What amperes measure and why you might need to check them

Amperes (often shortened to "amps") measure how much electrical current is flowing through a wire or device. Think of it like water pressure in a pipe — amperes tell you the volume of electricity moving, not the force behind it. A multimeter can measure amperes, but the way you set it up and connect it matters more than with other measurements, because connecting it wrong can damage the meter or create a safety hazard.

You might need to check amperes to troubleshoot a device that is not working, to see if a battery is still delivering power, or to verify that a circuit is drawing the amount of current it should. Unlike voltage (which you can measure without breaking a circuit), measuring amperes usually requires you to interrupt the circuit and put the multimeter in line with the current flow.

Key Takeaways

  • Amperes measure the amount of electrical current flowing through a device, and a multimeter can read this value if you set it to the correct mode and range.
  • To measure amperes, you must break the circuit and insert the multimeter in series (in line with the current), not in parallel like you do for voltage.
  • Start with the highest amperage range on your multimeter, then switch to a lower range if the reading is too small, to avoid damaging the meter.
  • Always turn off the power before you connect the multimeter, and never measure amperes across a battery or power source directly without a load in the circuit.

Setting your multimeter to measure amperes

Most multimeters have a dial or button that lets you choose what to measure. Look for a setting labeled with an "A" or "mA" (milliamps, which is one-thousandth of an amp). Digital multimeters usually have separate jacks for measuring amperes — often labeled "A" for higher currents and "mA" or "COM" for lower currents. Check your multimeter's manual to see which jack to use, because plugging the probe into the wrong jack can blow an internal fuse.

If your multimeter has multiple amperage ranges (like 2A, 20A, 200A), start with the highest one. If the reading is too small to see clearly, switch to the next lower range. This protects the meter from being overloaded on the first try.

Breaking the circuit and inserting the multimeter

To measure amperes, you must interrupt the flow of electricity and place the multimeter in the path so current flows through it. This is called measuring "in series." Turn off the power to the circuit first, then disconnect one wire from the device or battery. The multimeter's red probe goes to the positive side (the side that was connected to power), and the black probe goes to the negative side (the side that was connected to ground or the return path).

Once the probes are connected, turn the power back on. The multimeter will show the current flowing through the circuit. If the reading is very small or shows zero, the circuit may be broken, the device may not be drawing power, or you may have connected the probes backward (though most modern multimeters will just show a negative number instead of breaking).

Why you cannot measure amperes the way you measure voltage

Voltage can be measured by touching the probes to two points in a circuit without breaking it — the multimeter sits "in parallel" and does not interrupt the flow. Amperes cannot be measured this way because the multimeter's internal resistance is too low. If you connect an amperage-measuring multimeter in parallel across a power source, you create a short circuit, which can damage the meter, blow a fuse, or cause a spark.

This is the most common mistake people make when learning to use a multimeter. If you are unsure whether you should measure voltage or amperes, measure voltage first — it is safer and gives you useful information about whether power is reaching the device.

Reading the display and understanding what the number means

Once the multimeter is connected in series and power is on, the display will show a number. If it shows something like "0.5" and your dial is set to "2A," that means 0.5 amps are flowing. If the display shows "500" and your dial is set to "mA" (milliamps), that is 500 milliamps, which equals 0.5 amps — the same current, just displayed in smaller units.

If the display shows "1" with a small line or dash before it, the current is flowing backward (the probes are reversed). If it shows "0.00" or nothing, either no current is flowing, the circuit is broken, or the device is not powered on. If the display shows "OL" or flashes, the current is too high for the range you selected — switch to a higher range when ready.

Common mistakes and how to avoid them

The most dangerous mistake is connecting the multimeter in parallel (across the power source) instead of in series (in line with it). Always break the circuit first and insert the multimeter into the path. Another common error is forgetting to switch the dial to amperage mode — if you leave it on voltage and try to measure amperes, you will get a wrong reading or damage the meter.

A third mistake is selecting a range that is too low for the current you are measuring. If you set the dial to "2A" but the circuit draws 5 amps, the meter's internal fuse will blow. Always start high and work your way down. Finally, do not measure amperes across a battery or power supply with nothing else in the circuit — this creates a short and can damage both the meter and the power source. There must be a load (a device using the power) between the positive and negative terminals.

When to call someone else instead of measuring yourself

If the circuit involves high voltage (more than 48 volts), or if you are working on household wiring or a car's main electrical system, measuring amperes can be dangerous. High-current circuits can cause serious injury if something goes wrong. If you are not confident about where to break the circuit safely, or if the device is plugged into wall power, consider having an electrician or technician do the measurement.

For low-voltage circuits (like battery-powered devices, USB chargers, or small electronics), measuring amperes with a multimeter is usually safe as long as you follow the steps above. If you are testing something for the first time, do a voltage measurement first to confirm power is present, then move to amperage if you need more information.

Frequently Asked Questions

What is the difference between amps and milliamps?

One amp equals 1,000 milliamps. Milliamps are used for small currents, like those in USB devices or battery chargers. If your multimeter shows "500 mA," that is 0.5 amps. Most small electronics draw current in the milliamp range, so you will use the "mA" setting more often than the "A" setting.

Can I measure amperes while the device is plugged into the wall?

You can, but it is riskier than measuring a battery-powered device. You must turn off the power before you disconnect the wire and insert the multimeter. Never work on live household wiring unless you are trained to do so. If you are unsure, unplug the device first or have an electrician measure it for you.

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

A negative reading means the probes are reversed — the red probe is touching the negative side and the black probe is touching the positive side. The current value itself is correct; just flip the probes if you want the display to show a positive number. Most meters work fine either way.

Why does my multimeter show zero amps when I know the device is on?

The most likely reason is that the circuit is broken somewhere, or the device is not actually drawing power. Check that the device is turned on and that you have broken the circuit at the right point. Also verify that you are using the correct jack on the multimeter for amperage measurement — if you plugged the probe into the voltage jack by mistake, it will not read amperes.

Can a blown fuse in the multimeter be replaced?

Yes, most multimeters have a replaceable fuse inside, usually located near the jacks. Check your manual for the correct fuse type and amperage rating. You will need a small screwdriver to open the fuse holder. If you blow the fuse repeatedly, you are likely measuring a current that is too high for your meter — use a higher range or a different meter designed for higher currents.