Set your multimeter to measure amps before you touch any wires
A multimeter measures electrical current in amps when you turn the dial to the amp setting. The dial usually shows "A" or "mA" (milliamps, which are smaller units). You need to choose the right range — most multimeters have a 10-amp setting and a 200-milliamp setting. Start with the higher range if you do not know how much current you are measuring; if the reading is very small, switch to the lower range for a more precise number.
Before you connect the multimeter to anything, make sure the power is off. Unlike voltage or resistance measurements, measuring amps requires the multimeter to be in series with the circuit — meaning the current has to flow through the multimeter itself. This is different from how you measure voltage, where the multimeter sits across two points without interrupting the flow.
Key Takeaways
- Turn the dial to the amp setting (marked "A" or "mA") and choose a range higher than what you expect to measure.
- Insert the red probe into the amp jack (usually labeled "mA" or "A") and the black probe into the common jack before you start.
- Break the circuit at one point, connect the red probe to the positive side and the black probe to the negative side, and turn the power back on.
- If the reading shows a very small number or a decimal, switch to the milliamp range for a clearer reading.
- Never measure amps across a battery or power supply directly without a load in the circuit, as this can damage the multimeter.
Plug the probes into the correct jacks on the multimeter
The multimeter has three or four jacks on the front. The black probe always goes into the jack labeled "COM" or "common." The red probe goes into the jack labeled for amps — this is usually marked "mA" for milliamps or "A" for amps, depending on the range you chose on the dial. Some multimeters have separate jacks for different amp ranges; if yours does, match the jack to the dial setting.
This step matters because the multimeter routes the current through different internal resistances depending on which jack the red probe is plugged into. Using the wrong jack can give you a wrong reading or blow an internal fuse in the multimeter.
Break the circuit and connect the probes in series
To measure current, you have to insert the multimeter into the path the electricity is taking. Find the wire or component where you want to measure the current, and disconnect one end of it. This creates a break in the circuit. Touch the red probe to the side that connects back to the positive terminal (the power source), and touch the black probe to the side that connects to the rest of the circuit. The current now flows from the power source, through the red probe, through the multimeter, out the black probe, and back through the circuit.
If you reverse the probes — red to negative and black to positive — the multimeter will show a negative number instead of a positive one. The reading is still correct; it just tells you the current is flowing in the opposite direction from what you expected. Some multimeters will not let current flow at all if the probes are reversed, depending on the model.
Turn the power back on and read the display
Once the probes are in place, turn the power back on. The multimeter display will show a number with a unit next to it — either "A" for amps or "mA" for milliamps. If the number is very small (like 0.05 A) or shows mostly zeros, switch the dial to the milliamp range and reconnect the probes. Milliamps are one-thousandth of an amp, so a reading of 500 mA is the same as 0.5 A.
If the display shows "1" or "OL" (overload), the current is higher than the range you selected. Switch to a higher amp range on the dial and try again. If you are already on the highest range and still see overload, the current is too high for your multimeter to measure safely.
Common mistakes that damage the multimeter or give wrong readings
The most common mistake is measuring amps across a battery or power supply without any load in the circuit. When you do this, you are creating a short circuit through the multimeter, which can blow the internal fuse or damage the device permanently. Always make sure there is a load — a light, motor, resistor, or other component — drawing current before you measure.
Another mistake is leaving the probes plugged into the amp jacks when you are done measuring. If you then try to measure voltage with the probes still in the amp jacks, you will get a wrong reading or damage the multimeter. Always move the red probe back to the voltage jack (usually marked "V") when you switch to a different measurement.
A third mistake is measuring amps on a circuit with very high voltage. Even though you are measuring current, not voltage, the voltage in the circuit can arc across the probes or through your hands. If you are working with anything over 48 volts, wear insulated gloves and keep your fingers away from the probe tips.
When to use milliamps instead of amps
Most small electronics — phone chargers, LED circuits, sensor circuits — draw current in the milliamp range. A typical USB charger draws 500 to 2000 milliamps. If you measure this on the amp setting, you will see 0.5 to 2.0 A, which is harder to read precisely. Switching to the milliamp setting shows 500 to 2000 mA, which gives you more detail and makes it easier to spot small changes.
Larger devices like motors, heaters, and power tools draw current in the amp range. A typical household circuit breaker is rated for 15 or 20 amps. If you are measuring something that plugs into a wall outlet, use the amp setting unless the multimeter display shows mostly zeros.
What the reading tells you about your circuit
The amp reading shows how much current is flowing through that point in the circuit at that moment. If you measure the same circuit at different points, you should get the same reading — current does not get used up the way voltage does. If you measure different currents at different points, something is wrong: either you have a break in the circuit, a short circuit, or a component is drawing current in an unexpected way.
If the current is lower than you expected, check for loose connections, corroded contacts, or a failing power supply. If the current is higher than expected, check for a short circuit or a component that is drawing more power than it should. A multimeter reading is a snapshot of what is happening right now; if the circuit is unstable, the reading may jump around.
Frequently Asked Questions
Can I measure amps without turning off the power?
You can measure amps on a live circuit, but you must be careful. The power does not need to be off to insert the probes, but you should never touch the bare metal parts of the probes or the wires while the circuit is live. Wear insulated gloves and keep your fingers away from the connection points.
What happens if I measure amps across a battery directly?
You create a short circuit, which can blow the fuse inside the multimeter or damage it permanently. Always put a load (a light, resistor, or other component) in the circuit before you measure. If you need to measure battery current, measure it through the load, not directly across the battery terminals.
Why does my multimeter show a negative number?
The probes are reversed. The red probe is touching the negative side and the black probe is touching the positive side. Flip them around and the reading will be positive. The magnitude of the number is still correct; the negative sign just tells you the direction of current flow.
How do I know if I am using the right amp range?
Start with the highest range on the dial. If the reading is very small or shows mostly zeros, switch to a lower range. If the display shows "OL" or "1", the current is too high for that range — switch to a higher one. Once you get a reading between 1 and 9 on the display, you are in the right range.
Can I measure amps on a circuit with AC current?
Yes, if your multimeter has an AC amp setting. The dial usually shows "A~" for AC amps and "A" or "A—" for DC amps. Make sure you are on the correct setting for the type of current you are measuring. Most household circuits use AC; batteries and small electronics use DC.