Ohm's Law Calculator
Note about the Calculator
This calculator provides an approximation that is close enough for most practical LED projects. The actual formula for calculating the resistor value in an LED circuit is:
R = (Vs - Vf) / If
Where:
R = Required resistor value (Ω)
Vs = Supply voltage (V)
Vf = LED forward voltage drop (typically 1.8V-3.3V)*
If = LED forward current (typically 20mA = 0.02A)
*Common LED forward voltages:
Red: ~1.8V-2.1V
Green: ~2.0V-2.2V
Blue/White: ~3.0V-3.3V
The calculator above assumes a simplified calculation. For more precise results, subtract your LED's forward voltage (Vf) from your supply voltage before calculating.
Voltage (V)
Voltage is the electrical pressure or force that pushes electric current through a circuit. Think of it like water pressure in a pipe:
- Measured in Volts (V)
- Symbol: V or U
- Common sources: batteries, power supplies
Example: A 9V battery provides 9 volts of electrical pressure
Current (I)
Current is the flow of electric charge through a conductor. Like water flowing through a pipe:
- Measured in Amperes (A)
- Symbol: I
- Types: DC (Direct Current) and AC (Alternating Current)
Example: An LED might use 20mA (0.02A) of current
Resistor (R)
A resistor is a component that limits or controls the flow of electric current. Like a narrow section in a water pipe:
- Measured in Ohms (Ω)
- Symbol: R
- Common values: 100Ω to 1MΩ
Example: A 220Ω resistor is commonly used with LEDs
Ohm's Law
The relationship between Voltage, Current, and Resistance is defined by Ohm's Law:
V = I × R
Where:
V = Voltage (Volts)
I = Current (Amperes)
R = Resistance (Ohms)
Accurate LED Resistor Calculator
About the Calculators
The first calculator at the top provides a basic Ohm's law calculation. For LED circuits, the accurate calculator above uses the proper formula that accounts for the LED's forward voltage drop:
R = (Vs - Vf) / If
Where:
R = Required resistor value (Ω)
Vs = Supply voltage (V)
Vf = LED forward voltage drop (typically 1.8V-3.3V)*
If = LED forward current (typically 20mA = 0.02A)
*Common LED forward voltages:
Red: ~1.7V-2.1V
Green: ~2.0V-2.2V
Blue/White: ~3.0V-3.3V
Always choose a resistor value equal to or slightly higher than the calculated value. Never use a lower value as it may damage your LED.