Ohm's Law Calculator
Calculate voltage, current, resistance, and power using Ohm's Law. Enter any two values to find the rest instantly.
Calculator
Enter any two values to calculate the rest
How to Use This Calculator
This Ohm's Law calculator helps you find voltage, current, resistance, or power when you know any two of these values. Simply enter two known values and click "Calculate" to find the remaining values.
- Enter any two known values (voltage, current, resistance, or power)
- Select the appropriate unit for each value (mA/A, Ω/kΩ, mW/W)
- Click "Calculate" to see all four values
- Use "Reset" to clear all fields and start over
Ohm's Law Formulas
Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. Combined with the power equation, we can derive any value from two known values.
Basic Formulas
Power Variations
- P = V × I — Power from voltage and current
- P = I² × R — Power from current and resistance
- P = V² / R — Power from voltage and resistance
Units
- Voltage (V) — measured in Volts (V)
- Current (I) — measured in Amperes (A) or milliamperes (mA)
- Resistance (R) — measured in Ohms (Ω) or kilohms (kΩ)
- Power (P) — measured in Watts (W) or milliwatts (mW)
Practical Examples
Example 1: LED Current Limiting
You have a 5V power supply and want to power an LED with a 2V forward voltage at 20mA. What resistor value do you need?
Given: V = 5V - 2V = 3V (voltage across resistor), I = 20mA
Formula: R = V / I
Result: R = 3V / 0.02A = 150Ω
Example 2: Power Dissipation
A 1kΩ resistor has 12V across it. How much power does it dissipate?
Given: V = 12V, R = 1000Ω
Formula: P = V² / R
Result: P = 144 / 1000 = 0.144W = 144mW
Example 3: Current Draw
A device consumes 2.5W at 5V. What is the current draw?
Given: P = 2.5W, V = 5V
Formula: I = P / V
Result: I = 2.5 / 5 = 0.5A = 500mA
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points. It was discovered by German physicist Georg Simon Ohm in 1827 and is fundamental to electrical and electronics engineering.
When does Ohm's Law not apply?
Ohm's Law applies to "ohmic" or linear devices where resistance remains constant. It doesn't apply to non-linear devices like diodes, transistors (in saturation), or components whose resistance changes with temperature, voltage, or current.
What's the difference between mA and A?
Milliamperes (mA) and Amperes (A) are both units of electrical current. 1 Ampere equals 1000 milliamperes. Electronics often use mA since currents are typically small (20mA for an LED, 500mA for a USB device).
How do I choose the right resistor power rating?
Calculate the power dissipation using P = I²R or P = V²/R, then choose a resistor rated for at least 2x that power to ensure reliability and prevent overheating. Common ratings are 1/8W, 1/4W, 1/2W, and 1W.
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