LED Resistor Calculator
Calculate led resistor step by step: R = (Vsupply − n × Vf) ÷ I_LED; resistor power P = (Vsupply − n × Vf) × I_LED.
In short
Formula: R = (Vsupply − n × Vf) ÷ I_LED; resistor power P = (Vsupply − n × Vf) × I_LED.
Resistor value
150 Ω
- Voltage across resistor
- 3 V
- Resistor power
- 60 mW
- Suggested rating (2× power)
- 120 mW or more
What this calculator does
Calculate led resistor step by step: R = (Vsupply − n × Vf) ÷ I_LED; resistor power P = (Vsupply − n × Vf) × I_LED. Worked example, questions and limitations included.
Use it to turn Supply voltage, LED forward voltage, LED current, LEDs in series into a checked result you can compare, copy, or rerun with different assumptions.
The page shows the formula, a numeric worked example, and the assumptions that affect this electrical calculation.
Inputs and what they mean
- Supply voltage
- — number value.
- LED forward voltage
- — From the LED datasheet.
- LED current
- — From the LED datasheet.
- LEDs in series
- — number value.
How to use it
- Enter each value in the unit shown next to the box (use the unit converter first if your numbers are in other units).
- Check the breakdown to see every intermediate step.
- Read the limitations before relying on the result.
Formula
Current entered in mA is converted to amperes.
The supply must exceed the combined forward voltage.
Choose the next standard resistor value above the result to keep current at or below the target.
R = (Vsupply − n × Vf) ÷ I_LED; resistor power P = (Vsupply − n × Vf) × I_LED.
Inputs used: Supply voltage, LED forward voltage, LED current, LEDs in series.
Worked example
Red LED on 5 V at 20 mA
- Resistor voltage = 5 − 2 = 3 V.
- R = 3 ÷ 0.02 = 150 Ω.
- P = 3 × 0.02 = 60 mW.
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
Limitations and assumptions
- Forward voltage varies with temperature and batch.
- For high-power LEDs, a constant-current driver is better than a resistor.
- Supply ripple is ignored.
- Results are estimates for planning and learning. Real designs, installations and bids must be checked by a qualified, licensed professional against the codes that apply where you work.
- The result depends on the values you enter for this led resistor calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.
Common questions
What if the value isn't a standard resistor?
Pick the next higher standard value. The LED will run slightly dimmer but within its current limit.
Can I share one resistor between parallel LEDs?
It is not recommended. Forward voltages differ slightly, so one LED can hog current. Give each parallel LED its own resistor.
Where do forward voltage and current come from?
The LED's datasheet. They vary by colour and type, so no value is assumed here.
How do I use the LED Resistor Calculator?
Enter the required values for Supply voltage, LED forward voltage, LED current, LEDs in series. The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.
What formula does the LED Resistor Calculator use?
R = (Vsupply − n × Vf) ÷ I_LED; resistor power P = (Vsupply − n × Vf) × I_LED. The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.
Can the LED Resistor Calculator be used for exact decisions?
Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.
Related tools
Calculate electrical power step by step: P = V × I = I² × R = V² ÷ R (DC or resistive AC using RMS values).
Calculate voltage divider step by step: Vout = Vin × R2 ÷ (R1 + R2).
Calculate resistance step by step: R = V ÷ I, or for a conductor R = ρ × L ÷ A (A in m²).