Calculate the resistor needed for one or more LEDs in series from supply voltage, LED forward voltage, and target current, with resistor dissipation and wattage guidance.
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Lighting
An LED resistor calculator helps translate supply voltage, LED forward voltage, and target current into a practical resistor choice. This version also handles multiple LEDs in one series string and reports resistor dissipation plus a wattage recommendation with headroom.
This page sizes the current-limiting resistor for one or more LEDs in series from the supply voltage, the forward voltage of one LED, the number of LEDs in the string, and the desired operating current.
It also reports how much voltage the resistor must drop, how much power it dissipates, and a practical resistor wattage suggestion rather than stopping at the ideal ohm value alone.
Once the total LED forward drop is known, the resistor only needs to absorb the remaining voltage from the source. That remaining drop, divided by the target current, gives the resistor value.
If the supply voltage cannot exceed the total LED forward drop, a simple series resistor cannot establish the requested current and the calculator correctly rejects the setup.
R = (V_s - n × V_f) / I
Subtract the total forward drop of the LED string from the supply voltage, then divide the remaining drop by the desired LED current.
The resistor still turns part of the supply energy into heat, so resistor dissipation needs to be checked along with the target current. Choosing only the ohm value can still fail if the wattage is too low.
This calculator suggests a higher standard wattage with basic headroom so the result is immediately closer to a usable parts-bin choice.
P_R = V_R × I
Resistor dissipation follows from the resistor's own voltage drop and the current through the series string.
This calculator uses an ideal constant-current approximation around one forward-voltage point. It does not model LED I-V curves, temperature drift, resistor tolerance, PWM dimming, or supply variation.
Use it as a first-pass design and educational reference. If LED brightness, thermal behavior, or efficiency is critical, confirm the result against the LED data sheet and the real operating conditions.
Frequently asked questions
Because a simple series resistor only works when there is some voltage left over after the LED string forward drop. If the LEDs already consume the full supply voltage, there is no remaining drop for the resistor to control current.
Because the resistor must safely dissipate heat as well as provide the target resistance. A resistor with the correct ohm value can still overheat if its power rating is too small.
Only as a first-pass estimate. Higher-power LEDs are often better driven by a constant-current driver, and their thermal and electrical behavior is not captured by this simple resistor model.
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