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Physics

LED Resistor Calculator

R = (Vs − Vf) / If. Free online LED Resistor Calculator. Calculate led resistor online — fast, accurate, mobile-friendly, no signup needed.

R = (V_s − V_LED) / I_LED.
Resistor (Ω)
150

Derivation

  1. ├── 01GivenVs = 5, Vf = 2, If_ = 20
  2. ├── 02Formula(e.Vs-e.Vf) / (e.If_ / 1e3)
  3. ├── 03Substitute(e.5-e.2) / (e.20 / 1e3)
  4. └── 04Compute Resistor (Ω)150
Did you know?

Georg Simon Ohm published V = I·R in 1827 but was dismissed by German universities for 15 years before his work was accepted.

§01What is

Understanding the LED Resistor Calculator

The LED Resistor Calculator computes Resistor (Ω) from 3 inputs: source (v), led forward (v), led current (ma). R = (Vs − Vf) / If.

Physics is the toolkit for turning a real-world observation into a prediction. Whether it’s a falling object, a moving car, or a stressed beam, the equations here are the same ones every engineer relies on. The LED Resistor Calculator sits in that toolkit — it R = (Vs − Vf) / If. Enter your numbers above and the result updates instantly; every step of the math is shown in the Derivation panel so you can see exactly how the answer was reached.

§02The Formula

How it’s calculated

(e.Vs-e.Vf) / (e.If_ / 1e3)

Where

Vs
Source (V)
Vf
LED forward (V)
If_
LED current (mA)
§03Practical Example

Step-by-step walkthrough

Scenario

Apply the formula to a realistic set of inputs: Source (V) = 5, LED forward (V) = 2, LED current (mA) = 20.

  1. 01Start by noting the input — Source (V): 5.
  2. 02Start by noting the input — LED forward (V): 2.
  3. 03Start by noting the input — LED current (mA): 20.
  4. 04Substitute these values into the formula: (e.Vs-e.Vf) / (e.If_ / 1e3)
  5. 05Compute Resistor (Ω): the calculator returns 150.
  6. 06Cross-check the answer by opening the Derivation panel above — every line of math is shown so you can follow the computation end-to-end.
§04Variants

Common LED Resistor Problems

The formula gets rearranged depending on which variable you need. Here are the patterns you’ll run into in the real world — find the one that matches your problem and follow the worked steps.

01 · PATTERN

Source (V) halved

Vs = 2.5 (from 5)

Keep every other input at its default and halve the source (v). See how resistor (ω) responds.

  1. 01New Source (V): 2.5
  2. 02Baseline Resistor (Ω): 150
  3. 03New Resistor (Ω): 25
  4. 04Resistor (Ω) decreases by 83.3% → use this sensitivity to plan for real-world variation.
02 · PATTERN

Source (V) doubled

Vs = 10 (from 5)

Keep every other input at its default and double the source (v). See how resistor (ω) responds.

  1. 01New Source (V): 10
  2. 02Baseline Resistor (Ω): 150
  3. 03New Resistor (Ω): 400
  4. 04Resistor (Ω) increases by 166.7% → use this sensitivity to plan for real-world variation.
03 · PATTERN

LED forward (V) halved

Vf = 1 (from 2)

Keep every other input at its default and halve the led forward (v). See how resistor (ω) responds.

  1. 01New LED forward (V): 1
  2. 02Baseline Resistor (Ω): 150
  3. 03New Resistor (Ω): 200
  4. 04Resistor (Ω) increases by 33.3% → use this sensitivity to plan for real-world variation.
04 · PATTERN

LED forward (V) doubled

Vf = 4 (from 2)

Keep every other input at its default and double the led forward (v). See how resistor (ω) responds.

  1. 01New LED forward (V): 4
  2. 02Baseline Resistor (Ω): 150
  3. 03New Resistor (Ω): 50
  4. 04Resistor (Ω) decreases by 66.7% → use this sensitivity to plan for real-world variation.
§05FAQ

Frequently asked questions

Yes. The calculator implements the standard formula as documented and returns exact floating-point results. No approximations are used unless noted in the formula.
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