How to Measure Voltage With a Multimeter (AC and DC, Step by Step)

To measure voltage, set the dial to V with a wavy line for AC or V with a straight line for DC, plug the black lead into COM and the red lead into the socket marked V, then touch the probes across the two points you want to measure. Voltage is always measured in parallel, so the circuit stays live and connected while you test. Get those three things right and the reading appears in under a second.

Know Which Voltage You Are Chasing

Before you touch anything, decide whether the source is AC or DC. Anything coming from a wall socket, a light fitting, a consumer unit or a generator is alternating current. Anything from a battery, a solar panel, a car charging system or a low-voltage LED driver output is direct current. Picking the wrong one does not usually damage a decent meter, but it will give you a nonsense reading, often something close to zero on a live circuit, which is exactly the kind of false confidence that gets people hurt.

Setting Up the Meter

1. Check the leads. Look for cracked insulation, exposed metal near the plug ends, or bent probe tips. Damaged leads are the most common failure point on any meter.
2. Black to COM. The common socket is the reference point for every measurement. It never changes.
3. Red to the V socket. On most meters this is shared with resistance and continuity and is marked V with an omega symbol beside it. Do not leave the red lead in the 10A current socket, because touching that across a live supply creates a dead short.
4. Turn the dial to the right function. V with the wavy line for AC, V with the solid and dashed line for DC.
5. Set the range if the meter is manual. Choose the next range above what you expect. For mains testing that means the 600V or 750V setting, not 200V. Auto-ranging meters sort this out themselves.

Taking the Reading

Hold both probes in a way that keeps your fingers behind the finger guards. Never let a hand wander onto the metal tip. If you can manage it, keep one hand in a pocket while testing live mains so there is no route for current across your chest.

Touch the probes to the two points you want the difference between. For a battery, that is the positive and negative terminals. For a UK socket, that is live and neutral, which gives you the full supply voltage. Live to earth should read close to the same, and neutral to earth should read close to zero, usually under a couple of volts. If neutral to earth reads high, stop and get an electrician in.

Read the display once the digits settle. A number that drifts by a volt or two is normal. One that jumps wildly usually means poor probe contact rather than a fault in the circuit.

Polarity on DC

If you put the red probe on the negative terminal of a battery, the reading will show a minus sign. That is not an error and it will not hurt the meter. It simply tells you the probes are the other way round. Swap them if you want a positive number.

What the Numbers Actually Tell You

• Car battery at rest: 12.6V is fully charged, 12.4V is about 75 percent, 12.0V is roughly half and needs charging. Below 11.8V and the battery is flat.
• Car battery with the engine running: 13.7V to 14.7V shows the alternator is charging. Anything outside that points at a charging fault.
• AA or AAA alkaline: 1.5V fresh, 1.3V getting tired, under 1.1V and most devices will refuse to run it.
• 9V PP3: under 7.5V and it is done.
• UK socket: 230V to 253V is within the permitted range. A reading well below 220V under load can mean a poor connection somewhere upstream.

One thing worth understanding is that voltage alone does not prove a battery is good. A tired cell can show a decent open-circuit voltage and then collapse the moment anything draws current from it. If a battery reads fine but the device still will not run, test it again while the device is switched on and watch what the voltage does.

Safety Points That Actually Matter

Check the CAT rating printed on your meter and leads. CAT II is fine for sockets and appliances. Working at a consumer unit or near the incoming supply calls for CAT III or CAT IV. The rating is about surviving a voltage spike, not just the number on the dial.

Do not measure voltage with the dial set to resistance, current or continuity. On cheaper meters that can blow the internal fuse at best and destroy the meter at worst. Fit the probes before you go anywhere near live parts, and pull them clear before you change the dial setting.

If you are testing anything inside a consumer unit, behind a socket faceplate or at a light fitting, and you are not confident about what you are looking at, that is the point to call someone qualified. Measuring voltage is easy. Interpreting a strange result in a live installation is not.