How to Measure Current Across a Resistor with a Multimeter

How to Measure Current Across a Resistor with a Multimeter

ByKen

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How to measure current across a resistor with a multimeter starts with one correction: a meter reads the current through a resistor, and the word “across” belongs to voltage. For a direct reading, power the circuit down, open it at one end of the resistor, and put the meter in series on a current range. For an indirect reading, measure the voltage across the resistor and divide by its resistance. This guide covers low-voltage DC bench circuits only. Mains circuits can kill, the meter’s CAT rating must suit the circuit, and panel or service work belongs to a licensed electrician.

Quick answer

  • Voltage is measured across a part with the meter in parallel. Current is measured through a part with the meter in series, so the circuit has to be opened.
  • Direct method: power off, lift one resistor lead, set a DC current range, use the matching current jack, bridge the gap with the meter, then power on and read.
  • Indirect method: read the DC voltage across the resistor and divide by its resistance. The answer carries the resistor’s tolerance.
  • A meter left on a current range across a resistor or a source acts as a short circuit and can blow its fuse.

Electrical safety before you connect a meter

Work on mains circuits can kill. OSHA’s electrical page says that working with electricity can be dangerous, and that its standards address electric shock, electrocution, fires and explosions. This guide assumes a battery, a small bench supply or a low-voltage DC board. Nothing here asks you to probe an outlet, a breaker, a panel or service wiring.

The meter’s CAT rating must suit the circuit. Fluke’s meter-safety guide says the ratings usually sit near the input jacks, and it advises choosing a tool rated for the highest category you could use it in. Fluke’s category list puts receptacle-connected loads such as appliances and portable tools in CAT II, and the utility connection and service entrances in CAT IV. This guide names no meter, so there is no listing to quote. Your meter’s rating is whatever its markings or manual state, and where neither states a CAT rating, do not treat that meter as suitable for anything connected to a wall outlet.

Panel and service work belongs to a licensed electrician. The CPSC home-wiring guide calls removing a service-panel cover a job for a qualified electrician. A current range adds its own risk: the meter becomes a very low resistance, so a wrong hookup is a short circuit. The Fluke 87 manual says to turn the power off before connecting the meter for a current measurement.

“Across” versus “through”: where the probes go

The wording decides where the probes go. The Fluke 87 manual says to insert the meter in parallel for voltage and in series for current measurements. A University of Florida lab handout gives the reason: the ammeter must carry the same current as the component, so the branch is disconnected, the ammeter inserted, and the branch reconnected.

What you want Where the meter goes Open the circuit? Meter setting
Voltage across the resistor In parallel, one probe on each end No DC volts
Current through the resistor In series, bridging a gap in its branch Yes, at one lead DC current, red lead in a current jack
Resistance of the resistor Across it, unpowered, one end lifted Lift one lead Ohms

Method 1: put the meter in series (direct reading)

Our own assumption for this walk-through: a 5 V DC bench supply, a 1 kΩ resistor, and a meter with about 1.8 Ω across its mA range, the figure the Fluke 87 manual states for that model. Your meter’s figures will differ.

  • Step 1, predict the current. 5 V divided by 1,000 Ω is 0.005 A, or 5 mA. Fluke writes this as I = V / R. The prediction picks your jack and range.
  • Step 2, power down. The Fluke 87 manual and the Utah handout both put the power off before the meter goes in.
  • Step 3, set up. Black lead in COM, red lead in the current jack that covers the prediction, dial on DC current.
  • Step 4, open the circuit at the resistor. Lift one resistor lead from the supply side, or pull the jumper that feeds it.
  • Step 5, bridge the gap. Red lead to the end from the supply’s positive side, black lead to the resistor. SparkFun says swapped probes only make the reading negative.
  • Step 6, check the loop before power returns: supply positive, red lead, meter, black lead, resistor, supply negative. No lead may touch the supply’s other terminal.
  • Step 7, power on and read. SparkFun notes that the meter now acts as a piece of wire and averages over time, so the last digit moves.
  • Step 8, power off, remove the meter and restore the wire. Then put the red lead back in the voltage jack and the dial on volts.

Why the meter must not sit across the resistor on a current range

Probes across the resistor on a current range put the meter’s own small resistance in parallel with the resistor, which bypasses it. In the example, 5 V across about 1.8 Ω would ask for roughly 2.8 A (our own arithmetic, assuming an ideal source). The Fluke 87 manual’s table lists a 44/100 A fast fuse, or 0.44 A, beside its mA/µA input. The Utah handout warns that a current measurement can be more exciting if wired incorrectly. SparkFun reports that a blown fuse leaves the meter reading 0.00 in current mode, so a zero is a prompt to check the fuse.

Choosing the current range and jack

Meters split their current inputs differently, so use the manual for yours. The opened pages agree on a few points.

