Ohm's Law Calculator: Complete Guide to Voltage, Current, Resistance & Power
The Ohm's Law calculator is one of the most fundamental tools in electrical and electronics engineering. Rooted in Georg Simon Ohm's landmark 1827 discovery, Ohm's Law defines the precise relationship between voltage, current, and resistance in any linear electrical circuit. Whether you are a student learning circuit theory, a hobbyist prototyping with Arduino or Raspberry Pi, or a professional engineer designing power systems, mastering Ohm's Law is the essential first step.
What is Ohm's Law?
Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage applied across it and inversely proportional to its resistance. In mathematical form: V = I × R, where V is voltage in volts (V), I is current in amperes (A), and R is resistance in ohms (Ω). This deceptively simple equation underpins virtually all circuit analysis performed worldwide every day.
The Four Core Ohm's Law Formulas
When power is involved, three equivalent formulas apply: P = V × I (voltage times current), P = I² × R (current squared times resistance), and P = V² / R (voltage squared divided by resistance). Choose the formula based on which two values you already know.
Practical Examples Using This Calculator
- LED resistor sizing: Supply = 5 V, LED forward voltage = 2 V, desired current = 20 mA → drop = 3 V → R = 3 / 0.02 = 150 Ω
- Household circuit: 230 V mains, 10 A load → Power = 230 × 10 = 2,300 W (2.3 kW)
- Battery drain check: 9 V battery, 470 Ω resistor → I = 9 / 470 ≈ 19.1 mA, P ≈ 172 mW
- Motor driver: 12 V supply, 0.8 A motor → R ≈ 15 Ω, P = 9.6 W
How to Use This Ohm's Law Calculator
Select which quantity you want to solve for using the tabs at the top of the calculator. Enter any two known values and the live preview will instantly compute all four quantities. For engineering workflows requiring many calculations, use the bulk upload feature — format each line as solve_for,known1_name,value,known2_name,value (for example, V,I,0.5,R,100) and process the entire file in one click. Download results as CSV for further analysis.
Ohm's Law Limitations & Practical Tips
Ohm's Law applies to linear, resistive elements at constant temperature. Non-linear components such as diodes, transistors, and LED junctions do not strictly obey Ohm's Law — their resistance changes with voltage and current. Inductors and capacitors introduce reactive impedance (Z) at AC frequencies. For AC circuits, use the generalised form V = I × Z where Z is complex impedance. In DC circuits, always account for internal resistance of batteries and wire resistance for long cable runs. This calculator assumes ideal conditions; real-world results should always be verified by measurement.