Ohms Law Calculator
Calculate voltage, current, resistance or power from any two known values using Ohm's law.
Enter any two values (leave the unknown field blank) — V = I × R.
Voltage
12 V
Current
0.12 A
Resistance
100 Ω
Power
1.44 W
V × I
Leave one of the three fields empty and it will be computed from the other two.
Ohms Law Calculator on True Calculator gives you an instant, accurate answer with no sign-up and no app install. Calculate voltage, current, resistance or power from any two known values using Ohm's law. Every result shows the formula and a worked example so you can verify the calculation yourself, and all values are computed in your own browser — your numbers never leave your device.
Popular uses: ohms law calculator · voltage current resistance · v=ir calculator
How We Calculate
This calculator uses standard computing formulas and algorithms verified by our team. All calculations are performed instantly in your browser using JavaScript — no data is sent to any server.
Algorithms follow well-documented public specifications used by browsers and programming languages.
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 quantity you need — voltage, current or resistance.
- Step 2: Enter the two values you already know; leave the unknown field blank.
- Step 3: Read the computed quantity and the power in watts from the result cards.
- Step 4: Adjust inputs to test what happens if, say, resistance rises or voltage drops.
Worked Example
A 12 V LED strip draws current through a 100 Ω series resistor. Enter voltage 12 and resistance 100, leaving current blank. The calculator applies V = I × R and returns a current of 0.12 A (120 mA) and power of 1.44 W. Since common ¼-watt resistors handle 0.25 W, the 1.44 W figure reveals the resistor is badly underrated — you would need at least a 2 W part for this job.
Tips and Common Mistakes
- •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.
- •Remember current is measured in amperes; 0.12 A means 120 mA, a common mistake when ordering parts.
- •For quick mental checks, 1 Ω at 1 A gives 1 V and 1 W — a useful anchor for estimating circuits.
- ✗Avoid entering zeros for the value you want to find — the calculator treats zero as 'unknown', so leave the field empty.
- ✗Don't use 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.
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