Ohms Law Calculator

Ohms Law Calculator: Enter Two, Get Four

Ohm's Law relates voltage (V), current (I), and resistance (R) through the equation V = I × R. The power equation connects wattage (P) to voltage and current through P = V × I (equivalently P = I²R or P = V²/R). Enter any two of the four values above and this calculator for ohms law derives the remaining two instantly. It covers every fundamental electrical calculation from sizing a current-limiting resistor in a hobby circuit to estimating the power draw of an industrial motor and selecting the correct wire gauge for a building installation.

Calculating Ohm's Law in Practice

LED resistor sizing: a standard indicator LED has a forward voltage drop of approximately 2V and a rated forward current of 20mA. On a 5V supply, the resistor must drop the remaining 3V at 20mA. Applying the formula: R = V / I = 3 / 0.020 = 150 ohms. Select the nearest standard value (150Ω is an E24 series value) rated for at least P = I²R = 0.020² × 150 = 0.06W, well within a standard quarter-watt resistor's capacity. Fuse selection: a 120V appliance rated at 1,440W draws I = P / V = 1,440 / 120 = 12A, requiring a 15A breaker (the next standard size above operating current). Wire gauge: a 12V circuit drawing 10A over 50 feet of cable needs wire thick enough that the voltage drop stays below 3% of source voltage - enter 12V and 10A above and use the resulting resistance to cross-reference AWG tables for the required gauge at the given length.

Series and Parallel Resistance

Resistors wired in series add directly: 100Ω + 200Ω + 300Ω = 600Ω total. The same current flows through all series elements, and voltage distributes proportionally to resistance - the largest resistor drops the most voltage. This principle underpins resistive voltage dividers used in sensor circuits and analog reference generation. Resistors wired in parallel combine as reciprocals: 1/R_total = 1/R1 + 1/R2 + 1/R3. Two identical resistors in parallel yield exactly half the individual value. In parallel paths, all components see the same voltage, and current splits inversely to resistance - the lower-resistance path carries more current. A short circuit is the extreme case: a near-zero-resistance parallel path attracts nearly all available current, tripping overcurrent protection or causing catastrophic failure if none exists.

Power Dissipation, Heat, and Component Ratings

Every resistive element converts electrical energy to heat at a rate of P = I²R watts. A standard through-hole resistor is rated for 0.25W. A 100Ω resistor carrying 50mA dissipates P = 0.050² × 100 = 0.25W - exactly at its thermal limit. Operating at the rating ceiling continuously shortens the component's lifespan and risks eventual failure. Exceeding the rating causes discoloration, smoke, and in extreme cases ignition of adjacent components or the PCB itself. The calculator shows power dissipation for any combination of inputs so you can verify that every component in the circuit operates within its thermal specification. When dissipation exceeds the rating, the options are a higher-wattage resistor (0.5W, 1W, 2W, 5W, 10W packages exist), a heatsink, or a circuit redesign that reduces the current through the component.

Ohms Law Calculator Wheel and Quick Reference

The classic ohms law calculator wheel (also called the Ohm's Law pie chart) arranges twelve formulas in a circular diagram. The center divides into four quadrants for V, I, R, and P. Each quadrant contains three equivalent formulas: V = IR = P/I = √(PR); I = V/R = P/V = √(P/R); R = V/I = V²/P = P/I²; P = VI = I²R = V²/R. Memorizing the wheel is traditional in electrical training, but entering two known values above and reading the other two is faster, eliminates arithmetic errors, and handles decimal values and unit conversions (milliamps, kilohms, microwatts) without manual prefix juggling.

AC Circuits and the Limits of Basic Ohm's Law

This tool applies the DC (direct current) form of Ohm's Law, accurate for batteries, solar panels, DC power supplies, LED circuits, automotive 12V systems, and all purely resistive loads. AC (alternating current) circuits introduce impedance: resistance plus reactive components from capacitors (capacitive reactance, frequency-dependent) and inductors (inductive reactance, also frequency-dependent). For purely resistive AC loads - incandescent bulbs, space heaters, stovetop elements - the DC formula applies directly. For motors, transformers, fluorescent ballasts, and any load with significant capacitance or inductance, the relationship between voltage, current, and impedance involves phase angles and power factor, requiring a dedicated AC impedance calculator that accounts for frequency, reactance, and the distinction between real power (watts), reactive power (VAR), and apparent power (VA).

Frequently asked questions