Resistor & Ohm's Law Calculator
Instantly decode resistor colors, size LED resistors, and calculate Ohm's law
Ultimate Guide to Resistor Color Codes & Ohm's Law
Whether you are a student, hobbyist, or an engineer building an Arduino or ESP32 project, calculating exact resistance is a daily task. Our Pro Resistor and Ohm's Law Calculator eliminates manual errors and provides instant, accurate values for your electronic circuits.
How to Read a 4-Band Resistor Color Code?
A standard 4-band resistor has four color stripes. Reading them is easy once you know the formula: (Band 1 & Band 2) × Multiplier = Resistance.
- Band 1 & 2 (Significant Digits): These represent the first two numbers. For example, Brown(1) and Black(0) give you the number "10".
- Band 3 (Multiplier): This tells you how many zeros to add (or multiply by). If the 3rd band is Red (x100), the calculation is 10 x 100 = 1000 Ohms (1 kΩ).
- Band 4 (Tolerance): Represents the accuracy of the resistor. A Gold band means the actual resistance can vary by ±5%.
Why Do LEDs Need Resistors?
If you connect an LED directly to a 5V or 9V battery, it will draw too much current and instantly burn out. To prevent this, a resistor is placed in series to limit the current. The formula used in our LED calculator is based on Ohm's Law: R = (Vs - Vf) / If
Vs is your power source (e.g., 5V Arduino pin). Vf is the LED's forward voltage (usually 2V for Red and 3.3V for Blue/White). If is the current, safely kept around 20mA for standard 5mm LEDs. The calculator also computes the Wattage (Power dissipation) so you know if you need a standard 1/4W resistor or a thicker one.
Understanding Ohm's Law Formulas
Ohm’s law states that the current passing through a conductor is directly proportional to the voltage and inversely proportional to the resistance. It forms the foundation of all electronics:
- Voltage (V) = I × R: Use this to find out how much voltage is dropping across a component.
- Current (I) = V / R: Helps you ensure your circuit isn't drawing more Amps than your power supply can handle.
- Resistance (R) = V / I: Perfect for finding out what resistor size is needed for a specific voltage and current requirement.
Tip for Makers: Always use the closest higher standard resistor value if your exact calculated value (e.g., 85Ω) is not available in the market. Using a 100Ω resistor is perfectly safe!