EngineerHub.orgHow to use this calculator
Follow the three steps below. Start with the example values, replace them with your own data, and read the result together with the assumptions and limitations.
Inputs
Results
Background
Engineering basis, equations, assumptions, interpretation and practical limitations for this calculator.
R = (Vₛ − nVf) / I
P = I²R
Select a resistor power rating comfortably above the calculated dissipation—commonly at least twice the calculated value. Each parallel branch needs its own resistor.
Frequently Asked Questions
Practical questions about assumptions, inputs, interpretation and limitations.
LEDs connected in one series string can share one current-limiting resistor because the same current flows through the entire string. Separate parallel LED branches should normally have their own resistor so current sharing is not left to LED forward-voltage differences.
That occurs when the total LED forward voltage is equal to or greater than the supply voltage. In that condition a simple series resistor cannot establish the requested current; the supply voltage or LED arrangement must change.
Choose a standard resistor power rating above the calculated dissipation and include margin for ambient temperature, enclosure conditions and component tolerance. Using at least about twice the calculated dissipation is a common conservative screening choice.
E12 is a common preferred-value resistor series. The calculator selects the next higher standard E12 resistance so the resulting LED current does not exceed the value calculated from the exact resistance.
No. Forward voltage varies with LED type, current, temperature and manufacturing tolerance. Use a representative datasheet value and allow margin when the circuit must remain within a specific current range.