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Physics formula

How to calculate ohm's law

Ohm's law links voltage, current, and resistance in a simple electrical circuit.

See the formulaOpen calculator
Direct answer
Formula
V = I * R

Use the table below to match each symbol with the right input. Keep units consistent before you start.

Ohm's law formula
Start here when you only need the equation and variable names.
Formula
V = I * R

Ohm's law links voltage, current, and resistance in a simple electrical circuit.

Variable meanings
Check each symbol, meaning, and unit before you calculate.
SymbolNameDescriptionUnit
VVoltageElectrical potential difference.V
ICurrentFlow of electric charge.A
RResistanceOpposition to current.ohm
When to use this formula
Check that your situation matches the formula before you trust the result.
  • Good for resistive DC circuits and simple circuit checks where Ohm's law applies.
Step-by-step method
Follow these steps when you are solving it by hand.
  1. 1Use current in amperes.
  2. 2Use resistance in ohms.
  3. 3Multiply current by resistance to find voltage.
  4. 4Rearrange to I = V / R or R = V / I when needed.
Examples
These sample numbers show the order of operations and units.
Find voltage
I
2 A
R
12 ohm
  1. 1.V = 2 * 12
  2. 2.V = 24 V
Result
The voltage is 24 V.
Find current
V
9 V
R
3 ohm
  1. 1.I = V / R
  2. 2.I = 9 / 3
  3. 3.I = 3 A
Result
The current is 3 A.
Mistakes to avoid
Small input or unit errors can change the answer a lot.
  • Mixing metric and imperial units before solving the ohm's law formula.
  • Forgetting squared units such as m^2, m/s^2, or velocity squared.
  • Dropping negative signs when direction matters.
  • Rounding measured values too early in the calculation.
Open the Ohm's Law Calculator
Open the calculator for the answer without rewriting the formula.

Enter your values in the related calculator, then compare the output with the hand method above.

Open Ohm's Law Calculator
FAQs
Short answers for common formula questions.
Can I calculate Ohm's law by hand?

Yes. Write the units next to each value, convert rates or measurements first, and round only the final answer.

Why does my Ohm's law result differ from another calculator?

Most differences come from rounding, unit conversions, rate timing, or a slightly different version of the formula.

When is the Ohm's Law Calculator better than hand math?

Open the Ohm's Law Calculator to check several scenarios or skip the hand arithmetic.