03 // Bidirectional STEM Calculators // ENGINEERINGR = (V_s - V_f) / I_f

LED Current-Limiting Series Resistor Calculator

Because an LED has an exponential diode I-V curve, a series ballast resistor R must drop the excess supply voltage (V_s - V_f) at the desired forward operating current I_f: R = (V_s - V_f) / I_f.

R = (V_s - V_f) / I_f

Pick which value you want to calculate (or type into any field):Solving for: R — Series Limiting Resistor

Series Limiting ResistorCalculated

Required ballast resistance in Ohms

1100,000.00

DC Supply Voltage

Power rail voltage in Volts

1.5100

LED Forward Voltage Drop

Diode forward voltage in Volts (Red ≈ 2.0V, White/Blue ≈ 3.2V)

150

LED Forward Current

Target LED drive current in Amperes (0.02 A = 20 mA)

0.00055

Calculated Result · Series Limiting Resistor (R)

150Ω

Converted units:

Step-by-Step Calculation

01Canonical Law: R = (V_s - V_f) / I_f

02Given Inputs (Base Units): V_s = 5 V, V_f = 2 V, I_f = 0.02 A

03Solve for R (Series Limiting Resistor): R = 150 Ω

Because an LED has an exponential diode I-V curve, a series ballast resistor R must drop the excess supply voltage (V_s - V_f) at the desired forward operating current I_f: R = (V_s - V_f) / I_f.

Real-World Reference Values

Red / Amber AlGaInP LEDV_f = 1.8V – 2.2V @ 20 mA

150 Ω resistor on 5V rail

Blue / White InGaN LEDV_f = 3.0V – 3.4V @ 20 mA

91 Ω – 100 Ω resistor on 5V rail

Resistor Power RatingP_R = (V_s - V_f) × I_f

Use ≥ 2× calculated wattage for thermal derating

20-Row Comparison Table

Series Limiting Resistor (R) · DC Supply Voltage (V_s)

Compare across:

LED Current-Limiting Series Resistor Calculator — DC Supply Voltage → Series Limiting Resistor
#	DC Supply Voltage (V)	Series Limiting Resistor (Ω)	Δ %
01	1.5 V	0 Ω	-100.00%
02	2.397 V	19.852 Ω	-86.77%
03	3.2941 V	64.7039 Ω	-56.86%
04	4.1911 V	109.5559 Ω	-26.96%
05	5 VCurrent	150 Ω	0.00%
06	5.9852 V	199.2599 Ω	+32.84%
07	6.8822 V	244.1118 Ω	+62.74%
08	7.7793 V	288.9638 Ω	+92.64%
09	8.6763 V	333.8158 Ω	+122.54%
10	9.5734 V	378.6678 Ω	+152.45%
11	10.4704 V	423.5197 Ω	+182.35%
12	11.3674 V	468.3717 Ω	+212.25%
13	12.2645 V	513.2237 Ω	+242.15%
14	13.1615 V	558.0757 Ω	+272.05%
15	14.0586 V	602.9276 Ω	+301.95%
16	14.9556 V	647.7796 Ω	+331.85%
17	15.8526 V	692.6316 Ω	+361.75%
18	16.7497 V	737.4836 Ω	+391.66%
19	17.6467 V	782.3355 Ω	+421.56%
20	18.5438 V	827.1875 Ω	+451.46%

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Frequently Asked Questions

How do I enter LED current in milliamps (mA)?+

Switch the inline unit dropdown on I_f from Amperes (A) to Milliamperes (mA) and type 20 directly.

What happens if I connect multiple LEDs in series?+

Sum the forward voltages of all LEDs in the series string (V_f_total = N × V_f) and enter that sum as V_f.

When should I use a constant-current buck driver instead of a resistor?+

For high-power LEDs (>100 mA or >0.5W), a switching constant-current driver avoids wasting power as heat in a ballast resistor.