What you're measuring when you check amps

When you measure amps with a multimeter, you're measuring the amount of electrical current flowing through a circuit or device. Current is the movement of electricity itself — think of it like water flowing through a pipe. A multimeter set to measure amps (also called amperage or current) tells you how much electricity is moving at that moment.

The reason you'd check amps is usually to troubleshoot a device that isn't working right, confirm a power supply is delivering what it should, or verify that a circuit breaker is sized correctly for what you're plugging in. If a device draws way more amps than it's supposed to, something is wrong — either the device is failing or you're using it on the wrong power source.

Measuring amps is different from measuring voltage (which tells you the electrical pressure) or resistance (which tells you how much something opposes current flow). Your multimeter can do all three, but the steps and safety rules are different for each one.

Key Takeaways

  • To measure amps, you must break the circuit and insert the multimeter in series — the current flows through the multimeter itself, not around it.
  • Set your multimeter to the DC or AC amps setting before you connect it, and choose a range higher than what you expect the device to draw.
  • Connect the red probe to the positive side of the break and the black probe to the negative side, then turn the device on.
  • If the reading is very low or shows a dash, you may have chosen the wrong range or the circuit is broken.
  • Never measure amps across a component the way you measure voltage — this will damage the multimeter and create a safety hazard.

Why you must break the circuit to measure amps

The most important rule for measuring amps is that current must flow through the multimeter, not around it. This is called measuring "in series." If you touch the probes to two points on a live circuit without breaking it, you're measuring voltage, not amps — and if you try to force amps through a multimeter that's set to measure voltage, you'll blow the fuse inside the multimeter or damage it permanently.

Breaking the circuit means physically disconnecting one wire or terminal so that the only path for current to take is through your multimeter. For example, if you're testing a battery-powered device, you might disconnect one battery terminal, then touch one multimeter probe to the battery and the other to the device's battery contact. Now all the current has to flow through the multimeter to reach the device.

This is why measuring amps is more disruptive than measuring voltage. You have to stop the device from working, insert the multimeter, and then turn it back on. But it's the only safe and accurate way to do it.

Setting up your multimeter before you connect it

Before you touch anything to the circuit, set your multimeter to the correct mode and range. Turn the dial to either DC amps (marked as DCA or A with a straight line) or AC amps (marked as ACA or A with a wavy line). Most battery-powered devices and DC power supplies use DC; household outlets and wall adapters use AC. If you're not sure, start with DC.

Next, choose a range. Multimeters usually have several amp ranges — common ones are 200 milliamps (mA), 2 amps, 20 amps, and 200 amps. Pick a range that is higher than what you expect the device to draw. If you're testing a small USB device, use the 200 mA range. If you're testing a power tool, use the 20 amp range. If you guess too low, the multimeter will show an overload symbol (usually "OL" or a dash) and you'll have to try again with a higher range.

Some multimeters have an auto-ranging feature that picks the range for you — if yours does, you can skip this step, but it's still good to have a rough idea of what you expect so you know if the result makes sense.

Disconnecting the circuit and inserting the multimeter

Turn off the device you're testing. Locate the power source — this might be a battery, a power adapter, or a wall outlet. You need to break the connection between the power source and the device by removing one wire or terminal.

If you're testing a battery-powered device, the easiest approach is to remove one battery terminal. If you're testing something plugged into a wall outlet, unplug it, then cut or carefully strip a small section of one of the power wires (usually the black or red one) so you can access the bare wire inside. If you're not comfortable cutting wires, you can use a battery holder with removable terminals or a power supply with clip leads instead.

Once the circuit is broken, hold the multimeter so you can see the dial and the probes. Touch the red probe to the positive side of the break (the side connected to the power source) and the black probe to the negative side (the side connected to the device). If you're not sure which is which, look for markings on the battery, adapter, or wire. Red is almost always positive.

Turning on the device and reading the result

With the multimeter probes in place, turn the device on or reconnect the power. The multimeter will show a number on its display. This is the current in amps (or milliamps, depending on the range you chose). Leave the device running for a few seconds so you can see a stable reading.

