You can't measure resistance directly with a voltmeter, but you can calculate it
A voltmeter measures voltage (electrical pressure), not resistance. But if you know the voltage across a component and the current flowing through it, you can use Ohm's Law to find the resistance: Resistance = Voltage ÷ Current. This is the practical way most people work around the limitation — you measure what the voltmeter can tell you, then do the math.
The reason this matters: if you're troubleshooting a circuit and need to know whether a component has failed or changed resistance, a voltmeter alone won't give you that answer. You need either a multimeter (which has a resistance setting built in) or you need to measure voltage and current separately and calculate.
This guide walks you through both approaches — why a voltmeter can't do it alone, and how to get the answer you need with the tools you have.
Key Takeaways
- A voltmeter measures voltage only; it cannot directly measure resistance no matter how you connect it.
- To find resistance using a voltmeter, you measure the voltage across a component and the current through it, then divide voltage by current using Ohm's Law.
- A multimeter with a resistance setting (ohms mode) is the simpler tool if you have one, because it measures resistance directly without math.
- Measuring current requires breaking the circuit and inserting an ammeter in series, which is more involved than just touching a voltmeter to two points.
- If you only have a voltmeter and cannot measure current, you cannot calculate resistance accurately.
Why a voltmeter alone doesn't work
A voltmeter measures the difference in electrical potential (voltage) between two points. It tells you how much "push" the electricity has, but nothing about how hard it is to push electricity through a component. Resistance is the opposition to current flow — a completely different property.
Think of it like measuring water pressure in a pipe. A pressure gauge tells you how hard the water is being pushed, but it doesn't tell you whether the pipe is clogged or clear. You need to know both the pressure and the flow rate to figure out how much the pipe is restricting the water.
A voltmeter connected across a component will show you the voltage drop across it, but that voltage depends on both the resistance and the current. Two different resistors can have the same voltage drop if the current through them is different. So voltage alone is not enough.
The math: Ohm's Law and how to use it
Ohm's Law states that Resistance = Voltage ÷ Current, often written as R = V ÷ I. If you measure the voltage across a component with a voltmeter and measure the current through it with an ammeter, you can calculate the resistance.
Example: A resistor has 10 volts across it and 2 amps flowing through it. Resistance = 10 ÷ 2 = 5 ohms.
The voltmeter gives you the voltage part. You have to get the current another way — usually with an ammeter, which is a separate tool. Some multimeters have both a voltmeter and ammeter function, which makes this easier. But a voltmeter by itself cannot measure current.
Measuring voltage with a voltmeter (the part you can do)
Set your voltmeter to the DC or AC voltage range appropriate for what you're testing. Most circuits in homes and small devices use DC (direct current).
Turn off power to the circuit before you connect anything. Once power is off, touch the red probe to one side of the component and the black probe to the other side. Turn power back on. The voltmeter will show the voltage drop across that component.
Write down the voltage reading. This is half of what you need. The other half — the current — requires a different measurement that a voltmeter cannot make.
Measuring current (the part that requires more work)
To measure current, you need an ammeter or a multimeter set to amperage mode. Unlike a voltmeter, which you touch across a component, an ammeter must be placed in series — meaning you break the circuit and insert the ammeter into the path so current flows through it.
This is more involved than a voltmeter measurement. You have to disconnect a wire or trace, insert the ammeter leads into the break, and then turn power back on. If you insert an ammeter incorrectly or use the wrong range, you can damage the meter or the circuit.
If you have a multimeter, set it to the appropriate current range (usually milliamps for small circuits) and insert it in series. If you only have a voltmeter, you cannot measure current directly, and therefore cannot calculate resistance.
Using a multimeter instead (the simpler path)
If you have access to a multimeter, use it. A multimeter has a resistance setting (marked with the Greek letter omega, Ω, or the word "ohms"). This setting measures resistance directly without requiring you to measure voltage and current separately.
Turn off power to the circuit. Set the multimeter to the ohms range — usually you choose a range based on what you expect the resistance to be (low, medium, or high). Touch the red probe to one side of the component and the black probe to the other. The multimeter will display the resistance in ohms.
This is faster and more reliable than the voltmeter-plus-ammeter method, and it's why multimeters are standard tools for electronics work. A voltmeter is useful for troubleshooting power supply problems, but a multimeter is better for checking components.
When you're stuck with only a voltmeter
If a voltmeter is all you have and you need to know resistance, you have two options: get a multimeter, or measure current using a different method.
Some circuits have a known resistor already in series with the component you're testing. If you know the value of that resistor, you can measure the voltage across it, calculate the current (using Ohm's Law: Current = Voltage ÷ Resistance), and then measure the voltage across the unknown component and calculate its resistance.
This is a workaround, not a standard approach, and it only works if the circuit is set up that way. In most cases, if you need to measure resistance and only have a voltmeter, the practical answer is to use a multimeter instead.
Common mistakes when trying this
The biggest mistake is assuming that a high voltage reading means high resistance, or a low voltage reading means low resistance. Voltage depends on both resistance and current, so you cannot guess resistance from voltage alone. A component with very low resistance can have a high voltage drop if a lot of current is flowing through it.
Another mistake is trying to measure current with a voltmeter by connecting it in series. A voltmeter has very high internal resistance and is designed to be connected across a component, not in series. Connecting it in series will give you a wrong reading and may damage the meter.
If you're measuring current with an ammeter, the most common error is using the wrong range. Start with the highest range and work down to a range where the needle or display shows a clear reading. Using too low a range can damage the ammeter.
Frequently Asked Questions
Can I use a voltmeter to check if a resistor is burned out?
Not reliably with a voltmeter alone. A burned-out resistor usually has infinite resistance (open circuit), which means no current flows through it at all. A voltmeter across an open circuit will show the full supply voltage, but so will a voltmeter across a very high-value resistor with low current. A multimeter set to ohms mode will show infinite resistance (usually displayed as "OL" for overload) if the resistor is burned out.
What if I measure voltage but can't measure current — can I still find resistance?
No. You need both voltage and current to calculate resistance using Ohm's Law. If you can only measure voltage, you cannot determine resistance. You would need either a multimeter with an ohms setting or a way to measure current.
Is it safe to measure current with a voltmeter set to the wrong range?
No. A voltmeter is not designed to measure current and should never be connected in series to measure it. Connecting a voltmeter in series can damage the meter and give you a wrong reading. Use an ammeter or a multimeter set to amperage mode instead.
Why do multimeters have both voltage and resistance settings if they measure the same thing?
They don't measure the same thing. The voltage setting measures electrical potential between two points. The resistance setting applies a small internal voltage and measures how much current flows, then calculates resistance. They are different measurements that require different internal circuits.
Can I calculate resistance if I know the power and voltage?
Yes, using a different form of Ohm's Law: Resistance = Voltage squared ÷ Power, or R = V² ÷ P. But you would need to measure power, which requires a power meter or wattmeter — another tool beyond a voltmeter. In practice, measuring voltage and current is simpler.