What a multimeter does and why you need to know this

A multimeter is a handheld device that measures electrical properties in circuits, devices, and power sources. The most common measurement you'll make is voltage — the electrical pressure pushing current through a wire or component. Checking voltage tells you whether power is reaching a device, whether a battery still has charge, or whether a wire is live before you touch it.

You don't need to understand electricity deeply to use a multimeter safely. You need to know which dial setting to use, where to touch the probes, and what the number on the screen means. This guide walks you through each step so you can measure voltage on batteries, wall outlets, and device connections without guessing or risking a shock.

Key Takeaways

  • A multimeter has a dial with different settings — you must turn it to the voltage setting that matches what you're testing (DC or AC) before you take a measurement.
  • The two probes (red and black wires) go into the two holes at the bottom of the multimeter, and you touch the red probe to the positive side and the black probe to the negative or ground.
  • DC voltage (direct current) is what batteries use; AC voltage (alternating current) is what wall outlets use — testing the wrong type won't hurt the meter but you'll get a reading of zero or a very small number.
  • The number on the screen is the voltage; a 9-volt battery should read close to 9, a wall outlet should read around 110 to 120 volts in the US, and a phone charger cable should read close to what the label says.
  • Never touch both probes to the same wire at the same time on a live outlet or high-voltage source — always touch one probe to the positive side and one to the negative or ground.

Setting up your multimeter and inserting the probes

Before you measure anything, you need to plug the probes into the meter. Most multimeters have three holes at the bottom: one labeled COM (common), one labeled V with a squiggly line (AC voltage), and one labeled V with a straight line (DC voltage). The black probe always goes into the COM hole. The red probe goes into the V hole — it doesn't matter which one yet, because you'll choose the right dial setting next.

Once both probes are seated firmly in their holes, turn the dial on the front of the meter. You'll see different settings printed around the dial. Look for the voltage section, which usually shows V with a squiggly line (AC) and V with a straight line (DC). Start by pointing the dial at DC voltage if you're testing a battery or device that runs on batteries. Point it at AC voltage if you're testing a wall outlet or anything plugged into one.

If you're not sure which type of voltage you're measuring, start with DC. The worst that happens is the screen shows zero or a very small number, and you know to switch to AC instead. The meter itself won't be damaged.

Measuring voltage on a battery

A battery is the safest place to practice because the voltage is low and the risk of shock is nearly zero. Set your multimeter dial to DC voltage. A 9-volt battery should be tested on the 20V setting (or whatever the highest DC setting is if your meter doesn't have 20V). A AA or AAA battery should use the 2V setting. If your meter has an auto-ranging feature (usually a button labeled AUTO), you can press that instead and the meter will pick the right range for you.

Hold the battery in one hand. Touch the red probe to the positive terminal (the bump on top of most batteries). Touch the black probe to the negative terminal (the flat side). Look at the screen. A fresh 9-volt battery reads between 9 and 9.6 volts. A fresh AA battery reads about 1.5 volts. If the number is much lower — say a 9-volt battery reading 7 volts or less — the battery is weak and may not power a device reliably.

If the screen shows a negative number, you've reversed the probes. That's fine — just swap them and read the positive number instead. The meter won't be harmed.

Measuring voltage on a wall outlet

Wall outlets are higher voltage than batteries, so treat this step with respect. Set your multimeter dial to AC voltage and choose the 250V setting (or the highest AC setting your meter has). Do not use the DC setting — wall outlets are AC, and you need the right setting to get an accurate reading.

Look at a standard US wall outlet. The left slot is slightly larger than the right slot. The right slot is the hot wire (where the voltage comes from). The left slot is the neutral wire (the return path). Insert the red probe into the right slot and the black probe into the left slot. You should see a number between 110 and 120 on the screen. If you see zero or a very small number, the outlet is not powered — check whether a switch controls it or whether a breaker has tripped.

Never touch both probes to the same slot, and never let your fingers touch the metal parts of the probes while they're in the outlet. Once you have your reading, remove the probes and move away from the outlet.

