What a voltage meter does and why you need one

A multimeter is a handheld tool that measures voltage — the electrical pressure pushing current through a wire or device. When you set it to voltage mode and touch the probes to a circuit, it tells you how many volts are present. This matters because it tells you whether power is actually reaching a device, whether a battery still has charge, or whether a wire is live before you touch it.

Most multimeters measure three things: voltage, current, and resistance. You will use the voltage setting most often. A basic digital multimeter costs between $15 and $40 and works for nearly every household check you will need to do — testing batteries, outlets, phone chargers, and wiring.

Key Takeaways

  • Set the dial to the voltage range that matches what you are testing — AC for wall outlets and appliances, DC for batteries and USB chargers.
  • Touch the black probe to ground or the negative side and the red probe to the positive side or the wire you are testing.
  • The meter displays the voltage in numbers; zero means no power is present, and the reading should match what the device is rated for.
  • Always start with a higher voltage range than you expect, then switch down to get a more precise reading.
  • A reading of 120 volts at a US wall outlet is normal; anything much higher or lower means a problem with the outlet or the meter itself.

Understanding AC and DC voltage settings

Multimeters have two voltage settings: AC (alternating current) and DC (direct current). AC voltage is what comes from wall outlets in your home — the current switches direction 60 times per second. DC voltage is what batteries and USB chargers produce — the current flows in one direction only.

The dial on your meter will show symbols for both. AC looks like a wavy line (~). DC looks like a straight line with dots (⎓ or —). If you measure AC voltage with the meter set to DC, you will get a reading of zero or a very small number. If you measure DC voltage with the meter set to AC, the reading will be wrong or zero. Choose the right one before you touch the probes to anything.

Most household testing is AC — outlets, light switches, appliance cords. Battery testing and USB charger testing is DC. If you are not sure which one you need, start with AC, and if you get zero, switch to DC.

How to position the probes correctly

A multimeter has two probes: a black one and a red one. The black probe is ground or negative. The red probe is positive. For the meter to read voltage, you must touch the black probe to the negative side and the red probe to the positive side, or to the wire you are testing.

For a wall outlet, the black probe goes into the larger slot (neutral) and the red probe goes into the smaller slot (hot). For a battery, the black probe touches the flat end (negative terminal) and the red probe touches the bump (positive terminal). For a USB charger, the black probe touches the outer metal ring and the red probe touches the center pin.

If you reverse the probes on DC voltage, the meter will show a negative number instead of a positive one — the reading is still correct, just with a minus sign. On AC voltage, the order does not matter because the current is already switching direction.

Setting the voltage range on your meter

The dial on a multimeter has multiple voltage ranges: 200mV, 2V, 20V, 200V, and sometimes 1000V. Each range measures a different size of voltage. If you set the range too low, the meter will show "1" or "OL" (overload) and give you no useful reading. If you set the range too high, the reading will be less precise.

Start by guessing high. For a wall outlet, set the dial to 200V or higher. For a battery, set it to 20V. For a USB charger, set it to 20V. Once you touch the probes and get a reading, you can switch down to a lower range to see the voltage more precisely. For example, if you set the meter to 200V and it reads 120 volts at an outlet, you can switch to 20V and it will show 120.0 or 120.1 — more detail.

Never switch the range while the probes are touching a live circuit. Always move the probes away first, change the dial, then touch again.

Testing a wall outlet for voltage

To check whether a wall outlet has power, set the meter to AC voltage at 200V or higher. Touch the black probe into the larger slot and the red probe into the smaller slot. A normal US outlet will read between 110 and 125 volts. If you get zero, the outlet has no power — check the breaker or test another outlet nearby.

If the reading is much higher than 125 volts or much lower than 100 volts, the outlet itself may be damaged or the home's electrical service has a problem. If the reading bounces or flickers instead of staying steady, the outlet may have a loose connection.

Never test an outlet that is wet or in a bathroom without a GFCI breaker. If you must test a wet outlet, use a meter with a battery-powered display, not one that plugs in.

Testing batteries and USB chargers

For a battery, set the meter to DC voltage at 20V. Touch the black probe to the flat end (negative terminal) and the red probe to the bump (positive terminal). A new AA or AAA battery reads about 1.5 volts. A new 9-volt battery reads about 9 volts. If a battery reads much lower — say, 0.8 volts on an AA — it is nearly dead and should be replaced.

For a USB charger, set the meter to DC voltage at 20V. Plug the charger into an outlet. Touch the black probe to the outer metal ring of the USB connector and the red probe to the center pin. Most USB chargers output 5 volts. If you get zero, the charger is not working. If you get a very different number, the charger may be damaged.

USB-C chargers are trickier because the pins are small. If you cannot reach them safely with the probes, test the charger by plugging it into a phone and watching whether the phone charges. A meter reading is useful but not always necessary.

What to do if the reading seems wrong

If a wall outlet reads zero when you know it should have power, check three things. First, make sure the meter is set to AC voltage, not DC. Second, make sure the probes are in the right slots — the black probe in the larger slot, the red in the smaller. Third, try a different outlet in the same room to see if the problem is the outlet or the meter.

If a battery reads zero when it should have voltage, the battery is dead. If a USB charger reads zero when plugged in, the charger is broken or the outlet it is plugged into has no power. Test the outlet first with the AC setting to rule that out.

If the meter itself seems broken — it will not turn on, the display is blank, or it reads the same number for everything — check the battery inside the meter. Most multimeters use a 9-volt battery or two AA batteries. Replace it and try again.

Frequently Asked Questions

Can I hurt myself testing voltage with a meter?

A multimeter is designed to be safe for measuring household voltage. The probes are insulated, and you are not completing a circuit — you are just reading the voltage. Wall outlets at 120 volts are not dangerous to touch with a meter. High-voltage circuits like the main breaker panel or power lines are dangerous and should not be tested without training.

What does it mean if the meter shows a negative number?

On DC voltage, a negative number means you reversed the probes — the red probe is on the negative side and the black probe is on the positive side. The voltage is still correct; just flip the probes if you want a positive reading. On AC voltage, the meter does not show negative numbers because the current switches direction.

Why does my outlet read 115 volts instead of 120?

Household voltage varies between 110 and 125 volts depending on the time of day, how much power the neighborhood is using, and the distance from the power plant. A reading of 115 volts is normal and nothing to worry about. If the reading is below 100 or above 130, the outlet or the home's electrical service may have a problem.

Do I need to turn off power before testing with a meter?

No. A multimeter measures voltage without interrupting the circuit. You can test a live outlet, a working battery, or a plugged-in charger safely. The meter draws almost no current, so it will not damage the device or drain the battery.

What is the difference between a multimeter and a voltage tester?

A voltage tester is a simpler tool that only tells you whether voltage is present — usually with a light or a beep. A multimeter shows you the exact voltage number. For household use, a multimeter is more useful because it tells you whether a battery is weak, whether an outlet is delivering the right voltage, and whether a charger is working correctly.