LED Resistor Calculator: Find the Right Series Resistor for Any LED
Calculate the correct series resistor value for LED circuits. Enter supply voltage, LED forward voltage, and forward current to get the exact resistor you need.
Connecting an LED directly to a power supply without a current-limiting resistor is the fastest way to kill it. LEDs don't regulate their own current — the resistor does that job. Getting the right value takes 30 seconds of math, but if you're building something with dozens of LEDs, the calculator handles the repetition.
The CalcHub LED Resistor Calculator gives you resistor value in ohms, nearest standard resistor, and power dissipation for any LED circuit.
The Formula
R = (Vs − Vf) ÷ IfWhere:
- R = resistor value (ohms)
- Vs = supply voltage (V)
- Vf = LED forward voltage (V)
- If = LED forward current (A)
Resistor power rating: P = (Vs − Vf) × If
Always use a resistor rated at least twice the calculated power dissipation.
Forward Voltage by LED Color
LED forward voltage varies by the semiconductor material used:
| LED Color | Typical Vf | Notes |
|---|---|---|
| Red | 1.8–2.2V | Lowest forward voltage |
| Orange | 2.0–2.2V | Similar to red |
| Yellow | 2.0–2.4V | |
| Green (standard) | 2.0–2.4V | |
| Green (high-brightness) | 3.0–3.4V | GaN-based |
| Blue | 3.0–3.5V | GaN-based |
| White | 3.0–3.6V | Blue LED + phosphor |
| Infrared | 1.2–1.6V | Varies significantly |
| UV (violet) | 3.2–3.8V |
Worked Examples
Example 1: Single red LED on 5V USB power- Vs = 5V, Vf = 2.0V, If = 20mA (0.020A)
- R = (5 − 2.0) ÷ 0.020 = 150 ohms
- Power: (5 − 2.0) × 0.020 = 0.06W → ⅛W resistor is fine
- Vs = 9V, Vf = 3.3V, If = 20mA
- R = (9 − 3.3) ÷ 0.020 = 285 ohms → use nearest standard: 270Ω or 300Ω
- Power: 5.7 × 0.020 = 0.114W → use ¼W resistor
- Vs = 12V, Vf = 3.2V, If = 25mA
- R = (12 − 3.2) ÷ 0.025 = 352 ohms → use 360Ω standard
- Power: 8.8 × 0.025 = 0.22W → use ½W resistor
E24 Standard Resistor Values
The calculator shows the nearest standard E24 value. Common values around the LED range:
100, 110, 120, 130, 150, 160, 180, 200, 220, 240, 270, 300, 330, 360, 390, 430, 470Ω
When choosing between two equally close values, pick the higher one — it slightly reduces current and brightness but extends LED life.
Multiple LEDs in Series
You can string LEDs in series with one resistor:
R = (Vs − (Vf1 + Vf2 + ... + Vfn)) ÷ If
For three white LEDs on 12V:
R = (12 − (3.2 + 3.2 + 3.2)) ÷ 0.020 = (12 − 9.6) ÷ 0.020 = 120 ohms
Make sure Vs is at least 1V higher than the sum of forward voltages. If it's not, the LED chain won't light reliably.
Multiple LEDs in Parallel
Parallel LEDs each get their own resistor — don't share one resistor across multiple parallel LEDs. Slight Vf variations cause unequal current sharing and some LEDs may burn out.
What happens if my resistor value is too low?
Too little resistance means too much current. The LED will be very bright initially and will overheat and burn out quickly. Some LEDs fail within seconds of over-current conditions.
Can I use a potentiometer (variable resistor) to control brightness?
Yes, for low-power applications. Set it to roughly the correct resistance, then fine-tune. The minimum resistance should be calculated to prevent over-current at maximum brightness setting.
Do I always need a resistor with an LED?
Not always — LED driver ICs and constant-current supplies regulate current directly, eliminating the need for a series resistor. For automotive, higher-power LEDs, and LED strips, purpose-built drivers are more efficient than resistor current limiting.
Related Tools
- Series-Parallel Resistance Calculator — complex resistor network analysis
- Power Consumption Calculator — total power draw from LED arrays
- Battery Life Calculator — estimate runtime for battery-powered LED projects