Part of Circuit and resistor calculators
Calculate the resistor value, dissipation, and a practical minimum wattage for a single LED.
Enter the supply, LED forward voltage, and desired LED current.
Typical current entries can use engineering notation, such as 5m, 10m, or 20m.
Real LED forward voltage changes with current, temperature, colour, and production tolerance.
A series resistor is a practical way to limit current through a low-power indicator LED when the supply voltage is known and current accuracy does not need a dedicated current regulator. The useful design check includes resistor value, resistor power, supply tolerance, and LED forward-voltage spread.
Size current-limiting resistors for simple low-current indicators.
Check current against both LED ratings and the source or sink capability of the driving pin.
Confirm the resistor wattage after choosing the nominal current and resistor value.
Enter supply voltage, LED forward voltage, and desired LED current. Current supports engineering notation such as 5m, 10m, or 20m for milliamps.
Use the voltage actually applied to the LED-resistor string, including regulator tolerance where relevant.
Use the LED forward voltage at the target current, not just a generic colour estimate.
Choose a current that fits the LED rating, brightness need, battery budget, and driver capability.
The calculator uses the nominal DC resistor model. It assumes one LED and one series resistor in the same current path.
Subtract the LED forward voltage from the supply voltage, then divide by target LED current.
The resistor only controls current using the voltage left over after the LED forward drop.
The resistor dissipates the voltage across it multiplied by LED current.
Unit: volts (V)
The voltage feeding the LED and series resistor path.
Unit: volts (V)
The LED voltage at the chosen current, usually read from the datasheet or measured.
Unit: amps (A)
The desired LED current through the LED and resistor string.
Unit: ohms (Ω)
The resistor value needed to set the target current at the nominal operating point.
The example below is checked against the same LED series resistor helper used by the calculator.
Design question: A 5 V rail drives a 2.0 V indicator LED at 10 mA. What series resistor and resistor rating are needed?
Inputs: Vsupply = 5 V, Vf = 2.0 V, If = 10 mA, margin = 2× for resistor power.
Resistor drop: Vresistor = 5 V - 2 V = 3 V.
Series resistance: R = 3 V / 10 mA = 300 Ω.
Resistor power: P = 3 V × 10 mA = 30 mW. With 2× margin, required power is 60 mW.
Suggested rating: the next listed standard rating above 60 mW is 0.063 W.
The nominal resistor value only matches the nominal LED forward voltage and supply voltage. Real LED current moves when those values move.
The calculator assumes one LED and one resistor in series. Multiple LEDs need the combined forward-voltage drop.
This is intended for simple current-limited LEDs, not high-power lighting or precision current regulation.
Check both LED and resistor temperature rise when current, ambient temperature, or enclosure heating is significant.
Follow the next check based on whether the concern is current, power, tolerance, or units.
Check resistor dissipation, margin, and a practical wattage rating.
Sanity-check voltage, current, resistance, and power around the LED path.
Convert mA, mW, SI prefixes, and related engineering units.
Follow related resistor, divider, and pull-up workflows.
Review forward-voltage spread, supply tolerance, and resistor power margin.
Use typical values for a first-pass estimate, but check minimum and maximum forward-voltage values when current limits, brightness range, or production tolerance matter. Low forward voltage usually gives higher current for the same resistor.
If the resistor only drops a small part of the supply voltage, changes in LED forward voltage or supply voltage create a larger percentage change in current. More resistor headroom usually makes the current less sensitive to LED variation.
This simple resistor model is best for low-current indicators and similar circuits. High-power LEDs normally need current regulation, thermal design, and datasheet operating limits checked directly.
Engineering reference
Uses the DC resistor drop between supply voltage and LED forward voltage to size a series resistor and power rating.
R = (Vs - Vf) / IfP = (Vs - Vf) * IfInputs: Vs = 5 V, Vf = 2 V, If = 20 mA
Outputs: R = 150 ohm, P = 60 mW before rating margin