What this guide helps you decide
A basic LED series-resistor estimate subtracts the entered forward voltage from the supply and divides the remainder by the target current. Forward voltage is not a universal constant; it changes with device, current, and temperature.
Choosing the next higher available resistance generally reduces current. Resistor power and supply variation must be checked separately, and high-power LEDs commonly need regulated current rather than this simple model.
Calculate resistance, current, and dissipation together
For one LED and one resistor, R = (Vs − Vf)/I. Then verify actual current with the chosen standard resistor and calculate resistor power with I²R or voltage drop × current.
- Confirm supply voltage exceeds the selected LED forward-voltage assumption.
- Convert milliamps to amps before division.
- Calculate ideal resistance and choose an available value using a documented rounding direction.
- Recalculate current and resistor dissipation for the chosen part and supply range.
Worked scenario: one indicator LED
A 12 V source, assumed 2 V LED forward drop, and 20 mA target current are entered.
- Resistor voltage: 12 − 2 = 10 V; current = 0.020 A.
- Ideal resistance: 10/0.020 = 500 Ω; choosing 560 Ω gives about 17.9 mA under these assumptions.
- Resistor dissipation at 560 Ω is about 0.179 W, before applying an appropriate design margin and rating check.
Outcome: The simple model suggests 560 Ω as a conservative available value, but actual LED data, supply tolerance, thermal conditions, and component ratings still control the design.
LED estimate checklist
- Use a forward-voltage value for the actual LED and current range.
- Convert mA to A explicitly.
- Recalculate current after selecting a standard resistor.
- Check resistor power with margin.
- Use a regulated-current design where the application requires it.
Limits and responsible use
- The model excludes LED I–V curvature, temperature, supply tolerance, pulsed operation, multiple-device current sharing, and thermal design.
- Voltage-drop and resistor results are not electrical-safety approval or a substitute for component datasheets and qualified design review.
Authoritative references
These links support the definitions, conventions, or safety boundaries used in this guide. CalculatorToolset wrote the explanation and example independently.
- PhysicsOpenStax, Rice University
- Guide for the Use of the International System of Units (SI)National Institute of Standards and Technology
Frequently asked questions
Why choose a higher standard resistance?
In this simple model, higher resistance lowers current, which is usually a conservative direction when the exact ideal value is unavailable.
Can several parallel LEDs share one resistor?
That can produce unequal current sharing because forward voltages differ. Each branch or a regulated driver needs appropriate analysis.