How to Measure LED Current With a Multimeter
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People who search how to measure LED current with multimeter want to know how many milliamps their LED is drawing. There are two ways. Put the meter in series with the LED on its milliamp jack, or leave the circuit intact and measure the voltage across the series resistor, then divide by the resistor value. This guide covers both on battery-powered or low-voltage DC circuits, shows which is more accurate, and explains why the meter itself changes the answer. Mains work can kill, and panel or service work belongs to a licensed electrician.
Electrical safety first
Work on mains circuits can kill, so this guide covers only LEDs on battery-powered or low-voltage DC circuits. Mains LED bulbs, fixtures, drivers, outlets, breaker panels and service wiring are out of scope, and panel or service work belongs to a licensed electrician. The meter’s CAT rating must suit the circuit it touches. The Fluke 15B and 17B manual describes CAT II as equipment supplied from the fixed installation, such as TVs, PCs and portable tools, and CAT III as distribution panels, feeders and short branch circuits. Your own manual or listing gives your rating, and if it does not state a CAT rating, do not treat the meter as safe for household mains.
Current is the one measurement that can hurt the meter. The Fluke manual says to check the meter’s fuses and turn off power to the circuit before connecting the meter, to use the proper terminals, function and range, and to use caution above 30 V ac rms, 42 V ac peak or 60 V dc. A Randolph College lab note says fuses on these meters are blown frequently from misuse, usually when the meter is placed in parallel across a resistor instead of in series.
Quick answer
- To read current directly, turn the circuit off, move the red lead to the mA jack, set the dial to dc milliamps, break the circuit path, and connect the leads across the break.
- Apply power and read the display. The Fluke manual describes this sequence for the 15B and 17B.
- To avoid breaking the circuit, set the meter to DC volts, measure across the series resistor, and divide by the resistor’s value. Adafruit and SparkFun both use that Ohm’s law step in their LED guides.
- Never place the meter across the LED or the supply with the red lead in a current jack.
- Most 3 mm, 5 mm and 10 mm LEDs are rated for about 20 mA, according to Adafruit.
What sets the current in an LED circuit
An LED does not limit its own current. SparkFun says that an LED connected directly to a current source will draw as much as it is allowed and destroy itself, which is why a resistor goes in series. In that series loop, the same current flows through the resistor and the LED. Adafruit explains the arithmetic with Kirchhoff’s voltage law: the supply voltage equals the LED’s forward voltage plus the resistor’s voltage, and Ohm’s law then gives the current as the resistor voltage divided by the resistance.
The formula is I = (Vs minus Vf) / R. Adafruit says red, yellow, orange and light-green LEDs use between 1.85 V and 2.5 V, and takes 2.2 V as an average. In its own example a 9 V supply, a 2.2 V LED and a 1000 ohm resistor give 6.8 mA. The table below applies the formula to four circuits. The 2.0 V forward voltage in three rows is our assumption, inside Adafruit’s range, because the forward voltage of your LED comes from its datasheet.
| Supply | Forward voltage (assumed) | Resistor | Voltage across resistor | Expected LED current |
|---|---|---|---|---|
| 5 V | 2.0 V | 150 ohms | 3.00 V | 20.0 mA |
| 5 V | 2.0 V | 330 ohms | 3.00 V | 9.09 mA |
| 3 V | 2.0 V | 68 ohms | 1.00 V | 14.7 mA |
| 9 V | 2.2 V (Adafruit average) | 1000 ohms | 6.80 V | 6.80 mA |
SparkFun’s datasheet walk-through for a red 5 mm LED lists 20 mA for continuous current, a short-burst peak of 30 mA and a suggested stable range of 16 to 18 mA. Adafruit adds that for 99 percent of LEDs the optimal current is 20 mA, with 30 mA as an upper push. Treat those as examples. The datasheet for your LED sets the real limits, and the pages we opened do not cover high-power LEDs beyond Adafruit’s note that they can run at 1 or 2 amperes.