  • Not every meter has a milliamp or microamp jack, according to Fluke. Most digital multimeters stop at 10 A of DC or AC current, and larger currents need a clamp accessory, which is outside this guide.
  • When the current is unknown, the Fluke 87 manual says to connect to the A input first. Its mA/µA input covers up to 400 mA. SparkFun moves to the 10 A side when a circuit may draw close to or more than 200 mA.
  • Start at a high range and work down, as the Florida handout puts it. Fluke’s parasitic-drain article adds that the leads should be in the correct ports before you connect.
  • Your step 1 prediction tells you where to start.

Method 2: measure the voltage and calculate (indirect reading)

Ohm’s law lets you skip the broken circuit. Fluke notes that the rearranged formulas are especially useful when direct measurement is not possible, and a LibreTexts physics text describes the voltage drop across a resistor as produced by the current through it.

  • Step 1, read the voltage. Dial to DC volts and put one probe on each end of the resistor, in parallel.
  • Step 2, get the resistance. Read the color code, or switch off, lift one end and read ohms. The Florida handout says one end must be disconnected before a resistance measurement. The Fluke 87 manual says an in-circuit reading shows the total resistance through all possible paths.
  • Step 3, divide volts by ohms.
  • Step 4, state the spread. The Utah handout’s resistors have a 5% tolerance, and a color-code value passes that spread into your current.

An illustration with assumed readings: 4.93 V across the resistor and 987 Ω read with one end lifted give 4.93 / 987 = 0.004995 A, about 4.995 mA. The table shows what a 5% tolerance does at 5 V (our own calculation: 5 V divided by the resistance, with the resistance set to 1.05 or 0.95 times nominal).

Nominal resistor Current at nominal Real value 5% above Real value 5% below
560 Ω 8.929 mA 8.503 mA 9.398 mA
1 kΩ 5.000 mA 4.762 mA 5.263 mA
2.2 kΩ 2.273 mA 2.165 mA 2.392 mA
10 kΩ 0.500 mA 0.476 mA 0.526 mA

The voltmeter loads the circuit slightly. The Fluke 87 manual gives about 10 MΩ of input impedance and calls the error negligible, 0.1% or less, when the source impedance is 10 kΩ or less. For small resistors, lead resistance matters on the ohms range. The manual puts it at 0.1 to 0.2 Ω for a standard pair and points to Relative mode to subtract it.

Burden voltage: the meter changes the current it measures

A meter in series is a resistor in series. The Fluke 87 manual says its internal shunt resistors develop a burden voltage across the meter’s terminals, and it puts the terminal resistance at about 0.03 Ω for A, 1.8 Ω for mA and 100 Ω for µA. An Agilent (now Keysight) application note says the series burden voltage causes a measurement error, and gives that error in percent as 100 times Vb divided by Vs, where Vb is the burden voltage and Vs is the source voltage.

The table applies that formula to a 5 V source and 1.8 Ω on the mA range. All values are our own assumptions and calculations.

Resistor Current without meter Reading with meter Burden voltage Error, 100 × Vb / Vs
10 kΩ 0.5000 mA 0.4999 mA 0.90 mV 0.018%
2.2 kΩ 2.273 mA 2.271 mA 4.09 mV 0.082%
1 kΩ 5.000 mA 4.991 mA 8.98 mV 0.18%
560 Ω 8.929 mA 8.900 mA 16.02 mV 0.32%
100 Ω 50.00 mA 49.12 mA 88.41 mV 1.77%

With a 1 kΩ resistor the meter costs well under one percent, less than a 5% resistor’s tolerance. Near 100 Ω the error reaches almost two percent, and the reading falls below the current the circuit carried before the meter went in. If your manual lists burden voltage per range, use that figure instead.

How to decide between the two methods

  • Use the series hookup when you need the real branch current, when the resistor’s true value is uncertain, or when the resistor is large enough that the burden error is small.
  • Use voltage and Ohm’s law when lifting a lead is awkward, when the current is near the jack’s limit, or when the resistor is low in value.
  • Use both when something looks off. If the answers differ by more than tolerance plus burden error, suspect the resistor value, a blown fuse or a loose connection.

Mistakes to avoid

  • Probes across the resistor on a current range. Utah and SparkFun both describe the short-circuit and blown-fuse outcome.
  • Leaving the red lead in the current jack and then reading voltage. SparkFun says the 200 mA fuse in its example meter will blow, and it advises leaving the meter in a friendly state.
  • Putting an in-circuit ohms reading into the formula. Parallel paths change what the display shows.
  • Starting on a low range with an unknown current. The Florida handout and the Fluke 87 manual both point you to the high range first.
  • Connecting the meter while the circuit is live. Power comes off first.
  • Reading 0.00 as zero current. A blown fuse or an unbridged gap gives the same display.

Related guides

How we chose the steps and the sources

A step stays in only when a page opened on 2026-10-21 states it, and the safety-critical steps (series hookup, power off first, jack and range choice) needed two or more independent pages. The pages are the Fluke 87 manual and Fluke articles, an Agilent (now Keysight) application note, the Utah and Florida lab handouts, SparkFun, a LibreTexts physics text, OSHA and CPSC. Pages that returned an error or a bot challenge were left out. Table numbers are either quoted from a named page or labelled as our own assumption with the formula shown. No meter is recommended; the Fluke 87 figures appear because that manual states them.

