Ohm's Law Calculator
Free online Ohm's Law calculator. Enter any two of Voltage, Current, Resistance, or Power and compute the other two instantly. Includes visual triangle diagram, formula display, and series/parallel resistance calculator.
What is Ohm's Law?
Ohm's Law is the fundamental relationship between Voltage (V), Current (I), and Resistance (R) in an electrical circuit. First published by German physicist Georg Simon Ohm in 1827, the law states that the current through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance.
In formula form: V = I × R. This simple equation is the foundation of all circuit analysis and design in electrical engineering, industrial automation, and electronics. When combined with the Power Law (P = V × I), you can solve for any variable in a DC circuit given any two known quantities.
Ohm's Law & Power Formula — All 12 Permutations
The table below lists every possible way to express the relationship between Voltage (V), Current (I), Resistance (R), and Power (P):
| Unknown | Formula | Known Variables |
|---|---|---|
| Voltage (V) | V = I × R | Current & Resistance |
| Voltage (V) | V = P / I | Power & Current |
| Voltage (V) | V = √(P × R) | Power & Resistance |
| Current (I) | I = V / R | Voltage & Resistance |
| Current (I) | I = P / V | Power & Voltage |
| Current (I) | I = √(P / R) | Power & Resistance |
| Resistance (R) | R = V / I | Voltage & Current |
| Resistance (R) | R = V² / P | Voltage & Power |
| Resistance (R) | R = P / I² | Power & Current |
| Power (P) | P = V × I | Voltage & Current |
| Power (P) | P = V² / R | Voltage & Resistance |
| Power (P) | P = I² × R | Current & Resistance |
Practical Use Cases
- Circuit Design: Choose the correct resistor value and power rating to safely limit current in a circuit. For example, with a 12V supply and a desired 20mA through an LED, Ohm's Law gives R = 12V / 0.02A = 600Ω.
- Troubleshooting: Measure voltage across a known resistor with a multimeter, then calculate current via I = V / R — no need to break the circuit to insert an ammeter.
- LED Resistor Calculation: Given a supply voltage (Vs), LED forward voltage (Vf), and desired LED current (If):
R = (Vs - Vf) / If. Example: Vs=12V, Vf=2V, If=20mA → R = (12-2)/0.02 = 500Ω. - Load Calculation: Determine how much power a heater or motor draws. If a 10Ω heater runs on 230V, P = V²/R = 230²/10 = 5290W (approximately 5.3kW).
- Wire Sizing: Calculate voltage drop along a cable from its resistance per meter and the load current, then size the conductor accordingly using V = I × R.
- Industrial Control Panels: Verify that power supply ratings, fuse ratings, and contactor coil currents are correctly matched using the power law P = V × I.
Frequently Asked Questions
What is the difference between power and energy?
Power (P), measured in Watts (W), is the rate at which electrical energy is transferred or consumed. Energy (E), measured in Watt-hours (Wh) or Joules (J), is the total amount of work done over time: E = P × t. A 100W light bulb running for 10 hours consumes 1000Wh (1kWh) of energy.
How do I calculate the right resistor for an LED?
Use Ohm's Law with the formula R = (Vs - Vf) / If, where Vs is your supply voltage, Vf is the LED's forward voltage (typically 2-3V for red, 3-3.6V for white/blue), and If is the desired forward current (typically 20mA = 0.02A). Also check the resistor's power rating: P = I² × R. For example, a 5V supply with a 2V red LED at 20mA: R = (5-2)/0.02 = 150Ω, and P = 0.02² × 150 = 0.06W, so a standard 1/4W resistor is fine.
Can Ohm's Law be used for AC circuits?
Yes, but with important modifications. In AC circuits, impedance (Z) replaces resistance (R), and the formula becomes V = I × Z. Impedance includes both resistance and reactance (from inductors and capacitors) and is frequency-dependent. For purely resistive AC loads (like heaters or incandescent bulbs), Ohm's Law applies directly using RMS values. For circuits with motors, transformers, or capacitors, you need to account for power factor and phase angle.
Why are there two power formulas — P = V × I and P = I² × R?
Both formulas give the same result; they are algebraically equivalent through Ohm's Law. Use P = V × I when you know voltage and current (common for power supplies and appliances). Use P = I² × R when analyzing resistive losses in wires and components — this form clearly shows that doubling the current quadruples the heat generated, which is why overcurrent protection is critical in industrial wiring.