Ohm's Law Calculator – Voltage, Current, Resistance
Enter any two of voltage, current, resistance or power and get the rest. Includes kV, mV, mA, kΩ, MΩ, kW and MW unit support.
Fill exactly two fields — leave the other two blank or at 0. V = I × R, P = V × I.
Voltage
12 V
entered
Current
0.12 A
computed
Resistance
100 Ω
entered
Power
1.44 W
computed
Leave exactly 2 fields empty to calculate them.
Last updated: August 2026
How this calculator is verified
Checked by True Calculator automated test suite on
- Formula verified against a published worked example in the automated test suite
- Edge cases (zero, negative, boundary and unit-mismatch inputs) covered by unit tests
The full verification method is on our how we verify page. Found an error? Tell us and we will re-check it.
When to Use This Calculator
Use this calculator every time a circuit question comes down to voltage, current or resistance. Selecting a series resistor for an LED, sizing a fuse for a 12 V appliance, checking what current a heater element will draw from a 230 V Indian mains supply, or verifying that a power supply can deliver the current a project needs — all reduce to V = I × R. The power figure is the real payoff: it tells you whether a component will overheat before you burn it out. Students preparing for physics practicals, repair technicians and makers building small electronics all reach for this relation constantly, and the calculator removes the arithmetic from the equation.
How to Use This Calculator
- Step 1: Decide which two values you know — voltage, current, resistance or power — and leave the other two blank.
- Step 2: Enter the known values and pick their units, such as kV or mV for voltage and kΩ or MΩ for resistance.
- Step 3: Read all four values: the two you entered plus the two computed ones, including power in watts.
- Step 4: Adjust inputs to test what happens if, say, resistance rises or voltage drops.
Worked Example
A circuit runs on 18 V and draws 6 A. Enter voltage 18 and current 6, and the calculator returns resistance 3 Ω and power 108 W, since P = V × I. For an LED strip with a 12 V supply and a 100 Ω series resistor, current comes out as 0.12 A (120 mA) and power as 1.44 W — revealing that a ¼-watt resistor is badly underrated and a 2 W part is needed. The power input works too: 12 V at 36 W gives 3 A and 4 Ω.
Tips and Common Mistakes
- •Tip 1: Use a value slightly above the minimum power rating — a 2 W resistor for a 1.44 W dissipation is safer than a 1.5 W one.
- •Tip 2: Remember current is measured in amperes; 0.12 A means 120 mA, a common mistake when ordering parts.
- •Tip 3: For quick mental checks, 1 Ω at 1 A gives 1 V and 1 W — a useful anchor for estimating circuits.
- ✗Mistake 1: Entering zeros for the values you want to find — the calculator treats zero as 'unknown', so leave the fields blank.
- ✗Mistake 2: Using this tool for AC circuits with coils or capacitors; impedance, not plain resistance, applies there.
Frequently Asked Questions
What exactly is Ohm's law?
Ohm's law states that voltage equals current times resistance (V = I × R). Given any two of the three quantities you can always find the third, and power follows from V × I.
Why do I need to enter only two values?
The equation V = I × R has three variables, so two known values pin down the whole circuit. Leave the field you want to find blank and the calculator derives it from the other two.
How do I find the power dissipated by a resistor?
Power in watts equals voltage times current (P = V × I). For a 12 V circuit drawing 0.12 A, the resistor dissipates 1.44 W, which is why the calculator shows power alongside the other results.
Does Ohm's law work for AC circuits too?
It works for pure resistances, but inductors and capacitors introduce reactance, so you need impedance (Z) instead of plain resistance. This calculator is meant for DC and resistive AC loads.
What happens if I enter only one value?
With only one known value the equation cannot be solved, so the calculator shows a prompt instead of a result. Enter at least two of voltage, current and resistance.
You might also need
Related calculators from other categories
Everyday
Electricity Bill Calculator
Estimate your monthly and yearly electricity bill from each appliance's wattage, usage hours and your tariff. Add appliances to total your power costs.
Science & Weather
Engine Horsepower Calculator
Estimate engine power in HP, kW and PS from torque and RPM, using metric (N·m) or imperial (lb-ft) formulas.
Math
Exponent Calculator
Raise any number to any power, including fractional and negative exponents. Handles large exponents with scientific notation.
Science & Weather
Ideal Gas Law Calculator
Solve PV = nRT for pressure, volume, moles or temperature with the gas constant R = 8.314. Enter any three to find the fourth.