Ohm's Law Calculator - V, I, R & Power (Free Online Tool)
If you learn only one equation in electronics, make it this one: V = I × R. Ohm's Law ties together the three fundamentals of every electrical circuit — voltage, current and resistance — and, with the power equation, lets you find any one of them (plus power in watts) from the others. This free online Ohm's Law Calculator does it instantly: enter any two of voltage, current, resistance or power, and it computes the remaining two, with the formulas and worked examples to back it up.
Table of Contents
The Ohm's Law Calculator
Enter any two of the four values below and press Calculate — the tool fills in the other two using V = I·R and the power equations. Use the unit selectors for mV, mA, kΩ, etc.
⚡ Ohm's Law Calculator
What Is Ohm's Law?
Ohm's Law states that the voltage across a resistor equals the current through it times its resistance:
Discovered by Georg Ohm in 1827, it means current is proportional to voltage for a fixed resistance — double the voltage, double the current. It is the foundation of all circuit analysis, from sizing an LED resistor to designing power systems. The same "driving force ÷ resistance = flow" logic even mirrors fluid systems, where pressure drives flow against resistance — as in our pressure drop & head loss calculator.
The Three Formulas & the Triangle
Rearranging V = I·R gives three forms — remember them with the VIR triangle (V on top, I and R below):
| To find | Formula |
|---|---|
| Voltage (V) | V = I × R |
| Current (I) | I = V ÷ R |
| Resistance (R) | R = V ÷ I |
Cover the quantity you want in the triangle and the remaining two show the formula.
Power & the Ohm's Law Wheel
Add power (P, in watts) and you get the full "Ohm's Law wheel". The base power equation is:
Substituting Ohm's Law gives two more handy forms:
With these, any two of V, I, R, P give you the other two — exactly what the calculator does. Power tells you the heat a resistor must dissipate, which sets its wattage rating.
Units & Prefixes
| Quantity | Unit | Symbol | Common prefixes |
|---|---|---|---|
| Voltage | volt | V | mV, kV |
| Current | ampere | A | µA, mA |
| Resistance | ohm | Ω | kΩ, MΩ |
| Power | watt | W | mW, kW |
Worked Examples
1. Find the current
A 12 V battery across a 6 Ω resistor: I = V/R = 12/6 = 2 A, and P = V²/R = 144/6 = 24 W.
2. Size an LED resistor
A 5 V supply drives an LED needing 2 V at 20 mA. The resistor drops 5 − 2 = 3 V:
- R = V/I = 3 / 0.02 = 150 Ω
- P = V × I = 3 × 0.02 = 0.06 W — a standard ¼ W resistor is fine.
3. Find resistance from power
A 60 W bulb on 12 V: R = V²/P = 144/60 = 2.4 Ω, drawing I = P/V = 5 A.
DC vs AC: When Resistance Becomes Impedance
Ohm's Law applies directly to DC and to purely resistive AC circuits. When an AC circuit also has capacitance or inductance, resistance is replaced by impedance (Z), and the law becomes V = I × Z. For simple resistive loads (heaters, incandescent bulbs, resistors), the plain V = I·R works for both AC and DC.
Common Mistakes
- Mixing unit prefixes. mA with kΩ without converting — always use base units.
- Confusing series and parallel. Ohm's Law is per-element; combine resistances correctly first.
- Ignoring power rating. A resistor may be the right Ω but burn out if P exceeds its watts.
- Using V = IR on reactive AC loads. Use impedance Z for capacitive/inductive circuits.
- Assuming constant resistance. Filament and semiconductor resistance changes with temperature.
Frequently Asked Questions
What is Ohm's Law?
Voltage equals current times resistance (V = I·R). For a fixed resistance, current is proportional to voltage. It's the core relationship of all circuit analysis.
What are the three formulas of Ohm's Law?
V = I·R, I = V/R, and R = V/I — the same equation rearranged. With P = V·I you can find any quantity from any two knowns.
How do you calculate power using Ohm's Law?
P = V·I, or equivalently P = I²R or P = V²/R. These three forms make up the power wheel.
What is the unit of resistance?
The ohm (Ω). One ohm passes one ampere when one volt is applied. Common multiples: kΩ (1000 Ω) and MΩ (1,000,000 Ω).
Does Ohm's Law apply to AC circuits?
Directly for DC and purely resistive AC. With capacitance or inductance, use impedance Z instead of R: V = I·Z.
What is a real example of Ohm's Law?
An LED resistor: a 5 V supply, 2 V LED at 20 mA needs R = 3/0.02 = 150 Ω, dissipating 0.06 W — a ¼ W resistor suffices.
Conclusion
Ohm's Law — V = I × R — plus the power equations is the toolkit behind every circuit. Know any two of voltage, current, resistance and power, and you can find the rest. Keep your units consistent, respect resistor power ratings, and switch to impedance for reactive AC, and you'll handle the vast majority of everyday electrical calculations. Use the calculator above for instant, validated answers.
For more engineering, simulation and fluid-mechanics tutorials plus free calculators, explore Free CFD Tutorial. If this tool helped you, please share it with your fellow students and engineers.
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