Ohms Law Calculator
Free Ohms Law Calculator. Free online tool with accurate results using verified formulas. Includes worked examples, FAQ, and instant calculations.
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy
Ohms Law Calculator Formula
V = I × R | P = V × I | P = I²R | P = V²/R
Ohm's Law states that voltage equals current times resistance (V = IR). Combined with the power formula (P = VI), any two known electrical values can determine the other two. This calculator uses all 12 derived formulas from the Ohm's Law power wheel.
Ohms Law Calculator — Worked Examples
Example 1: LED Circuit Design
Problem:Calculate the resistor needed for a blue LED (3.2V forward voltage, 20mA) on a 9V battery.
Solution:Known: V_supply = 9V, V_LED = 3.2V, I = 20mA = 0.02A\nVoltage across resistor: V_R = 9 - 3.2 = 5.8V\nR = V / I = 5.8 / 0.02 = 290Ω\nNearest standard: 300Ω\nPower: P = V × I = 5.8 × 0.02 = 0.116W → Use 1/4W resistor
Result:R = 300Ω (1/4W) | I = 19.3mA | P = 0.116W
Example 2: Household Circuit
Problem:A 120V circuit has a 15A breaker. What is the maximum load resistance and power?
Solution:V = 120V, I_max = 15A\nMinimum resistance: R = V / I = 120 / 15 = 8Ω\nMaximum power: P = V × I = 120 × 15 = 1,800W\n80% rule: Continuous load should not exceed 80% = 1,440W (12A)
Result:R_min = 8Ω | P_max = 1,800W | Continuous max = 1,440W
Ohms Law Calculator — Frequently Asked Questions
What is Ohm's Law?
Ohm's Law is the fundamental relationship in electrical circuits that relates voltage (V), current (I), and resistance (R). Discovered by Georg Simon Ohm in 1827, it states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance: V = I × R. This means doubling the voltage doubles the current (if resistance stays the same), and doubling the resistance halves the current (if voltage stays the same). Ohm's Law is the foundation of all electrical engineering and is used in everything from designing simple LED circuits to planning power distribution systems.
What are the formulas for Ohm's Law?
The three basic Ohm's Law formulas are: V = I × R (voltage equals current times resistance), I = V / R (current equals voltage divided by resistance), R = V / I (resistance equals voltage divided by current). Adding power (P) gives additional formulas: P = V × I, P = I² × R, P = V² / R. From these you can derive: V = P / I, V = √(P × R), I = P / V, I = √(P / R), R = V² / P, R = P / I². The Ohm's Law wheel (or power wheel) arranges all 12 formulas in a circle for easy reference. You need any two known values to calculate the other two.
What are volts, amps, ohms, and watts?
These are the four fundamental electrical units: Voltage (V, volts) is the electrical pressure or potential difference that pushes electrons through a circuit — like water pressure in a pipe. Current (I, amps/amperes) is the flow rate of electrons — like the flow rate of water. Resistance (R, ohms/Ω) opposes current flow — like a narrow section in a pipe. Power (P, watts) is the rate of energy consumption — like the work done by the water flow. A helpful analogy: voltage is the height of a waterfall, current is how much water flows, resistance is rocks blocking the flow, and power is the energy of the falling water.
What is the difference between AC and DC in Ohm's Law?
Ohm's Law (V = IR) applies directly to DC (direct current) circuits. For AC (alternating current), the relationship becomes more complex because AC circuits include reactive components (inductors and capacitors) that create impedance (Z) rather than simple resistance. The AC version is V = I × Z, where Z is impedance measured in ohms but includes both resistance and reactance. For purely resistive AC loads (like heaters), Ohm's Law works the same as DC. For circuits with capacitors or inductors, you must account for phase angles and use complex numbers. Power calculations in AC also differ: real power (W) = V × I × cos(φ), where φ is the phase angle.