LED Current Limiting Resistor Calculator | Series Resistor
Calculate the correct current limiting resistor for an LED. Enter supply voltage, LED forward voltage, and LED current to get the required resistor value and power rating.
How to Use
- Enter the supply voltage (Vs) — this is your power source voltage (e.g. 5V, 12V, 3.3V).
- Enter the LED forward voltage (Vf) — check your LED datasheet (typically 1.8–3.5V depending on color).
- Enter the desired LED current in mA — typically 10–20mA for standard LEDs, 1–5mA for low-power indicator LEDs.
- Click 'Calculate' to get the required resistance value R = (Vs − Vf) / I.
- Select the next higher standard resistor value (E12/E24 series) for your circuit.
- Use a resistor rated at least 2× the calculated power dissipation (P = I² × R) for safety.
About LED Circuits
Why LEDs Need Current Limiting Resistors
LEDs are not resistive devices — they have a nearly fixed forward voltage drop (Vf) regardless of current. Without a current limiting resistor, even a small increase in voltage causes an exponential rise in current, quickly destroying the LED. The series resistor absorbs the excess voltage (Vs − Vf) and limits current to a safe level. This simple R + LED series circuit is the most common LED driving method.
Forward Voltage by LED Color
Different LED colors have different forward voltages: Red ≈ 1.8–2.2V, Yellow/Orange ≈ 2.0–2.2V, Green ≈ 2.0–3.5V (varies by technology), Blue ≈ 3.0–3.5V, White ≈ 3.0–3.5V (white LEDs use a blue chip with phosphor). UV LEDs can have Vf up to 4V. Always check the datasheet for the specific LED you are using.
Calculating the Resistor
The formula is R = (Vs − Vf) / I, where Vs is supply voltage, Vf is LED forward voltage, and I is desired current in amps. Power dissipated in the resistor is P = (Vs − Vf) × I = I² × R. For example, a 5V supply with a red LED (Vf=2V) at 20mA: R = (5−2)/0.02 = 150Ω, P = 3×0.02 = 60mW. Use a 0.25W resistor (safety margin).
Multiple LEDs in Series
For multiple LEDs in series, subtract the sum of all forward voltages: R = (Vs − ΣVf) / I. Ensure the supply voltage exceeds the total Vf sum. For parallel LEDs, each LED should have its own series resistor to balance current, as slight Vf differences cause uneven current sharing. Never connect LEDs in parallel without individual resistors.
Key Features
- Calculates exact resistor value using R = (Vs − Vf) / I
- Shows minimum power rating and 2× safety recommendation
- Suggests nearest standard E12/E24 resistor value
- Covers all LED types: standard, high-power, RGB, indicator
Common Applications
- Indicator LEDs on PCBs and panel mounts
- Arduino and Raspberry Pi LED projects
- Status indicator lights in industrial equipment
- LED lighting prototypes and hobby electronics
- Educational electronics labs