Ohm's Law Calculator: Voltage, Current, Resistance, and Power
Ohm's Law is the foundational relationship of electrical circuit analysis, describing how voltage, current, and resistance interact. First formulated by German physicist Georg Ohm in 1827, this simple law underlies the design and analysis of virtually every electrical circuit from household wiring to microprocessors. Our Ohm's Law Calculator solves for any unknown variable when two others are known.
Ohm's Law
V = I × R
Where: V = Voltage in Volts (V), I = Current in Amperes (A), R = Resistance in Ohms (Ω).
Rearranged: I = V ÷ R and R = V ÷ I.
Example: A circuit has 12V across a 4Ω resistor. Current = 12 ÷ 4 = 3A. Power dissipated = V × I = 12 × 3 = 36W.
The Power Equations
The four power relationships derived from Ohm's Law:
P = V × I (power in watts = voltage × current)
P = I² × R (power from current and resistance)
P = V² ÷ R (power from voltage and resistance)
These form the "Ohm's Law wheel" — given any two of V, I, R, P, you can find the other two.
Practical Applications
Fuse selection: A circuit supplying 2,000W at 230V draws I = 2,000 ÷ 230 ≈ 8.7A. Select a 10A fuse.
LED resistor calculation: LED forward voltage 2V, supply 5V, desired current 20mA. Required resistor R = (5−2) ÷ 0.02 = 150Ω.
Power dissipation: A 100Ω resistor with 5V across it dissipates P = 25 ÷ 100 = 0.25W — use a ½W rated resistor for safety margin.
Wire gauge selection: Higher current circuits need lower resistance wire (heavier gauge) to prevent voltage drop and heat generation in wiring.
AC vs. DC Circuits
Ohm's Law applies directly to DC (Direct Current) circuits. For AC (Alternating Current), the equivalent of resistance is called impedance (Z), which includes both resistance (R) and reactance (from capacitors and inductors). The relationship V = I × Z still holds, but calculations involve complex numbers. Our calculator handles DC circuits directly; AC impedance calculations are available in the advanced mode.
How to Use This Calculator
- Enter any two known values (Voltage, Current, Resistance, or Power) with their units.
- Click Calculate to find all remaining values.
Conclusion
Ohm's Law is the first tool every electronics student learns and the last tool every engineer puts down. Our Ohm's Law Calculator provides instant solutions for all four electrical quantities, making circuit analysis, component selection, and electrical problem solving fast and error-free for students, hobbyists, and professionals.