HOW-TO GUIDE
How to Test a Capacitor With a Multimeter (and Why the Reading Looks Wrong)
By the OhmSavvy Research Team ·
Stored charge can hurt you after the power is off
- Unplug the equipment, then check the capacitor terminals in DC volts before touching them.
- Bleed the charge through a resistor, never a screwdriver. A dead short can weld metal, throw sparks and damage the part.
- Microwave ovens, old CRT sets, flash units and large power supplies can hold lethal energy. If you are not sure how to discharge a specific part, leave it to a qualified technician.
Discharge first: how much energy is stored
The energy in a charged capacitor is E = ½CV², so voltage matters far more than size. A 100 µF capacitor on a 25 V rail holds about 0.03 joules, roughly nothing. A 470 µF bulk capacitor sitting at 400 V after the rectifier in a mains power supply holds ½ × 0.00047 × 400² ≈ 37.6 J, enough for a painful and dangerous shock and enough to blow the input protection on many meters if you put them in a resistance or capacitance range across it. The chart shows how quickly the numbers grow. These are worked examples from the formula, not measurements.
Bars use a logarithmic scale: each step is ten times the last.
A resistor discharges the part along an exponential curve with time constant τ = R × C. After five time constants, under 1% of the starting voltage is left. For the 470 µF supply capacitor and a 10 kΩ resistor, τ = 10,000 × 0.00047 = 4.7 s, so allow about 24 s. The resistor briefly sees V²/R = 400² ÷ 10,000 = 16 W at the moment of contact, which is why a 1/4 W part can crack; use a power resistor with insulated clip leads, or a purpose-made discharge tool. Then confirm the terminals read close to 0 V DC before going further.
Why a capacitor reading looks wrong
Before you order a replacement, rule out the measurement. These six causes explain most cases where a healthy capacitor appears to "test bad". The left column is what you see; work across the row.
| Symptom | Likely cause | How to check | Fix |
|---|---|---|---|
| Reads 15–20% low or high | Normal factory tolerance | Look for a tolerance letter or ±% on the body | Accept anything inside the band |
| Tiny part reads 0 or a few pF too many | Lead and stray capacitance, or below the meter's resolution | Read the meter with leads open; check its lowest range | Use REL/zero, short leads, or an LCR meter |
| OL or blank on a big electrolytic | Above the meter's maximum range | Compare the marked µF with the manual's top range | Use a meter with a higher range |
| Reads far higher in-circuit | Other capacitors in parallel | Lift one leg and measure again | Always confirm out of circuit |
| Number jumps or the meter warns | Leftover charge on the part | DC volts across the terminals | Discharge through a resistor, then retest |
| Value fine, circuit still faulty | Aged electrolytic with high ESR | Bulged vent, ESR meter, or swap test | Replace with same µF, voltage and temperature rating |
A capacitance reading is a range check, not a pass/fail score
1Power down and discharge
Unplug the equipment and wait a minute; many supplies have bleeder resistors that do some of the work. Measure DC volts across the capacitor. If it reads more than a volt or two, bleed it through a resistor as described above and measure again. Leftover charge upsets a capacitance reading, so an erratic number at this stage is a clue to discharge again rather than a fault in the part.
2Lift one leg, or remove the part
Desolder one lead of a through-hole capacitor and bend it clear of its pad, or remove a surface-mount part fully. One free leg is enough to break every parallel path. If that is not practical, treat the in-circuit result only as a hint and read the section below on what parallel parts do to it.
3Read the markings
Note the capacitance, voltage and tolerance. Electrolytics print the value directly, such as 470 µF 25 V, often with M for ±20%. Film and ceramic parts use a three-digit code plus a letter: J is ±5%, K is ±10% and M is ±20%. Write the acceptable window down before you measure so the meter cannot anchor your judgment.
4Zero the leads on small values
For anything under about 1 nF, hold the leads where you will use them, with the tips apart, and note what the meter shows. That is the capacitance of the leads and the meter input. If your meter has a relative (REL or Δ) button, press it now so the offset is subtracted automatically.
5Measure in capacitance mode
Select the capacitance range, connect red to the positive lead of a polarized part, and wait for the reading to settle. Large values can take several seconds to appear.
6No capacitance mode? Use ohms as a short/open check
Set a high resistance range and connect the probes. A good electrolytic reads low at first and climbs toward OL as the meter's small test current charges it. A reading that sticks near 0 Ω means a short; an instant OL on a large electrolytic suggests an open part. Small ceramics charge too fast for this to show anything, so it only works on larger values.
In-circuit readings and parallel parts
Capacitors in parallel add. When the meter's probes sit across a capacitor that is still soldered in, it also sees every other capacitor connected between the same two nodes, which on a power rail can be several. Resistors across the same nodes leak away the meter's charging current, which makes the reading unstable or simply wrong.
The arithmetic is simple: 10 µF + 1 µF + 0.1 µF = 11.1 µF, an 11% error before the resistor makes things worse. On a motherboard rail with dozens of decoupling capacitors, the number can be many times the marked value of any single part. An in-circuit reading that is lower than the marked value is more telling, because parallel parts can only add capacitance, so a low reading points at the target or at leakage. Even then, confirm it with the part lifted.
