter. A capacitor marked "100n" should read close to 100 nanofarads, which is the same as 0.1 microfarads.

Capacitors also have a voltage rating, printed separately — for example, "25V" or "450V". This tells you the maximum voltage the capacitor can safely handle. The voltage rating does not affect your capacitance reading, but it matters when you replace the capacitor, because you need a replacement with at least the same voltage rating as the original.

Some capacitors also have a tolerance marked, like "±10%" or "±20%". This tells you how far off the actual value is allowed to be from the printed value, even when the capacitor is brand new. A capacitor marked "10µ ±20%" could legally be anywhere from 8µ to 12µ and still be considered good. Use this tolerance as a guide when comparing your meter reading to the marked value.

What different readings mean

A reading of zero or very close to zero means the capacitor is open — it has failed and is no longer storing any charge. This capacitor must be replaced. An open capacitor is the most common failure mode and is usually caused by age, heat, or overvoltage.

A reading that is much higher than the marked value (for example, 15µ when the capacitor is marked 10µ) usually means the capacitor is leaking — it is allowing charge to escape and is no longer reliable. A leaking capacitor should be replaced, even if it has not completely failed yet.

A reading that is lower than the marked value but not zero (for example, 8µ when the capacitor is marked 10µ) could mean the capacitor has aged and lost some of its capacity, or it could be within the tolerance range. Check the tolerance marking on the capacitor. If the reading is within the tolerance, the capacitor is still good. If it is outside the tolerance, the capacitor has degraded and should be replaced.

If the meter shows a very high reading that keeps climbing (sometimes called "ramping up"), the capacitor is likely shorted — it is allowing current to flow freely instead of storing charge. A shorted capacitor must be replaced when ready, because it can damage other components in the circuit.

Reinstalling the capacitor or replacing it

If your test shows the capacitor is still good, you can resolder it back onto the circuit board. Use the same soldering technique you used to remove it — heat the joint until the solder flows, then remove the iron and let the solder cool. A good solder joint looks shiny and smooth, not dull or blobby.

If the test shows the capacitor has failed, you will need a replacement. Take the old capacitor to an electronics shop or order a new one online using the value and voltage rating printed on the old one. Make sure the replacement has the same capacitance value (or very close, within tolerance) and a voltage rating at least as high as the original. The physical size and shape do not have to match exactly, as long as it fits on the board.

Solder the new capacitor in place, making sure the positive and negative leads go to the correct pads on the board. If the capacitor is polarized (has a positive and negative lead), installing it backwards will cause it to fail when ready or even explode. Most electrolytic capacitors are polarized; most film capacitors are not.

Frequently Asked Questions

Can I test a capacitor while it is still soldered to the circuit board?

You can, but the reading will not be accurate because other components connected in parallel with the capacitor will affect the measurement. If the capacitor reads zero or very low, it is definitely bad. If it reads close to the marked value, the capacitor is probably fine, but a test after removal is more reliable.

What does it mean if the multimeter shows "OL" or "1" on the capacitance setting?

OL means "overload" — the capacitance is too high for the range you selected. Switch to a higher range (for example, from 20µ to 200µ). A reading of 1 usually means the meter is not making good contact with the capacitor leads, so try cleaning the leads with a pencil eraser and testing again.

Why does the reading keep changing when I hold the probes on the capacitor?

Capacitors charge slowly, especially larger ones. Hold the probes steady for two to three seconds and wait for the reading to stabilize before you write it down. If the reading keeps climbing indefinitely, the capacitor is shorted and needs replacement.

Is it dangerous to test a capacitor?

A charged capacitor can deliver a painful shock, so always discharge it first by touching a screwdriver across both leads. Once discharged, testing is safe. Wear safety glasses when you solder, because solder can splatter, and work in a well-ventilated area because solder fumes are not healthy to breathe.

Can a capacitor fail partially, or does it either work or not work?

Capacitors can fail gradually. A capacitor can lose capacity over time (showing a lower reading than marked), develop a leak (showing a higher reading), or fail completely (showing zero). A capacitor that has lost 30 to 40 percent of its capacity may still work in some circuits but fail in others, so it is usually safer to replace it if the reading is significantly off.