Measuring voltage on device cables and connectors

Many devices use barrel connectors or USB cables to deliver power. You can check whether power is reaching the connector without opening the device. First, look at the label on the power adapter or charger — it will say something like "12V" or "5V" and whether it's DC or AC (almost all device chargers are DC).

Set your multimeter to DC voltage and choose a setting higher than the label says. If the label says 12V, use the 20V setting. If it says 5V, use the 20V setting. Unplug the cable from the wall outlet. Touch the red probe to the center pin or the positive wire of the connector, and touch the black probe to the outer ring or the negative wire. Plug the cable back into the wall outlet and look at the screen immediately — it should read close to the voltage on the label.

If it reads zero or much lower than expected, the power adapter may be failing. If it reads significantly higher than the label (for example, 18V when the label says 12V), do not plug it into your device — the adapter is faulty and could damage the device.

Understanding what the reading means

The number on the screen is the voltage in volts. For a battery, it tells you how much charge is left. For a wall outlet or power adapter, it tells you whether power is flowing and whether it's at the right level. A reading of zero means no voltage is present — either the source is dead, the connection is broken, or you've set the meter to the wrong type (DC instead of AC, or vice versa).

A reading that's close to but slightly lower than expected is usually normal. A 9-volt battery reading 8.5 volts is still good. A wall outlet reading 115 volts instead of 120 is fine. A reading that's significantly lower — more than 10 percent below what you expect — suggests the source is weak or the connection is loose. A reading that's higher than expected suggests a problem with the power source itself.

If you see a negative number, you've touched the probes to the terminals backwards. The actual voltage is the positive version of that number. Swap the probes and take the reading again to confirm.

Common mistakes and how to avoid them

The most common mistake is setting the dial to the wrong type of voltage. If you're testing a wall outlet and the screen shows zero, switch from AC to DC (or vice versa) and try again. The meter won't be damaged by testing the wrong type — you just won't get a useful reading.

The second mistake is choosing a dial setting that's too low for what you're measuring. If you set the meter to 2V and try to measure a 9-volt battery, the screen will show "1" or "OL" (overload) and you won't get a real reading. Always start with the highest setting available, or use the auto-ranging feature if your meter has one. You can always switch to a lower setting once you know the approximate voltage.

The third mistake is touching both probes to the same point on a live outlet or high-voltage source. This creates a short circuit and can damage the meter or cause a spark. Always touch one probe to positive and one to negative or ground. On a wall outlet, this means one probe in the right slot and one in the left slot, never both in the same slot.

Frequently Asked Questions

What's the difference between DC and AC voltage?

DC (direct current) flows in one direction and is what batteries use. AC (alternating current) switches direction back and forth and is what wall outlets use. Most devices that plug into the wall use an adapter to convert AC to DC for the device itself. Always check the label on the power adapter to see which type you need to measure.

Can I hurt myself checking voltage with a multimeter?

Batteries and low-voltage device chargers are safe. Wall outlets carry enough voltage to cause a painful shock if you touch both wires at once, but the multimeter probes are designed to let you measure safely if you follow the steps — one probe in each slot, never touching the metal parts of the probes. If you're nervous, start with a battery to get comfortable with the meter first.

Why does my reading keep changing?

Voltage can fluctuate slightly, especially on wall outlets and devices under load. A reading that bounces between 118 and 122 volts on a wall outlet is normal. If it's bouncing wildly or dropping suddenly, the outlet or power source may have a loose connection. Try the measurement again in a different outlet to see if the problem follows the outlet or the meter.

What does "OL" on the screen mean?

OL stands for overload and means the voltage is higher than the dial setting you chose. Turn the dial to a higher setting and try again. For example, if you see OL on the 20V setting, switch to 200V or 250V and measure again.

Do I need to turn off a device before checking its voltage?

For batteries and unplugged chargers, it doesn't matter. For wall outlets and devices plugged in, the outlet should stay powered so you can measure the voltage flowing through it. Just be careful not to touch both probes to the same point and keep your fingers away from the metal probe tips.