LED Resistor Calculator
Enter the supply voltage, LED forward voltage, and desired forward current to calculate the correct current-limiting resistor value and wattage.
Typical: 10–20 mA for standard 5 mm LEDs; 350–700 mA for high-power LEDs
LEDs in series share the same current but their V_f values add up
—
Exact resistance
—
Nearest E24 value
—
Power dissipated
—
Min. resistor rating
How to Calculate the LED Resistor
The current-limiting resistor drops the excess voltage between the supply and the LED's forward voltage. Using Ohm's Law:
$$R = \frac{V_s - (V_f \times N)}{I_f}$$
Where:
- $V_s$ = supply voltage
- $V_f$ = LED forward voltage
- $N$ = number of LEDs in series
- $I_f$ = desired forward current in amps
The resistor dissipates power equal to $P = I_f^2 \times R$. Always use a resistor rated for at least twice the calculated wattage for thermal safety.
Typical LED Parameters
| LED type | Forward voltage (V_f) | Typical I_f | Notes |
|---|---|---|---|
| Red (5 mm) | 1.8–2.2 V | 20 mA | Most common indicator LED |
| Orange / Yellow | 1.9–2.2 V | 20 mA | |
| Green (standard) | 2.0–2.4 V | 20 mA | |
| Blue | 2.9–3.5 V | 20 mA | Requires higher V_s |
| White | 2.9–3.5 V | 20 mA | Blue die with phosphor |
| Infrared (IR) | 1.2–1.5 V | 20–100 mA | Remotes, sensors |
| High-power LED | 3.0–3.5 V | 350–700 mA | Needs heatsink |
LEDs in Series vs. Parallel
Series — the same current flows through all LEDs; their V_f values add up; one resistor for the whole string. Use this calculator with N = number of LEDs.
Parallel — each LED has its own current path; each needs its own resistor. Never connect LEDs in parallel with a shared resistor — small V_f differences cause current imbalance.
E24 Standard Resistor Values
When the calculated value doesn't match a standard resistor, choose the nearest higher value to keep current at or below the desired level. Common E24 values (×1 decade): 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91.
Frequently Asked Questions
Do I always need a resistor with an LED?
Yes, for any LED connected to a fixed voltage source, you need a current-limiting resistor (or another current-limiting method like a constant-current driver). Without one, the LED will draw excessive current, overheat, and fail within seconds.
Can I use a resistor that is too high in value?
Yes — a higher resistance than calculated will reduce the current below I_f, making the LED dimmer but not damaging it. This is acceptable for indicators where exact brightness is not critical.
What if my resistor value is 0 or negative?
The supply voltage equals or is below the total forward voltage of the LEDs in series. You need a higher supply voltage — you cannot add a resistor to make this circuit work.