Is the reading inside tolerance?
Capacitors are made to much looser tolerances than resistors. Typical figures, which vary by series and maker, are ±20% for aluminum electrolytics, ±10% or ±20% for tantalum, ±5% or ±10% for film and anywhere from ±5% to a very wide −20/+80% for some high-capacitance ceramics. Ceramic X7R and Y5V parts also lose capacitance with DC bias and temperature, so a datasheet is the final word. The table turns typical markings into the reading window you should accept.
| Marked value | Tolerance | Accept a reading between | Typical type |
|---|---|---|---|
| 100 µF | ±20% (M) | 80 – 120 µF | Aluminum electrolytic |
| 470 µF | ±20% (M) | 376 – 564 µF | Aluminum electrolytic |
| 10 µF | ±10% (K) | 9 – 11 µF | Tantalum |
| 1 µF | ±5% (J) | 0.95 – 1.05 µF | Film |
| 100 nF (104) | ±10% (K) | 90 – 110 nF | X7R ceramic |
Remember that the meter has its own error on top. Fluke specifies the 115's 100 µF to 1,000 µF readings at 1.9% + 2 counts, and its accuracy above 1,000 µF is much looser. Stack that on the part tolerance and a reading a few percent outside the printed window is not proof of failure. Small pF parts also run into resolution: the 115's lowest capacitance range is 1,000 nF with 1 nF steps, so a 47 pF part is simply too small for it, and the display will read near zero even though the capacitor is fine. Use an LCR meter or a meter with a pF range for those.
Aging electrolytics and ESR
Aluminum electrolytics wear out. Heat slowly dries the electrolyte, and the first thing that suffers is equivalent series resistance (ESR), the small resistance in series with the ideal capacitor. A filter capacitor whose ESR has risen will still show its marked microfarads on a multimeter, yet it can no longer smooth ripple, so a supply hums, a monitor clicks on and off, or a router reboots. Your multimeter cannot measure ESR. Look for a domed or leaking vent on top of the can, measure ESR with a dedicated ESR meter, or swap the part for one with the same capacitance, an equal or higher voltage rating and the same temperature rating (105 °C if the original was). If the capacitance reads well below tolerance as well, the part is far gone and the decision is easy.
Which meter to use
Any meter with a capacitance range will check value against tolerance. The figure that separates them is the top of the range. The Fluke 179 reads to 10,000 µF, enough for large audio and supply filter cans; the Fluke 115 goes to 9,999 µF; the Fluke 17B+ and 15B+ stop at 1,000 µF, which covers most board-level work but not a big power supply reservoir. For capacitors you meet on HVAC equipment, see the fan motor capacitor guide, and for small ceramics, the ceramic capacitor guide. If you are new to meters, start with how to use a multimeter.
Meters with a capacitance range
Capacitance from 1 nF to 9,999 µF, True RMS, CAT III 600V. Hands-on tested.
Capacitance to 10,000 µF for big filter capacitors, plus CAT IV 600V.
Compact meter with capacitance, temperature and built-in NCV.
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Frequently asked questions
Can you test a capacitor with a voltmeter?
Only partly. A plain voltmeter has no capacitance range, so it can't give you a microfarad value. What it can do: charge the capacitor briefly through a resistor and watch the voltage climb toward the source voltage, then discharge it and watch the voltage drop — a cap that won't hold a charge at all, or that reads zero resistance to charge, is shorted or open. For an actual value against tolerance, you need a meter with a capacitance (F) range.
Can a capacitor read the correct microfarads and still be bad?
Yes. A capacitance range measures stored charge, not internal resistance. An electrolytic that has dried out can still show close to its marked value while its equivalent series resistance has climbed enough to cause ripple, hum or a power supply that will not start. If the symptom fits and the value looks fine, check ESR with a dedicated ESR meter or simply replace the part.
How long should a multimeter take to measure a large capacitor?
Small values settle almost instantly, but large electrolytics take longer because the meter charges them with a modest test current and times the result. Several seconds for a few thousand microfarads is normal. Wait for the number to stop changing before you write it down, and do not move the probes while it is still counting.
Do I need to observe polarity when testing a capacitor?
For electrolytic and tantalum capacitors, put the red lead on the positive terminal and black on the negative, marked by the stripe on the can. The meter's test voltage is low, so a reversed connection is unlikely to damage the part, but it can give a less stable reading. Ceramic and film capacitors are not polarized, so either way round works.
What does OL mean in capacitance mode?
OL in capacitance mode usually means the value is above the range the meter can measure, or the part is shorted so the meter cannot charge it. Check the maker's maximum capacitance in the manual. If the capacitor is well within that limit and still shows OL, measure it in ohms; a near-zero resistance confirms an internal short.
Can I test a capacitor with an analog multimeter?
An analog meter on a high ohms range gives a clear charge kick on electrolytics: the needle swings toward zero ohms, then drifts back toward infinity as the part charges. A larger capacitor gives a slower return. That shows the part is not open or shorted, but it gives no number, so a digital capacitance range is still needed to check value against tolerance.
