Resistance Calculator
Calculate electrical resistance, voltage, or current using Ohm's law, plus the power dissipated by the circuit.
How is Resistance Calculated?
Electrical resistance is calculated using Ohm's law: R = V / I, where V is the voltage in volts and I is the current in amps. The power dissipated by the circuit is P = V × I. Depending on which two values you know, the calculator can find resistance, voltage, or current.
Example
If a 12 V supply pushes a current of 2 A through a circuit, the resistance is 12 / 2 = 6 ohms, and the power is 12 × 2 = 24 watts.
Common Use Cases
- Choosing the correct resistor value for LEDs and other components.
- Checking that a circuit won't exceed the power rating of a component.
- Verifying the current drawn by an appliance from a known voltage and resistance.
Resistor Color Code Reference
- Black = 0
- Brown = 1
- Red = 2
- Orange = 3
- Yellow = 4
- Green = 5
- Blue = 6
- Violet = 7
- Grey = 8
- White = 9
FAQs
What does Ohm's law state?
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage and inversely proportional to the resistance: V = I × R.
How do I read a resistor color code?
Each color represents a digit (0-9), and the first two bands spell out the significant digits, the third is the multiplier, and the fourth is the tolerance. For example, brown-black-red = 1 0 × 100 = 1000 ohms.
Why does power matter when choosing a resistor?
A resistor converts electrical energy to heat. If the power (P = V × I) exceeds the resistor's power rating (usually 1/8 W to 1 W), it will overheat and can burn out.