LED Resistor Calculator

LED Resistor Formula

Worked Example

One 2 V LED at 20 mA from a 5 V supply:

1. R = (V_s − n V_f) ÷ I = (5 − 1 × 2) ÷ 0.02 A
2. R = 3 ÷ 0.02 = 150 Ω
3. P = (V_s − nV_f) × I = 60 mW

Current with that resistor: 20 mA · Power in the resistor: 60 mW · Resistor rating: 0.125 W is enough

GATE tip

Diode questions in GATE use the same idea: replace each conducting diode with its forward drop (0.7 V for silicon), then apply Ohm’s and Kirchhoff’s laws to what remains.

More Analog and Digital Electronics Calculators

• Op-Amp Gain Calculator

  Inverting −R_f/Rᵢₙ, non-inverting 1 + R_f/Rᵢₙ

  difference (R_f/Rᵢₙ)(V₂ − V₁); in-amp 1 + 2R/R_G; summing −(R_f/Rᵢₙ)ΣV

  Example: An inverting amplifier with Rᵢₙ = 10 kΩ, R_f = 100 kΩ and 0.2 V in: Voltage gain = -10.

  Open the op-amp gain calculator →

• ADC Resolution Calculator

  LSB = Vref / 2ⁿ, SNR = 6.02n + 1.76 dB

  code = floor(Vᵢₙ/LSB); ENOB = (SINAD − 1.76)/6.02

  Example: A 12-bit ADC with a 3.3 V reference converting 1.65 V: Resolution (1 LSB) = 0.805664 mV.

  Open the adc resolution calculator →

• 555 Timer Calculator

  f = 1/(ln 2 (R₁ + 2R₂)C) ≈ 1.44/((R₁ + 2R₂)C)

  duty = (R₁ + R₂)/(R₁ + 2R₂); monostable T = ln 3 × RC ≈ 1.1RC

  Example: An astable 555 with R₁ = 1 kΩ, R₂ = 10 kΩ and C = 10 µF: Frequency f = 6.86998 Hz.

  Open the 555 timer calculator →

• Decibel Calculator

  dB = 10 log₁₀(P₂/P₁) = 20 log₁₀(V₂/V₁)

  dBm = 10 log₁₀(P / 1 mW)

  Example: A power ratio of 100: Gain = 20 dB.

  Open the decibel calculator →

Frequently asked questions

Sources

• The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures .

Last reviewed: 2026-10-15. Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.