Calculate the exact and standard commercial series current-limiting resistor required for your LED circuit, along with power rating recommendations.
Supply Voltage must be greater than LED Forward Voltage.
Required Series Resistor
150 Ω
(Closest Standard E24 Value)
Exact Calculated Value:150 Ω
Resistor Power Dissipation:60 mW
Suggested Resistor Rating:1/4 W (0.25 W)
How to Use
1
Select LED Color
Pick your LED color from the dropdown. This automatically fills in typical forward voltage (Vf) and forward current (If).
2
Set Supply Voltage
Enter your power supply voltage (Vs) (e.g. 5V, 9V, 12V). Make sure it is greater than the LED's forward voltage.
3
Get Resistor & Rating
Get the exact computed resistance, E24 standard value, and power rating recommendation instantly.
LED Resistor Explanation
Formula:R = (Vsource - Vforward) / Iforward
LEDs are current-driven devices. Connecting them directly to a voltage supply without a resistor will cause thermal runaway and destroy the LED. The series resistor absorbs the excess voltage and limits the current to a safe level (typically 20mA).
Typical Forward Voltage Drop Drop (Vf) table:
LED Color
Typical Voltage Drop (Vf)
Forward Current (If)
Red
1.8 V – 2.0 V
20 mA
Yellow / Orange
2.0 V – 2.2 V
20 mA
Green
2.1 V – 3.2 V
20 mA
Blue / White
3.2 V – 3.6 V
20 mA
LED Resistor Formula Table
Parameter
Formula / Description
Resistor Value (R)
R = (V_source - V_forward) / I_forward
Resistor Power (P)
P = I_forward² × R
Recommended Rating
Select a resistor with a power rating of at least 2 × Resistor Power (P) for safety margins.
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