What we could not verify (what the sources do not say)

  • Burden figures exist here for one named meter model. The pages give no typical figure for other meters.
  • The fuse rating and CAT rating of your meter come from its own manual or markings, which the pages cannot state for you.
  • Resistor power ratings are not given. The resistor’s datasheet caps the voltage you should apply.
  • How a particular bench supply behaves when shorted through a meter is not covered.
  • AC current in mains circuits is not covered and is left to a qualified electrician.

Frequently asked questions

Can I measure current by touching the probes across the resistor?

Not on a current range. On DC volts the same probe position reads voltage, and on a current range it makes a short circuit. For current, put the meter in line with the resistor or calculate it from the voltage.

Do I have to cut a wire to put the meter in series?

You have to open the circuit, but lifting a lead from a breadboard or removing a jumper is enough. Never cut or open mains wiring for this.

Which method is more accurate?

The sources do not rank them. Burden voltage pulls the series reading low, mostly on small resistors. The voltage method carries the resistor’s tolerance unless you read its value in ohms.

Why does the meter show 0.00 in current mode?

Either the gap is not bridged or the meter’s fuse has opened. SparkFun’s tutorial explains how to change the fuse in a later section.

Is this safe for household wiring?

No. This guide stops at batteries, bench supplies and low-voltage DC boards. Outlets, breakers, panels and service wiring are work for a licensed electrician.

Bottom line

Current is measured through a resistor, with the meter in series on a current range and the power off while you connect it. When the circuit cannot be opened, measure the voltage across the resistor and divide by its resistance, taken from the color code or an out-of-circuit ohms reading. Keep to low-voltage DC, and leave mains, panel and service work to a licensed electrician.

Sources

  • Fluke 87 True RMS Multimeter Users Manual, series and parallel connection, burden voltage, input terminals: https://assets.fluke.com/manuals/87______umeng0800.pdf (opened 2026-10-21)
  • SparkFun, How to Use a Multimeter, current in series, fuse and jack behavior: https://learn.sparkfun.com/tutorials/how-to-use-a-multimeter/all (opened 2026-10-21)
  • University of Utah ECE 2200/2210 Lab 1, ammeter in series, voltmeter in parallel, resistor tolerance: https://my.ece.utah.edu/~ece2210/Lab%201%20Measuring%20Circuit%20Values.pdf (opened 2026-10-21)
  • University of Florida EEL 3111 Lab 1, meter connections and meter loading: https://mil.ufl.edu/3111/labs/lab1_u11_Res_Laws.pdf (opened 2026-10-21)
  • Agilent/Keysight application note 5988-5512EN, burden voltage error formula (copy hosted at xdevs.com): https://xdevs.com/doc/HP_Agilent_Keysight/Resistance%3B%20DC%20Current%3B%20AC%20Current%3B%20and%20Frequency%20and%20Period%20Measurement%20Errors%20in%20DMMs%205988-5512EN%20%5B4%5D.pdf (opened 2026-10-21)
  • Fluke, What is Current?, the 10 A limit and clamp accessories: https://www.fluke.com/en-us/learn/blog/electrical/what-is-current (opened 2026-10-21)
  • Fluke, What Is Ohm’s Law?, I = V / R: https://www.fluke.com/en-us/learn/blog/electrical/what-is-ohms-law (opened 2026-10-21)
  • Fluke, Dials, Buttons, Symbols and Display of a Digital Multimeter, current jacks: https://www.fluke.com/en-us/learn/blog/digital-multimeters/multimeter-dial-button-jacks-display (opened 2026-10-21)
  • Fluke, Guide to Digital Multimeter Safety, CAT ratings: https://www.fluke.com/en-us/learn/blog/safety/multimeter-guide (opened 2026-10-21)
  • Fluke, IEC Category Ratings, category list: https://www.fluke.com/en-us/learn/blog/safety/iec-category-ratings-use-the-right-tools-for-the-job (opened 2026-10-21)
  • OSHA, Electrical, general hazard statement: https://www.osha.gov/electrical (opened 2026-10-21)
  • CPSC, Guide to Home Wiring Hazards, service panel: https://www.cpsc.gov/s3fs-public/518.pdf (opened 2026-10-21)
  • Fluke, How to Find Parasitic Battery Drain with a Multimeter, correct ports and voltage drop across fuses: https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-find-parasitic-battery-drain-with-a-multimeter (opened 2026-10-21)
  • LibreTexts (OpenStax College Physics), Ohm’s Law: Resistance and Simple Circuits: https://phys.libretexts.org/Bookshelves/College_Physics/College_Physics_1e_(OpenStax)/20%3A_Electric_Current_Resistance_and_Ohm’s_Law/20.02%3A_Ohms_Law_-_Resistance_and_Simple_Circuits (opened 2026-10-21)

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