Method 1: put the meter in series
The Fluke manual gives the sequence. Turn the rotary switch to the current position and press the yellow button to choose dc. Connect the red lead to the A or mA terminal, based on the current to be measured, and the black lead to COM. Then “Break the circuit path to be measured.” Connect the leads across the break, apply power and read the display. A Southern Methodist University lab handout describes the same step in plain words: you “temporarily break the circuit, insert the ammeter in series at the break,” then close the circuit again so the current flows through the meter.
For an LED loop, the break can be anywhere in the single series path: between the supply and the resistor, or between the resistor and the LED. The current is the same at every point in a series circuit, so the reading does not depend on where you break it. That is our reasoning from the series-loop description above.
Pick the terminal first. The 15B and 17B mA terminal reads to 400 mA and is protected by a 500 mA fuse, and the A terminal reads to 10 A with a separate 11 A or 10 A fuse. A 20 mA LED belongs on the mA terminal, which has a 40.00 mA range with 0.01 mA resolution. If you do not know the current, as with an LED strip or power LED, the Randolph College note advises starting with the high-current input before trying the sensitive one. Check the mA fuse before you rely on it, using the fuse test in the manual. That test is a resistance check, and our guide to using a continuity tester covers the same kind of reading.
Method 2: measure volts across the resistor
This method leaves the circuit intact. Set the meter to DC volts with the red lead in the voltage jack. Touch the probes across the series resistor, with the circuit powered, and read the voltage. Then divide by the resistor’s value. Adafruit uses exactly this step, dividing 6.8 V by 1000 ohms to get 6.8 mA. For the first row in the table, 3.00 V across 150 ohms is 20.0 mA.
You need the resistor’s true value. Adafruit tells readers to verify it by checking the color stripes. The Fluke resistance guide says that for the best result the component should be removed from the circuit, because other parts in parallel lower the reading. So measure the resistor with the circuit off and the resistor lifted, or use the marked value and accept its tolerance. The pages we opened do not state a resistor tolerance.
The voltage input also has a big advantage. The Fluke manual gives an input impedance above 10 megohms on dc volts, and a Fluke article notes that input resistances of 1 or 10 megohms generally keep circuit loading small. The meter draws almost nothing from the LED circuit. For more on current meters generally, see our roundup of milliamp meters and of dc current amp meters.
What the meter does to a small circuit
A meter placed in series adds its own resistance. A Fluke article calls the voltage dropped across the meter’s input the burden voltage, and gives a figure for one model: 1.8 mV per mA on the 400 mA range of the Fluke 87V. It derives a combined shunt and fuse resistance of about 1.8 ohms. The article says that in a 3 V circuit drawing 100 mA the burden is 6 percent of the supply, and that the effect can be ignored at 1 mA. The pages we opened do not give a burden voltage for the 15B or 17B.
The table applies the 87V figure of 1.8 ohms to the circuits above, as an illustration for any meter with a similar shunt. The voltage-method uncertainty uses the manual’s dc volts accuracy of 0.5 percent plus 3 digits, and the mA-range uncertainty uses the 40.00 mA range accuracy of 1.5 percent plus 3 digits. All numbers here are our arithmetic.
| Circuit | Current with no meter | Current with a 1.8 ohm meter in series | Change | mA-range meter error, plus or minus | Voltage-method meter error, plus or minus |
|---|---|---|---|---|---|
| 5 V, 150 ohms | 20.00 mA | 19.76 mA | -1.2% | 0.330 mA | 0.120 mA |
| 5 V, 330 ohms | 9.09 mA | 9.04 mA | -0.5% | 0.166 mA | 0.055 mA |
| 3 V, 68 ohms | 14.71 mA | 14.33 mA | -2.6% | 0.251 mA | 0.118 mA |
| 9 V, 1000 ohms | 6.80 mA | 6.79 mA | -0.2% | 0.132 mA | 0.064 mA |
Two points follow. The burden effect is small on a 5 V or 9 V circuit and grows on a low-voltage one, as in the 3 V row. And on these examples, the meter’s own error is smaller on the voltage method. That is only part of the voltage method’s error, because the resistor’s tolerance is not in the table. If your resistor is marked with a wide tolerance, the in-series reading may be the better one.
