Calculating your resistor value is straightforward once you know the LED Resistor formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the LED Resistor Calculator.
What is LED Resistor?
The LED Resistor calculation tells you your resistor value from a few simple inputs. The figure you are solving for here is the resistor value.
The LED Resistor formula
The core formula is:
Resistor value = (Supply voltage - LED forward voltage) ÷ (LED current ÷ 1000)Here is what each input means:
- Supply voltage — a value measured in V. Example: 9 V.
- LED forward voltage — a value measured in V. Example: 2 V.
- LED current — a value measured in mA. Example: 20 mA.
How to calculate it step by step
- Write down the supply voltage (for example, 9 V).
- Write down the led forward voltage (for example, 2 V).
- Write down the led current (for example, 20 mA).
- Apply the formula above to get your resistor value.
- Double-check the result with the LED Resistor Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Supply voltage | 9 V |
| LED forward voltage | 2 V |
| LED current | 20 mA |
| Resistor value | 350.0 |
| Resistor power dissipated | 0.1400 |
With supply voltage of 9 V, led forward voltage of 2 V and led current of 20 mA, the resistor value works out to 350.0.
Example 2
With supply voltage of 18 V, led forward voltage of 2 V and led current of 20 mA, the resistor value works out to 800.0.
| Result | Value |
|---|---|
| Resistor value | 800.0 |
| Resistor power dissipated | 0.3200 |
Example 3
With supply voltage of 4.5 V, led forward voltage of 2 V and led current of 20 mA, the resistor value works out to 125.0.
| Result | Value |
|---|---|
| Resistor value | 125.0 |
| Resistor power dissipated | 0.0500 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
Prefer not to do the maths by hand? — the LED Resistor Calculator does it instantly, for free, with the formula and a worked example built in.
Related calculators
Continue exploring electrical calculators with these tools: Wire Resistance Calculator, Watts to Amps Calculator, kVA to kW Calculator, kWh to Joules Calculator, Resistance from Power Calculator.