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General EE Utilities

LED RESISTOR CALCULATOR

Calculate the current limiting resistor for LEDs. Enter supply voltage, LED forward voltage, and desired current to find the right resistor value.

Calculate the correct current limiting resistor for your LED circuit. Enter the supply voltage, LED forward voltage, and desired current — the calculator shows the exact resistance, nearest E24 standard values, power dissipation, and a circuit diagram. Supports single and series LED configurations.

LED Presets

Circuit Parameters

Max 2 at 5V

Results

Required Resistance

150 Ω

Exact: 150.00 Ω

Next Lower (E24)

150 Ω

I = 20.0 mA

Next Higher (E24) ✓

150 Ω

I = 20.0 mA

Voltage Across R

3.00 V

Total LED Vf

2.0 V

Power in Resistor

60.0 mW

Min Power Rating

1/16 W

Circuit Diagram

5V+R = 150 Ω

Common LED Forward Voltages

LED ColorTypical VfRangeTypical If
Red2.0V1.8 – 2.2V20 mA
Orange2.1V1.9 – 2.2V20 mA
Yellow2.1V1.9 – 2.3V20 mA
Green2.2V2.0 – 3.5V20 mA
Blue3.2V2.8 – 3.6V20 mA
White3.2V3.0 – 3.6V20 mA
IR (940nm)1.2V1.0 – 1.6V20 mA
UV (405nm)3.4V3.2 – 3.8V20 mA

Values are typical for standard through-hole LEDs. High-brightness and power LEDs may have different specifications. Always check the manufacturer's datasheet.

Formulas Used

This tool computes exact resistance, E24 practical choices, resulting current, and resistor power dissipation for series LED strings.

\[R = \frac{V_{supply} - nV_f}{I_f}\]

\[P_R = (V_{supply} - nV_f) \times I_f\]

\[I_{actual} = \frac{V_{supply} - nV_f}{R_{selected}}\]

\[n_{max} = \left\lfloor\frac{V_{supply} - 0.5V}{V_f}\right\rfloor\]

Uses 0.5V minimum headroom for resistor regulation (same assumption as calculator logic).

n = number of series LEDs
Vf = forward voltage per LED
If = target LED current.

Why LEDs Need Current Limiting Resistors

LEDs are current-driven devices, not voltage-driven. Unlike incandescent bulbs that self-regulate through increasing resistance as they heat up, an LED's resistance drops as it warms — creating a positive feedback loop that leads to thermal runaway and destruction. A current limiting resistor breaks this cycle by providing a stable voltage drop that scales with current.

The calculation is straightforward: subtract the LED's forward voltage (Vf) from the supply voltage to find the voltage that must be dropped across the resistor, then divide by the desired current. The result gives you the exact resistance needed. In practice, you'll round up to the nearest standard resistor value to ensure the LED runs at or slightly below its rated current.

When running multiple LEDs in series, the forward voltages add together. This is more efficient than using individual resistors for each LED, as the single resistor dissipates less total power. However, the supply voltage must be higher than the sum of all forward voltages — otherwise there's no headroom for the resistor to regulate current.

For aerospace and defense indicator circuits, LED selection involves more than just color. Specifications for radiation tolerance, operating temperature range (-55°C to +125°C), and MTBF must be verified against mission requirements. Calpak USA's engineering team can help design indicator and illumination circuits for MIL-spec applications. Contact us for design support.

Quick Reference: Common LED Resistor Values

Standard 20mA LEDs with common supply voltages

LED Color3.3V Supply5V Supply12V Supply
Red (2.0V)68Ω150Ω510Ω
Green (2.2V)56Ω150Ω510Ω
Blue (3.2V)6.8Ω91Ω430Ω
White (3.2V)6.8Ω91Ω430Ω

Values use nearest E24 standard resistor (rounded up). Actual current will be slightly below 20mA. Always verify with the LED manufacturer's datasheet.

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