Ohm's Law
Solve V = IR for voltage, current, or resistance. Free Ohm’s law calculator with clear unit handling and step-by-step circuit working.
Key formula
V = IR
How do you use Ohm's law (V = IR)?
Ohm's law is V = I × R. If you know two values, solve for the third: I = V / R or R = V / I. Keep volts, amperes, and ohms consistent (convert milliamps and kilo-ohms first).
About this tool
Ohm's law links voltage, current, and resistance with V = IR — the relationship students search as “ir formula” or “v = ir.” This page keeps the rearrangements visible so you can solve for V, I, or R without losing mA and kΩ conversions.
How to use this calculator
- Choose the variable you want to solve for and confirm which values are already known.
- Enter inputs in consistent units before calculating so the formula is applied correctly.
- Check whether the result is physically reasonable by comparing it with the size of the inputs.
- Use the step-by-step breakdown to see how the equation was rearranged and evaluated.
Worked examples
Example 1: I from 12 V and 4 Ω (IR formula)
I = V / R = 12 / 4.0 = 3.0 A. Sanity check: lower R would draw more current from the same 12 V source.
Example 2: R from 9 V and 30 mA
Convert current first: 30 mA = 0.030 A. Then R = V / I = 9 / 0.030 = 300 Ω. Skipping the mA→A conversion and using 9 / 30 gives 0.3 Ω — wrong by a factor of 1000.
Example 3: V from 0.025 A and 1.2 kΩ
R = 1.2 kΩ = 1200 Ω. V = I R = (0.025)(1200) = 30 V.
Frequently asked questions
What is the Ohm's law formula (V = IR)?
V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. Rearrangements are I = V / R and R = V / I. Searches for “ir formula” or “v = ir” mean this same relationship.
How do you use Ohm's law step by step?
Convert to V, A, and Ω, write the form that isolates the unknown, substitute, then multiply or divide. Check that USB-scale currents are often tens to hundreds of mA.
How do I convert mA and kΩ for Ohm's law?
1 mA = 0.001 A and 1 kΩ = 1000 Ω. Convert to amperes and ohms before dividing or multiplying, unless every quantity stays in a consistent milli/kilo set that you track carefully.
Does Ohm's law work for every component?
It models ohmic resistors well under the conditions stated in the problem. Diodes, transistors, and many real devices are non-linear, so V/I is not a single fixed R for all voltages.
How is Ohm's law related to electric power?
Power often uses P = V I, and with Ohm's law you can also write P = I² R or P = V² / R. Use the electric power calculator when the unknown is power rather than V, I, or R alone.