Ohm's Law Calculator
Solve V = IR for voltage, current or resistance.
What is Ohm's Law?
Ohm's law states that the current through a conductor is proportional to the voltage across it: V = IR. One ohm is the resistance that carries one ampere at a voltage of one volt. It is the starting point of every circuit calculation.
How it works
V = IR, rearranged for whichever field is blank. Each field has its own unit list — volts, millivolts or kilovolts; amps, milliamps or microamps; ohms, kilo-ohms or mega-ohms — and everything is converted before the division. That matters in practice, because a circuit described in mA and kΩ would otherwise need two conversions before the arithmetic even starts.
- Enter what you know. Every field recalculates as you type — there is no submit button. Leave the quantity you want blank and the page solves for it.
- Read the answer and the working. The result appears above the fold; underneath it is the formula with your numbers substituted, so the arithmetic can be checked rather than trusted.
- Check the units and the size. Units are shown on every value and selectable per field, so a result in kJ, kPa or km/h is converted rather than assumed. Sanity-check the magnitude against a known landmark such as 1 g/cm³ for water.
Examples
A textbook problem with a known answer
Every tool here is checked against standard example problems — the molar mass of C6H12O6 is 180.16 g/mol, and free fall from 20 m takes about 2.02 s at g = 9.8 m/s².
A result you want to reason backwards from
Leave the quantity you already know blank instead: given the final answer, most tools recover the starting value — the mass needed for a target concentration, or the height a fall must have had.
Common mistakes
Forgetting that a formula is only as good as its subscripts
H2O and H2O2 are different compounds. The formula field shows its parse — element by element — so a mis-typed subscript is visible before it reaches the arithmetic.
Mixing units in the middle of a calculation
The most common source of a wrong answer is a value in grams next to a formula expecting kilograms, or Celsius next to a gas constant expecting kelvin. This category converts on the way in and labels what it used.
Frequently asked questions
What is the current through a 5 Ω resistor at 10 V?
2 A. I = V ÷ R = 10 ÷ 5 — load the example and the rearrangement appears in the working.
Does Ohm's law apply to everything?
No — it describes ohmic conductors, whose resistance is constant. Diodes, filament lamps and transistors are non-ohmic: their resistance changes with voltage, so Ohm's law is a local approximation rather than a rule.
What does resistance actually resist?
The flow of charge. A resistor converts electrical energy into heat, and the resistance determines how much current flows for a given voltage.