Ohm's Law Calculator

Ohm's Law is the fundamental relationship in electrical circuits that describes how voltage, current, and resistance interact. This calculator helps you solve for any unknown value when you know two of the three variables.

Understanding Ohm's Law

The Basic Formula

V = I × R

Where:

• V = Voltage (measured in Volts)
• I = Current (measured in Amperes)
• R = Resistance (measured in Ohms)

All Formulas Derived from Ohm's Law

Power Relationships

Power (P) measured in Watts can be calculated using:

The Ohm's Law Wheel

The Ohm's Law Wheel (also called the Power Wheel) is a visual tool that shows all 12 formulas relating V, I, R, and P:

Voltage Formulas

• V = I × R
• V = P ÷ I
• V = √(P × R)

Current Formulas

• I = V ÷ R
• I = P ÷ V
• I = √(P ÷ R)

Resistance Formulas

• R = V ÷ I
• R = P ÷ I²
• R = V² ÷ P

Power Formulas

• P = V × I
• P = I² × R
• P = V² ÷ R

History of Ohm's Law

Ohm's Law was discovered by German physicist Georg Simon Ohm in 1827. He published his findings in his book "Die galvanische Kette, mathematisch bearbeitet" (The Galvanic Circuit Investigated Mathematically).

Interestingly, his work was not immediately accepted, and it took several years before the scientific community recognized its importance. Today, the unit of electrical resistance—the Ohm (Ω)—is named in his honor.

Practical Applications

Electronics Design

• Selecting appropriate resistor values
• Designing voltage dividers
• Calculating LED current-limiting resistors
• Sizing power supplies

Electrical Safety

• Understanding how resistance affects current flow
• Calculating safe operating parameters
• Determining wire gauge requirements

Troubleshooting

• Testing circuit components
• Identifying shorts or open circuits
• Measuring unknown resistance values

Common Electrical Values

Standard Voltages

Wire Resistance (per 1000 ft copper)

Frequently Asked Questions

What is the difference between Ohm's Law and Watt's Law?

Ohm's Law relates Voltage, Current, and Resistance (V = IR). Watt's Law relates Power to these quantities (P = VI). Together, they form the complete set of formulas in the Ohm's Law Wheel.

Can Ohm's Law be used for AC circuits?

Yes, but for AC circuits, you need to consider impedance (Z) instead of just resistance (R). Impedance includes both resistance and reactance from capacitors and inductors.

What happens when resistance is zero?

With zero resistance (a short circuit), theoretically infinite current would flow. In practice, the power source has internal resistance, but short circuits can still cause dangerous current levels and fires.

Why do we use different wire gauges?

Thicker wires have lower resistance, allowing more current to flow without excessive heating. The wire gauge must be appropriate for the expected current to prevent fire hazards.

Note: This calculator assumes ideal DC circuits. For AC circuits, complex impedance calculations may be required. Always consult a qualified electrician for electrical work.