Reading the result
Compare the measured current with I = (Vs minus Vf) / R. By our reasoning from that formula, a reading well below the expected figure suggests a supply voltage lower than you assumed, a larger resistor than marked, or a forward voltage above your assumed value. A reading well above it suggests a smaller resistor or a lower forward voltage. The pages we opened give no diagnostic limits, so treat a difference of a few percent as ordinary, since the table already shows the meter alone can account for one or two percent.
Mistakes to avoid
- Connecting the meter in parallel in current mode. The Randolph College note says this is the usual way fuses blow.
- Leaving the red lead in the mA jack. The manual says to use the proper terminals, function and range.
- Powering the circuit before connecting the meter. The manual says to turn power off first.
- Skipping the resistor. SparkFun says an LED without one can destroy itself.
- Assuming the meter changes nothing. The Fluke burden article shows the shunt can reduce the current in a low-voltage loop.
- Measuring mains LED products. This guide does not cover them.
How we chose
This is a method, not a product list, so there are no rankings here. We kept only steps and figures that one of the pages we opened on 2026-10-22 states: the Fluke 15B and 17B manual, a Southern Methodist University lab handout, a Randolph College lab note, a Fluke article on burden voltage, a Fluke resistance guide, an Adafruit LED guide and a SparkFun LED tutorial. The tables of expected current, burden effect and meter error are our own arithmetic from those figures, with the assumptions stated beside them.
What we could not verify (what the sources do not say)
The pages we opened do not give the burden voltage of the 15B or 17B, so the 1.8 ohm figure is an illustration from a different model. They do not give a resistor tolerance, and they do not explain how a dc meter reads an LED that is dimmed by rapid switching, so a dimmed LED may not read as the table predicts. They do not give a safe method for measuring current in mains LED products, and we give none. The 20 mA and 30 mA figures are examples from tutorials, not limits for your LED.
Frequently asked questions
Can I measure LED current without breaking the circuit?
Yes, indirectly. Measure the voltage across the series resistor with the meter on DC volts and divide by the resistor’s value. The pages we opened describe the same Ohm’s law step, though they do not call it a measurement method.
Which jack do I use for LED current?
For a typical indicator LED, the mA jack. On the Fluke 15B and 17B that terminal reads to 400 mA and has its own fuse. For an unknown or larger current, start with the A terminal.
Why does my LED look dimmer with the meter connected?
The meter adds resistance in series. Fluke calls the drop across it the burden voltage, which can matter in a low-voltage circuit.
How much current should an LED carry?
Adafruit says about 20 mA for most 3 mm, 5 mm and 10 mm LEDs, and SparkFun’s example datasheet lists 20 mA continuous. Your LED’s datasheet sets the actual limit.
Can I measure the current of an LED strip the same way?
Only on low-voltage DC strips, and the strip may draw more than the mA jack allows. Start with the high-current input, as the Randolph College note advises.
Bottom line
For an LED on a battery circuit, either put the meter in series on the mA jack with the power off, or read the volts across the series resistor and divide by its value.
Sources
- https://assets.fluke.com/manuals/15b17b__umeng0400.pdf (opened 2026-10-22)
- https://www.physics.smu.edu/tneumann/110X_Spring2025/files/em_lab_6.pdf (opened 2026-10-22)
- https://physics.randolphcollege.edu/psheldon/classes/p3331/multimeter.html (opened 2026-10-22)
- https://www.fluke.com/en-us/learn/blog/electrical/can-you-live-with-the-burden (opened 2026-10-22)
- https://learn.adafruit.com/all-about-leds/forward-voltage-and-kvl (opened 2026-10-22)
- https://learn.sparkfun.com/tutorials/light-emitting-diodes-leds/all (opened 2026-10-22)
- https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-measure-resistance (opened 2026-10-22)
