Ohms Law Calculator

Enter any two of Voltage (V), Current (I), Resistance (R), or Power (P) and instantly calculate the other two. Supports multiple units with full formula display. Free, private, no data leaves your browser.

Enter exactly 2 known values. Leave the other fields empty.

Ohm's Law Formulas (All 12)

V =
I × R
P / I
√(P × R)
I =
V / R
P / V
√(P / R)
R =
V / I
V² / P
P / I²
P =
V × I
V² / R
I² × R

Ohm's Law Wheel

V = I × R P = V × I P = I² × R V I × R P / I √(P×R) I V / R P / V √(P/R) R V / I V² / P P / I² P V × I I² × R V² / R
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How Ohms Law Works

Ohm's Law is the fundamental equation governing electrical circuits. It describes the relationship between voltage (V), current (I), and resistance (R) as V = I × R. This means voltage equals the product of current and resistance. When you know any two values, you can calculate the third. Power (P) extends this relationship: P = V × I, P = I² × R, and P = V² / R. These four quantities — voltage, current, resistance, and power — form the complete electrical calculation framework that every engineer, electrician, and hobbyist uses daily.

This Ohms Law calculator supports multiple units for each parameter. Enter voltage in volts or millivolts, current in amps, milliamps, or microamps, resistance in ohms, kilohms, or megohms, and power in watts, milliwatts, or kilowatts. The calculator automatically converts to base units, performs the calculation, and displays results in the most readable format.

Ohms Law Triangle Explained

The Ohm's Law triangle is a visual memory aid used by students and professionals alike. Arrange V at the top, I at the bottom-left, and R at the bottom-right. To find any value, cover it: if two values are side by side, multiply them; if one is above the other, divide the top by the bottom. For example, cover V and you see I × R (voltage equals current times resistance). Cover I and you see V over R (current equals voltage divided by resistance). Cover R and you see V over I (resistance equals voltage divided by current).

The power triangle works the same way with P at the top, V at the bottom-left, and I at the bottom-right. Combined, these give you all 12 Ohm's Law equations shown in the formula grid above. Memorizing the triangles lets you solve any DC circuit problem without looking up formulas.

Practical Applications of Ohms Law

Ohm's Law is essential in circuit design, troubleshooting, and component selection. Use it to size resistors for LED circuits — if your LED needs 20 mA at 2V from a 12V supply, calculate R = (12 - 2) / 0.02 = 500Ω. Use it to determine wire gauge by calculating current draw. Use power calculations to select properly rated components — a resistor dissipating more than its rated wattage will overheat and fail.

In home electrical systems, Ohm's Law helps understand why circuit breakers trip (too much current), why voltage drops over long cable runs, and how to calculate energy consumption. Electronics hobbyists use it constantly when working with Arduino, Raspberry Pi, and breadboard prototyping projects.

Ohms Law Units and Conversions

Voltage is measured in volts (V), with millivolts (mV) for small signals. Current is measured in amperes (A), with milliamps (mA) and microamps (µA) common in electronics. Resistance is measured in ohms (Ω), with kilohms (kΩ) and megohms (MΩ) for larger values. Power is measured in watts (W), with milliwatts (mW) for low-power circuits and kilowatts (kW) for industrial applications. This calculator handles all these unit conversions automatically.