Set the multimeter to voltage mode and touch the probes to the circuit you want to measure

A multimeter measures electrical properties of circuits and devices. To check voltage, you turn the dial to the voltage setting, then touch the two probes (the thin wires attached to the meter) to the points you want to measure. The meter displays the voltage between those two points on its screen.

The key is choosing the right voltage setting before you start. Multimeters have separate settings for AC voltage (the kind that comes from wall outlets) and DC voltage (the kind that comes from batteries). If you measure AC voltage on a DC setting, or vice versa, you will get a wrong reading or no reading at all.

The process takes about 30 seconds once you know which setting to use. You do not need to turn anything off or disconnect anything — voltage measurement is passive, meaning the meter only reads what is already there.

Key Takeaways

  • AC voltage (wall outlets, power cords) and DC voltage (batteries, USB chargers) need different meter settings, and using the wrong one gives you no reading or a wrong number.
  • The black probe always touches ground or the negative side; the red probe touches the point you want to measure, and the meter shows the voltage between them.
  • Start with a higher voltage range than you expect (like 200V for a wall outlet) and work down, because starting too low can damage the meter or show nothing.
  • A reading of zero does not always mean there is no voltage — it can mean the meter is on the wrong setting, the probes are not making contact, or the battery in the meter is dead.

Understand AC and DC voltage settings

The dial on your multimeter has separate markings for AC and DC. AC voltage is marked with a wavy line that looks like a sine wave. DC voltage is marked with a straight line with dots underneath, or sometimes just the letters DC. These are not interchangeable — using the wrong one will give you a blank screen or a meaningless number.

AC voltage comes from wall outlets, extension cords, and anything plugged into them. DC voltage comes from batteries, phone chargers, car batteries, and USB power supplies. If you are not sure which one you need, look at what you are measuring: if it plugs into a wall, use AC. If it runs on batteries or has a power adapter, use DC.

Some multimeters have an auto-ranging feature that picks the right range for you. Most do not. If yours does not, you will see numbers like 2V, 20V, 200V, and 600V next to the voltage markings. These are the maximum voltage the meter can read on that setting. Start with the highest number (usually 600V) and work your way down to a setting that shows a clear reading.

Position the probes correctly

The multimeter has two probes: a black one and a red one. The black probe is the reference point, also called ground or negative. The red probe is what you use to measure. In almost every case, the black probe touches the negative side of the circuit (the ground, the battery's minus terminal, or the return wire), and the red probe touches the point you want to measure.

For a battery, touch the black probe to the minus terminal and the red probe to the plus terminal. For a wall outlet, touch the black probe to the neutral hole (the longer slot on the left) and the red probe to the hot hole (the shorter slot on the right). For a USB cable, the black probe touches the ground wire (usually black) and the red probe touches the power wire (usually red).

The probes have sharp tips so they can pierce through insulation or sit in tight spaces. Push them firmly into contact with the point you are measuring. A loose connection will give you a reading that jumps around or shows nothing at all. If the reading is unstable, press harder or try a different spot on the same wire.

Read the display and understand what the number means

Once the probes are in place, the meter displays a number. That number is the voltage between the black probe and the red probe, measured in volts. A wall outlet in the United States reads around 120V on AC. A standard AA battery reads around 1.5V on DC. A car battery reads around 12V on DC.

The number tells you how much electrical pressure is pushing between those two points. A higher number means more pressure. Zero does not always mean there is no voltage — it can mean the meter is on the wrong setting, the probes are not touching properly, or the battery inside the meter is dead. If you get zero when you expect a reading, try moving the probes, switching to a different voltage range, or checking the meter's battery.

Pay attention to the units shown on the screen. Most meters display volts (V), but some older analog meters show millivolts (mV) or other units depending on the range you selected. A reading of 120 on the 200V range means 120 volts. A reading of 120 on the 2V range means 0.12 volts, which is very different.