A normal reading will be a positive number. If the display shows "OL" or a dash, the current is higher than the range you chose — turn off the device, disconnect the multimeter, and try again with a higher range. If the display shows 0 or a very small number like 0.001, either the device isn't drawing power (check that it's actually on), the circuit is broken somewhere else, or you've chosen a range that's too high to read small currents accurately.

Once you have a reading, write it down and turn the device off. Disconnect the multimeter probes carefully, then reconnect the circuit the way it was. If you cut a wire, you'll need to solder it back together or use a wire connector.

Common mistakes that damage the multimeter or give wrong results

The most common mistake is measuring amps the same way you measure voltage — by touching the probes to two points on a live circuit without breaking it. This will blow the fuse inside the multimeter or damage the amp-measuring circuit. If you do this, the multimeter will stop responding or show nonsense numbers. Check the fuse (usually inside a compartment on the back) and replace it if it's burned out.

Another mistake is choosing a range that's too low. If you expect a device to draw 5 amps but you set the multimeter to 200 milliamps, the display will show "OL" immediately. This won't damage anything, but it's frustrating. Always start with a range higher than you think you need, then switch to a lower range if the reading is very small.

A third mistake is forgetting to turn the device on after you insert the multimeter. If nothing is running, no current flows, and the multimeter reads 0. Make sure the device is actually powered up and working before you assume the reading is correct.

Finally, some people mix up the probe connections. If you touch the red probe to the negative side and the black probe to the positive side, most digital multimeters will show a negative number instead of a positive one. This won't damage anything, but it means you've reversed the polarity. Just swap the probes and try again.

When to use DC amps versus AC amps

DC amps (direct current) is what you use for batteries, solar panels, car electrical systems, and anything powered by a DC power adapter. AC amps (alternating current) is what you use for devices plugged into a wall outlet in your home or office. If you're not sure which one your device uses, look at the power adapter or the label on the device itself — it will usually say "DC" or "AC" somewhere.

If you choose the wrong one, the multimeter will usually show 0 or a very low number because it's looking for the wrong type of current. This won't damage the multimeter, but you won't get a useful reading. Just turn the dial to the other setting and try again.

Some multimeters have a setting that measures both DC and AC at the same time (sometimes marked as "A" without a symbol). This is less accurate than choosing the right one, but it works in a pinch if you're not sure which type you're dealing with.

Frequently Asked Questions

What's the difference between amps and milliamps?

One amp equals 1,000 milliamps. Milliamps (mA) are used for small devices like phone chargers and USB devices that draw less than one amp. Amps are used for larger devices like power tools and household appliances. Your multimeter will show the reading in whichever unit matches the range you chose — if you're on the 200 mA range, it shows milliamps; if you're on the 20 amp range, it shows amps.

Can I measure amps without breaking the circuit?

No. Measuring amps requires the current to flow through the multimeter itself, which means breaking the circuit. If you try to measure amps by touching the probes to two points on a live circuit, you'll damage the multimeter. If you want to check power without breaking anything, measure voltage instead — that tells you whether power is present.

What if the multimeter shows a negative number?

A negative reading means you've reversed the probe connections. The red probe should touch the positive side and the black probe should touch the negative side. Swap them and try again. A negative reading won't damage the multimeter, but it means the current is flowing in the opposite direction from what you set up.

How do I know if the amps reading is normal?

Check the device's manual or the label on the power adapter — it will usually list the expected current draw in amps or milliamps. If your reading is close to that number, the device is working normally. If it's much higher, something inside the device may be failing. If it's much lower or zero, the device may not be getting power or may be turned off.

Will measuring amps hurt me?

Measuring amps the correct way (in series, with the circuit broken) is safe as long as you're working with low-voltage devices like batteries and USB power supplies. High-voltage circuits (like household wiring or car batteries) can cause serious injury or death if you make a mistake. If you're not trained to work with high-voltage systems, do not attempt to measure them yourself.