Calcylator
Ohm's Law

Ohm's law:
volts equal amps times ohms

Find the voltage across a resistor from the current and resistance, rearrange the law for current or resistance, and avoid the milliamp and kilohm slips.

Calcylator Editorial Team

Updated · 5 min read

Ohm's law: voltage equals current times resistance

Ohm's law says the voltage across a resistor equals the current through it multiplied by its resistance. A current of 0.02 A through 150 ohms gives 0.02 × 150 = 3 volts.

The Calcylator tool asks for two inputs, the current in amps (A) and the resistance in ohms (Ω), and returns the voltage in volts (V). It solves for voltage only, so to find current or resistance you rearrange the same formula.

Ohm's law =V = I × R
V:
voltage in volts (V)
I:
current in amps (A)
R:
resistance in ohms (Ω)
Rearranged: I = V ÷ R, and R = V ÷ I.

  • Current

    0.5 A

  • Resistance

    220 Ω

  • Calculation

    0.5 × 220

Voltage

110 V

A 220 ohm element carrying half an amp drops 110 volts.

Finding current or resistance

Ohm's law is one relationship that can be turned three ways. If you know two of the three quantities, you can always find the third.

  • Voltage: V = I × R. Use the calculator directly.
  • Current: I = V ÷ R. A 12 V supply across 4 Ω drives 3 A.
  • Resistance: R = V ÷ I. A 3.3 V drop at 0.02 A means 165 Ω.

A quick way to remember it is the triangle method: cover the quantity you want and read what is left. Cover V and you see I next to R, so multiply. Cover I and you see V over R, so divide.

Units: milliamps and kilohms

The calculator expects base units. Many circuit values are written with prefixes, and forgetting to convert is the biggest source of wrong answers. Divide milliamps by 1,000 to get amps, and multiply kilohms by 1,000 to get ohms.

Common conversions
Written asIn base unitsExample calculation
20 mA0.02 A0.02 A × 150 Ω = 3 V
3.5 mA0.0035 A0.0035 A × 1,000 Ω = 3.5 V
2.2 kΩ2,200 Ω0.01 A × 2,200 Ω = 22 V
47 Ω47 Ω2.5 A × 47 Ω = 117.5 V

Worked example: a resistor for an LED

Hobbyists use Ohm's law to pick a series resistor for an LED. Assume a 5 V supply, an LED that drops about 2 V, and a target current of 20 mA. The resistor must drop the remaining 3 V.

Resistance is R = 3 ÷ 0.02 = 150 Ω. The usual next step is to choose the nearest standard resistor value, and check the LED's own data sheet, since forward voltage and maximum current vary between parts.

The law also works for a series circuit. If 0.03 A flows through a 100 Ω and a 220 Ω resistor in series, the same current passes through both, so they drop 3 V and 6.6 V, and the total across the pair is 9.6 V. In series the current is shared and the voltages add.

Common mistakes with Ohm's law

  • Typing milliamps as if they were amps. 20 mA is 0.02 A, not 20 A.
  • Applying the law to parts that do not behave like a plain resistor, such as diodes, where resistance changes with voltage.
  • Mixing the voltage across one part with the total supply voltage in a series circuit.
  • Dividing when you should multiply, or the other way round.
  • Ignoring safety. Mains voltage can be dangerous, and calculations are no substitute for a qualified electrician.

Common questions

What is Ohm's law?

Ohm's law states that voltage equals current times resistance, written V = I × R. A current of 2 amps through a 10 ohm resistor produces a 20 volt drop. It links the three basic electrical quantities for ordinary resistors.

How do I calculate voltage from current and resistance?

Multiply the current in amps by the resistance in ohms. For 0.5 A and 220 Ω, the voltage is 0.5 × 220 = 110 V. Convert milliamps to amps first by dividing by 1,000, and kilohms to ohms by multiplying by 1,000.

How do I find current with Ohm's law?

Divide voltage by resistance: I = V ÷ R. A 12 volt supply across a 4 ohm load gives 12 ÷ 4 = 3 amps. The Calcylator tool solves for voltage, so use this rearranged version by hand for current.

How do I convert milliamps to amps for Ohm's law?

Divide the milliamps by 1,000. So 20 mA is 0.02 A, and 3.5 mA is 0.0035 A. Enter the amp value in the calculator. Skipping this step makes the voltage 1,000 times too large.

Does Ohm's law work for every component?

No. It holds for resistors and other ohmic parts where resistance stays constant. Diodes, LEDs and lamps change resistance with voltage or temperature, so the simple formula is only a rough guide for them.

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