Start with the highest voltage range to avoid damaging the meter

If you do not know how much voltage you are measuring, always start with the highest range on the dial. For most multimeters, that is 600V for AC or DC. Take your reading, then switch to a lower range if the number is much smaller than the range allows.

Starting too low can damage the meter. If you set it to 2V and measure a 120V outlet, you can blow out the internal fuse or damage the circuitry. Starting high is safe — the worst that happens is you get a reading like 120 on a 600V scale, which is perfectly readable. Then you can switch to 200V or 20V to get a more precise number if you need it.

Once you have a reading on a high range, you can move down to get more detail. If you measure 120V on the 600V range, switch to 200V to see the reading more clearly. If it shows 120 on the 200V range, you could switch to 20V, but at that point the reading is clear enough for most purposes.

Common mistakes and how to fix them

The most common mistake is using the wrong setting. If you get no reading or a reading that does not make sense, check that you are on AC or DC (whichever matches what you are measuring) and that the range is high enough. A blank screen usually means the range is too low or the meter is on the wrong mode entirely.

The second mistake is loose probe contact. If the reading jumps around or flickers, the probes are not pressing firmly enough. Push them harder into the contact point, or try touching a different spot on the same wire. For thin wires, you may need to wrap the probe around the wire instead of pressing straight on.

The third mistake is measuring AC voltage on a DC setting or vice versa. This will give you zero or a very small number even when voltage is present. If you measure a wall outlet and get zero, switch from DC to AC. If you measure a battery and get zero, switch from AC to DC.

If the meter still shows nothing after you have checked the setting and the probe contact, the battery inside the meter may be dead. Most multimeters have a small battery compartment on the back. Open it and replace the battery (usually a 9V battery or a set of AA batteries, depending on the meter model). After replacing it, try measuring again.

When to use voltage measurement in real situations

Voltage measurement tells you whether power is reaching a device. If a lamp does not turn on, you can measure the voltage at the outlet to confirm the outlet works. If a phone charger is not charging, you can measure the voltage coming out of the charger to see if it is producing power. If a car battery seems dead, you can measure it to see if it still has charge.

Voltage measurement also helps you find broken wires or loose connections. If a wire should carry 12V but measures 0V, the wire is broken or disconnected somewhere. If it measures 12V at one end but 0V at the other, the break is somewhere in between.

You can also use voltage to identify which wire is which in a bundle of unmarked wires. If you know one wire should carry 5V and another should carry 12V, you can measure each one to figure out which is which. This is especially useful when working with USB cables, power supplies, or old equipment where the labels have worn off.

Frequently Asked Questions

Can I measure voltage while the device is turned on?

Yes. Voltage measurement is passive — the meter only reads what is already there and does not change the circuit. You can measure a wall outlet, a running device, or a powered-up circuit without any risk. The device does not need to be off.

What if the meter shows a negative number?

A negative number means you have the probes reversed. The red probe is touching the negative side and the black probe is touching the positive side. The voltage is still there — you just measured it backwards. Swap the probes and you will see the same number as positive.

Why does my multimeter show different readings each time I measure the same thing?

Voltage can fluctuate slightly, especially on AC circuits. Wall outlets vary by a few volts depending on how much power is being used in your home. If the reading jumps wildly, the probes are probably not making solid contact. Press them harder or try a different spot on the wire.

Is it safe to measure voltage on a live circuit?

Yes, measuring voltage is safe. The meter draws almost no current and does not interfere with the circuit. However, be careful not to touch the metal tips of the probes to each other or to your skin while measuring high-voltage circuits like wall outlets, as that can cause a shock.

How do I know if my multimeter battery is dead?

Most multimeters show a battery symbol on the screen when the battery is low. If you get no reading on any setting, or if the readings are very dim or flickering, the battery is probably dead. Open the battery compartment on the back of the meter and replace the battery with a fresh one of the same type.