Ohm's Law Calculator

About the Ohm's Law Calculator

An Ohm's Law calculator instantly solves for any one of four electrical quantities — voltage (V), current (I), resistance (R), or power (P) — when any two are known, using the complete set of relationships derived from Ohm's Law and the power equation. Ohm's Law (V = I × R) is the most fundamental relationship in electrical engineering, used by electricians, electronics engineers, physics students, hobbyists building circuits, and makers designing Arduino, LED, and sensor projects. Our calculator covers all twelve mathematical combinations derived from the four Ohm's Law variables, shows which specific formula was applied, converts between common units (ohms/kilohms/megohms; amps/milliamps/microamps; volts/millivolts; watts/milliwatts), and includes a practical guide to common circuit calculations like LED current-limiting resistor sizing.

How It Works

Ohm's Law: V = I × R. Power equations: P = V × I = I² × R = V² / R. From any two known quantities, find the other two: Given V and R: I = V/R; P = V²/R. Given V and I: R = V/I; P = V×I. Given V and P: I = P/V; R = V²/P. Given I and R: V = I×R; P = I²×R. Given I and P: V = P/I; R = P/I². Given R and P: V = √(P×R); I = √(P/R). Example: LED circuit with 5V supply, red LED (V_forward = 2.0V, I_forward = 20 mA). Voltage across resistor = 5.0 − 2.0 = 3.0V. Required resistance = V/I = 3.0V / 0.020A = 150Ω. Power dissipated by resistor = I²×R = (0.020)² × 150 = 0.06W = 60mW — well within a standard 1/4W resistor's rating.

Tips & Best Practices

• ✓LED resistor sizing: always calculate! A 5V supply directly connected to an LED (V_forward ≈ 2V) would draw: I = (5-2)/0 = infinite (theoretically). The resistor limits current to the safe operating value. R = (V_supply − V_forward) / I_target.
• ✓Short circuit: if R approaches 0, current I = V/R approaches infinity. This is why short circuits are so dangerous and why fuses and circuit breakers are essential protection — they interrupt the circuit before excessive current causes fire.
• ✓Open circuit: if R approaches infinity (circuit broken), I = V/∞ = 0. No current flows regardless of voltage. Measuring high resistance between two circuit nodes identifies an open (broken) connection.
• ✓Ohm's Law is only exactly valid for linear resistive elements. Non-linear devices (diodes, LEDs, transistors) don't follow it directly — but Ohm's Law still applies to the resistive elements in circuits containing non-linear components.
• ✓Power handling: a resistor's power rating must exceed the calculated P = I²R or V²/R. Safety margin: use a resistor rated at least 2× the calculated dissipation. A circuit dissipating 0.06W needs at least a 1/8W (0.125W) resistor; use 1/4W for margin.
• ✓Kirchhoff's Voltage Law (KVL): the sum of all voltages around any closed loop equals zero. This extends Ohm's Law to complex multi-component circuits where direct application isn't possible.
• ✓Thevenin's theorem: any complex circuit can be simplified to a single voltage source (V_th) and series resistance (R_th) for analysis purposes — a powerful tool that relies on Ohm's Law at every step.
• ✓AC vs DC Ohm's Law: for AC circuits, resistance is replaced by impedance (Z), which includes inductive reactance (X_L = 2πfL) and capacitive reactance (X_C = 1/2πfC). The magnitude relationships remain Ohm's Law analogous: |V| = |I| × |Z|.

Who Uses This Calculator

Electronics students solving circuit analysis problems and building intuition about voltage, current, and resistance. Arduino, Raspberry Pi, and breadboard hobbyists calculating component values for LED, sensor, and motor circuits. Electricians verifying circuit calculations for NEC compliance. Ham radio operators designing matching networks and amplifier stages. Physics students completing electricity and circuits coursework. Teachers demonstrating fundamental electrical relationships with concrete numbers.