Ohm's Law Calculator
Instant electrical calculations for circuits.
Ohm's Law Calculator
Fill in any 2 values to calculate the remaining 2.
Ohm's Law Explained
Ohm's Law is the fundamental relationship in electrical circuits, connecting voltage (V), current (I), and resistance (R). Whether you're designing circuits, troubleshooting electronics, or calculating wire sizes, this law is essential.
The Three Forms
From V = I × R, we derive: I = V / R (to find current) and R = V / I (to find resistance). Combined with power (P = V × I), you can solve any basic circuit problem with just two known values.
Practical Examples
Example 1 — LED circuit: An LED needs 20mA at 2V forward voltage from a 12V supply. Required resistor: R = (12-2) / 0.020 = 500Ω. Use a standard 470Ω or 560Ω resistor.
Example 2 — Household circuit: A 2000W heater on 230V draws: I = P/V = 2000/230 = 8.7A. This is within the 16A rating of a standard European circuit breaker.
Example 3 — Speaker impedance: A 50W amplifier driving an 8Ω speaker produces: V = √(P×R) = √(50×8) = 20V RMS, with current I = 20/8 = 2.5A RMS.
Power Formulas
Power can be calculated three ways: P = V × I (voltage × current), P = I² × R (current squared × resistance), P = V² / R (voltage squared ÷ resistance). These are essential for sizing components, calculating heat dissipation, and determining energy costs.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that voltage (V) equals current (I) multiplied by resistance (R): V = I × R. It is the fundamental relationship in electrical circuits.
How do I calculate amps from volts and ohms?
Divide voltage by resistance: I = V / R. For example, 12 volts across 4 ohms gives 3 amps of current flow.
Can I calculate watts with this tool?
Yes, the calculator includes power formulas: P = V × I, P = I² × R, and P = V² / R. Enter any two values to find the others.
What units does the calculator support?
The calculator works with volts, amps, ohms, watts, milliamps, kilohms, and megohms with automatic unit conversion between them.