What a multimeter does and why you need one

A multimeter is a handheld tool that measures electrical properties of devices and circuits. The most common use is checking voltage — the electrical pressure in a wire or component. When you want to know if power is actually reaching a device, if a battery still has charge, or if a power adapter is working, a multimeter tells you the answer in seconds.

You do not need an expensive one. A basic digital multimeter costs $10 to $25 and handles nearly every household check: whether an outlet has power, if a battery is dead, whether a charging cable is broken, or if a power supply is failing. The reading appears on a small screen, and the tool is safe to use on low-voltage devices like phones, computers, and home electronics.

Key Takeaways

  • A multimeter has a dial or buttons to select what you are measuring, and two probes (red and black wires) that touch the thing being tested.
  • To check voltage, turn the dial to the voltage setting that matches what you expect — DC voltage for batteries and power adapters, AC voltage for wall outlets.
  • Touch the red probe to the positive side and the black probe to the negative or ground, then read the number on the screen.
  • A reading of zero or much lower than expected means the power source is dead or the connection is broken.

The parts of a multimeter and what they do

Every multimeter has the same basic layout. The dial in the center (or buttons on some models) lets you choose what to measure. The two probes — a red wire and a black wire — are what you touch to the thing being tested. The display is a small screen that shows the reading as a number.

The dial has different sections marked with symbols. You will see a V with a straight line (DC voltage), a V with a wavy line (AC voltage), an A (current, which you rarely need), and an Ω (resistance, also less common for basic checks). Most of the time you are turning the dial to one of the voltage settings.

The probes plug into two holes at the bottom of the meter. The red probe always goes into the hole marked with a plus sign or the word "VΩmA". The black probe goes into the hole marked with a minus sign or "COM" (common ground). This is the standard setup on every multimeter — the colors and labels never change.

DC voltage versus AC voltage and which one to use

DC voltage (direct current) is what batteries, phone chargers, laptop power adapters, and USB cables use. It flows in one direction only. AC voltage (alternating current) is what comes out of wall outlets in your home — it switches direction back and forth 60 times per second.

If you are checking a battery, a phone charger, a power adapter, or anything that plugs into USB, you want the DC setting. If you are checking a wall outlet or a device that plugs directly into the wall, you want the AC setting. The dial will have both options clearly marked. If you pick the wrong one, the meter straightforward shows zero or a very low number — it will not hurt the meter or the device.

Most basic multimeters have a dial with multiple DC voltage ranges: 2V, 20V, 200V, and sometimes 600V or higher. For checking batteries and small power adapters, the 20V setting works for almost everything. For checking wall outlets, use the 200V or 600V AC setting. If you are not sure which range, start with the highest one — the meter will show a reading, and you can switch down to a more precise range if needed.

How to check voltage on a battery or power adapter

Start with a battery. Turn the dial to DC voltage and pick the 20V range (or the closest range above the voltage you expect). A AA battery is 1.5V, a 9V battery is 9V, and a phone charger is usually 5V to 20V depending on the model.

Touch the red probe to the positive terminal (the bump on top of a AA battery, or the plus side of a battery holder). Touch the black probe to the negative terminal (the flat end of a AA battery, or the minus side). The screen will show a number. A healthy AA battery reads around 1.5V. A dead one reads 0.9V or lower. A phone charger that is plugged in but not connected to anything typically reads its rated voltage — a 5V charger shows about 5V, a 20V charger shows about 20V.

If the reading is zero or much lower than expected, the battery is dead or the power adapter is broken. If the reading is close to what you expect, the power source is working. That is the whole check — it takes five seconds.

How to check voltage on a wall outlet

Turn the dial to AC voltage and pick the 200V or 600V range. Wall outlets in North America supply 120V, so either range works.

Insert the red probe into the smaller slot of the outlet (the "hot" side). Insert the black probe into the larger slot (the neutral side). Do not touch the metal parts of the probes with your fingers — hold the plastic handles only. The screen will show a number around 110V to 125V. If it shows zero or very low, the outlet has no power. If it shows the expected voltage, the outlet is working.

Some outlets have a third round hole for ground. You do not need to check that for a basic voltage test — the two slots are enough. If you want to be extra careful, you can touch the black probe to the metal outlet box or the screw in the center of the outlet cover instead of the neutral slot, and you will get the same reading.

What the numbers mean and when something is wrong

The number on the screen is the voltage present at that moment. For a battery, compare it to what the battery is supposed to be: a AA battery should read 1.5V when new, a 9V battery should read 9V, a phone charger should read its rated voltage (usually printed on the adapter itself). If the reading is within 10 to 15 percent of that number, the power source is fine. If it is much lower, the battery is weak or the adapter is failing.

For a wall outlet, expect 110V to 125V in North America. Anything in that range is normal. Below 100V might mean the outlet is on a circuit with too much load, or there is a wiring problem — try a different outlet to compare. Zero means no power at all, which usually means the breaker is off or the outlet is dead.

If you are checking a power adapter and it shows zero even when plugged in, the adapter is broken. If it shows voltage but the device it powers is not charging or working, the problem might be in the cable or the device itself — you would need to check the cable next by testing both ends.

Common mistakes and how to avoid them

The most common mistake is picking the wrong voltage type. If you set the dial to AC and check a battery, you get zero. If you set it to DC and check a wall outlet, you also get zero or a very low number. The fix is straightforward: switch the dial and try again. It does not damage anything.

The second mistake is touching the metal tips of the probes with your bare fingers while checking a live outlet. This is not dangerous at 120V household voltage — your skin resistance is too high for current to flow — but it is a bad habit. Always hold the plastic handles and keep your fingers away from the metal probe tips.

The third mistake is forgetting which probe is which. The red probe is always positive, the black probe is always negative or ground. If you reverse them on a DC source like a battery, the screen shows 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 switches direction anyway.

Frequently Asked Questions

Is it safe to use a multimeter on a live outlet?

Yes. Household voltage (120V in North America) is safe to measure with a multimeter. The probes are insulated and the current is too low to cause harm. Just hold the plastic handles and do not touch the metal probe tips. Never use a multimeter on high-voltage sources like the terminals of a car battery or the inside of a power supply — those can be dangerous.

What does it mean if the voltage keeps changing on the screen?

A flickering or changing reading usually means a loose connection. Make sure the probes are making solid contact with the terminals or outlet slots. If you are checking a battery, press the probes firmly against both terminals. If you are checking an outlet, push the probes all the way in. A stable reading means good contact.

Can I use a multimeter to check if a USB cable is broken?

Partially. Plug the charger into a wall outlet and touch the red probe to the center pin of the USB connector and the black probe to the outer shield. You should see the charger's rated voltage (usually 5V). If you see zero, the cable or charger is broken. This tells you if power is reaching the end of the cable, but not whether the data lines inside are working.

Why does my multimeter show a different voltage than what is printed on the adapter?

The voltage printed on an adapter (like "5V 2A") is the rated output when the adapter is under load — actually powering a device. When the adapter is plugged in but not connected to anything, it often reads slightly higher, sometimes 5.5V or 6V instead of exactly 5V. This is normal. As long as it is within 10 to 15 percent of the rated voltage, the adapter is working.

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

No. You can check voltage on a powered-on device or outlet. The multimeter draws almost no current, so it does not affect how the device works. For safety, just avoid touching metal parts of the probes and keep them away from moving parts or exposed circuits inside a device.