Set your multimeter to the voltage setting that matches what you're testing

A multimeter measures electrical current, resistance, and voltage. To check for voltage, you need to turn the dial to a voltage setting — usually marked with a V symbol. Most multimeters have two voltage options: DC (direct current, marked with a straight line) and AC (alternating current, marked with a wavy line). Batteries and electronics inside devices run on DC. Wall outlets and power lines run on AC.

If you don't know which type you're measuring, start with AC — that's what comes from your wall. If the reading shows zero or doesn't change, switch to DC. Many modern multimeters have an auto-ranging feature that picks the right scale for you; older ones require you to select a range like 20V, 200V, or 600V. When in doubt, start with the highest range to avoid damaging the meter.

Key Takeaways

  • Turn the dial to either DC or AC voltage — AC for wall outlets and power cords, DC for batteries and device circuits.
  • Touch the black probe to ground or a neutral point and the red probe to the point you're testing; the meter will show the voltage between them.
  • A reading of zero does not always mean no voltage — check that both probes are making solid contact and that you have the right AC or DC setting selected.
  • Never touch both probes to a live circuit at the same time if you're testing high voltage, and keep one hand in your pocket to avoid creating a path across your chest.

Connect the probes correctly to get an accurate reading

Every multimeter comes with two probes: a black one and a red one. The black probe goes into the COM (common) port on the meter, and the red probe goes into the port marked V or VΩmA. These are usually already connected when you buy the meter, but check that they're plugged in firmly.

To measure voltage, touch the black probe to a ground point — usually the negative terminal of a battery, the metal case of a device, or a ground wire. Touch the red probe to the point where you want to know the voltage. The meter will display the difference between those two points. If the number is negative, you have the probes reversed; flip them and the reading will be positive.

Test a known source first to make sure the meter works

Before you test something you're unsure about, verify that your multimeter is working. The easiest way is to test a battery you know is good — a AA, AAA, or 9V battery from a device that's currently running.

Set the meter to DC voltage and choose a range that covers the battery's voltage (AA and AAA are 1.5V, 9V batteries are 9V). Touch the black probe to the negative terminal and the red probe to the positive terminal. You should see a reading close to the battery's rated voltage. If you see zero or a very low number, the battery may be dead, or the probes may not be making good contact. Clean the battery terminals with a dry cloth and try again.

Measure voltage in a wall outlet safely

Wall outlets in North America carry about 120V AC. To measure it, set your multimeter to AC voltage and select a range of at least 200V (or let auto-ranging choose for you). Insert the red probe into the shorter slot of the outlet — that's the hot wire. Insert the black probe into the longer slot — that's the neutral. You should see a reading between 110V and 125V.

Do not touch the metal parts of the probes while they're in the outlet. If you need both hands free, use probe holders or clip leads to hold the probes in place. Never insert both probes into the same slot, and never touch both probes to a live circuit at the same time with your bare hands — this creates a path for current to flow through your body.

Understand what different readings mean

A reading that matches the expected voltage means the circuit is live and working. A reading of zero or close to zero usually means either the circuit is off, the connection is broken, or you have the wrong AC/DC setting selected. Try flipping between AC and DC to see if the reading changes.

A reading that's much lower than expected — like 50V when you expected 120V — can mean a loose connection, a failing power supply, or a damaged wire. A reading that's higher than expected is less common but can happen if the meter is picking up electrical noise. If you're testing a device and the voltage is way off, the power supply or circuit board may be failing.

Use the right probe placement for different components

When testing inside a device, you're usually measuring voltage across a specific component or between two points on a circuit board. The black probe should touch a ground point — often a metal case, a ground wire, or a point marked GND on the board. The red probe touches the point you're testing. If you're measuring across a component like a resistor or capacitor, touch one probe to each end of the component.

For power supplies and adapters, locate the output terminals — usually marked with + and − symbols. Touch the black probe to the negative terminal and the red probe to the positive terminal. The reading should match the voltage printed on the adapter's label. If it's significantly lower, the power supply may be failing.

Troubleshoot when you're not getting a reading

If your meter shows zero when you expect voltage, first check that both probes are making solid contact with the points you're testing. Dirty or corroded terminals can block the connection. Clean the terminals with a dry cloth or a pencil eraser and try again.

Next, verify you have the right AC or DC setting. Many beginners measure AC circuits with the DC setting selected and see nothing. Flip the dial and check again. If you're testing a battery and still see zero, the battery may be dead. Test it in a device to confirm — if the device works, the battery is fine and the meter may need new batteries or repair.

Frequently Asked Questions

Can I measure voltage while a device is plugged in?

Yes. Most voltage measurements are done on live circuits — that's the point of checking voltage. Just avoid touching the metal parts of the probes and keep one hand in your pocket so current cannot flow across your chest if you accidentally touch something live.

What's the difference between the DC and AC settings?

DC (direct current) flows in one direction and is used by batteries and electronics inside devices. AC (alternating current) switches direction 50 or 60 times per second and is what comes from wall outlets. Using the wrong setting will give you a zero or incorrect reading.

Why does my meter show a different voltage than the label says?

Voltage can vary slightly depending on where you measure it and the load on the circuit. A power supply rated for 12V might show 12.5V with no load and 11.8V under heavy load. Readings within 10 percent of the label are usually normal. If the reading is much lower, the power supply may be failing.

Do I need to turn off the device before testing voltage?

No. Voltage testing is a non-invasive measurement — you're only touching the probes to the circuit, not inserting them into it. The device can stay powered on. However, if you're testing inside a device, unplug it first to avoid accidental contact with live parts.

What should I do if the meter shows a negative voltage?

A negative reading means you have the probes reversed. Swap them — touch the black probe to the point you were testing with red, and the red probe to the point you were testing with black. The reading will flip to positive and show the same voltage value.