Answer first
What this calculator tells you
Calculate the current-limiting resistor for an LED and the next standard E12 value. Pick a resistor that lights an LED at the current you want, without burning it out. Formula: R = (supply voltage − LED forward voltage) ÷ LED current; power = current² × R; then round up to the next E12 value. At the worked-example inputs, the resistor needed (ohms) is 500. Holding every other input steady, moving supply voltage (v) from 10 to 14 moves the result from 400 to 600.
Transparent method
The formula
Pick a resistor that lights an LED at the current you want, without burning it out.
Worked example
Example inputs
How to interpret the result
An LED has almost no resistance of its own once it turns on, so a resistor must set its current. Subtract the LED's forward voltage from the supply and divide by the current you want. On 12 volts with a 2 volt LED at 20 milliamps that is 500 ohms. Resistors come in standard values, so you round up to the next one, 560 ohms, which runs the LED a little below its full current.
At the worked-example inputs the resistor needed (ohms) is 500. It rises with supply voltage (v) and falls as LED current (ma) and LED forward voltage (v) increase.
These are exact physics and chemistry formulas. Real-world results add tolerances from component quality, temperature and measurement error that this calculator does not model.
Before you rely on it
What to check
Look up the forward voltage on the LED's datasheet. Colors differ, and the wrong value shifts the current you get.
The common error
Where people go wrong with LED resistor calculator
Rounding down to the nearest standard value. That pushes more current through the LED than planned and shortens its life.
Sensitivity evidence
How supply voltage (v) changes the resistor needed (ohms)
Holding every other input at the worked-example value, moving supply voltage (v) from 10 to 14 moves the resistor needed (ohms) from 400 to 600: a spread of 200, or 40% of the worked-example result.
| Supply voltage (V) | Resistor needed (ohms) | Resistor power (watts) | Next standard E12 value (ohms) |
|---|---|---|---|
| 10 | 400 | 0.16 | 470 |
| 11 | 450 | 0.18 | 470 |
| 12worked example | 500 | 0.2 | 560 |
| 13 | 550 | 0.22 | 560 |
| 14 | 600 | 0.24 | 680 |
Every input, tested
Which input moves the resistor needed (ohms) most
Of the 3 inputs, supply voltage (v) moves the resistor needed (ohms) most (100 across the range tested) and LED forward voltage (v) moves it least (20).
| Input | Tested from | To | Resistor needed (ohms) at each end | Swing |
|---|---|---|---|---|
| Supply voltage (V) | 11 | 13 | 450 to 550 | 100 (20%) |
| LED current (mA) | 18 | 22 | 555.6 to 454.5 | 101 (20%) |
| LED forward voltage (V) | 1.8 | 2.2 | 510 to 490 | 20 (4.0%) |
Two variables at once
Resistor needed (ohms) by supply voltage (v) and LED forward voltage (v)
Across the grid the resistor needed (ohms) runs from 380 to 620. Moving supply voltage (v) from 10 to 14 shifts it by 200 at the middle column, and moving LED forward voltage (v) from 1.6 to 2.4 shifts it by 40 at the middle row, so supply voltage (v) is the bigger lever here.
| Supply voltage (V) \ LED forward voltage (V) | 1.6 | 2 | 2.4 |
|---|---|---|---|
| 10 | 420 | 400 | 380 |
| 11 | 470 | 450 | 430 |
| 12 | 520 | 500 | 480 |
| 13 | 570 | 550 | 530 |
| 14 | 620 | 600 | 580 |
The highlighted cell is the worked example: 500.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Supply voltage (V) | 12 | Enter the supply voltage (v) used in this calculation. |
| LED forward voltage (V) | 2 | Red is about 2 V and white or blue about 3 V. Check the LED's datasheet. |
| LED current (mA) | 20 | Enter the LED current (ma) used in this calculation. |
| Resistor needed (ohms) | 500 | |
| Resistor power (watts) | 0.2 | |
| Next standard E12 value (ohms) | 560 | |
Inputs, definitions and assumptions
Supply voltage (V)
Enter the supply voltage (v) used in this calculation. The prefilled worked-example value is 12.
LED forward voltage (V)
Red is about 2 V and white or blue about 3 V. Check the LED's datasheet. The prefilled worked-example value is 2.
LED current (mA)
Enter the LED current (ma) used in this calculation. The prefilled worked-example value is 20.
How to use this calculator
- 1Verify the inputs. Gather supply voltage (v), LED forward voltage (v) and LED current (ma) from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the resistor needed (ohms) at 500. Store your own version of it as Scenario A.
- 3Test one change. Start with supply voltage (v), the input with the biggest effect here: moving supply voltage (v) from 11 to 13 takes the resistor needed (ohms) from 450 to 550, a swing of 20% of the worked-example figure.
- 4Check the extremes. At half the example supply voltage (v) (6) the resistor needed (ohms) is 200; at double (24) it is 1,100.
People also ask
Frequently asked questions
How do you calculate LED resistor?
R = (supply voltage − LED forward voltage) ÷ LED current; power = current² × R; then round up to the next E12 value. At the worked-example inputs the resistor needed (ohms) is 500.
What does the LED resistor result mean?
Pick a resistor that lights an LED at the current you want, without burning it out. At the worked-example inputs the resistor needed (ohms) is 500. It rises with supply voltage (v) and falls as LED current (ma) and LED forward voltage (v) increase.
How much does supply voltage (v) change the resistor needed (ohms)?
Holding every other input at the worked-example value, moving supply voltage (v) from 10 to 14 moves the resistor needed (ohms) from 400 to 600, a spread of 200.
What are the limits of this LED resistor calculator?
These are exact physics and chemistry formulas. Real-world results add tolerances from component quality, temperature and measurement error that this calculator does not model. The tables on this page test supply voltage (v) only from 10 to 14; a value outside that range is not tabulated here.
Which input moves the resistor needed (ohms) most in the LED resistor calculator?
Ranked by how far each moves the resistor needed (ohms) across the range tested: supply voltage (v) (100, 20%), LED current (ma) (101, 20%) and LED forward voltage (v) (20, 4.0%).
If I double supply voltage (v) in the LED resistor calculator, does the resistor needed (ohms) double?
Doubling it from 12 to 24 takes the resistor needed (ohms) from 500 to 1,100, which is 2.20 times the worked-example figure. So the result grows faster than the input does. Halving it to 6 gives 200.
How much does LED forward voltage (v) matter in the LED resistor calculator?
The worked example uses 2. With the other inputs left at the worked example, moving LED forward voltage (v) from 1.8 to 2.2 takes the resistor needed (ohms) from 510 to 490, a swing of 4.0% of the worked-example figure.
How much does LED current (ma) matter in the LED resistor calculator?
The worked example uses 20. With the other inputs left at the worked example, moving LED current (ma) from 18 to 22 takes the resistor needed (ohms) from 555.6 to 454.5, a swing of 20% of the worked-example figure.
Which inputs change the resistor power (watts) in the LED resistor calculator?
At the worked-example inputs it is 0.2. Supply voltage (v) takes it from 0.18 to 0.22, LED forward voltage (v) takes it from 0.204 to 0.196 and LED current (ma) takes it from 0.18 to 0.22.
Which inputs change the next standard e12 value (ohms) in the LED resistor calculator?
At the worked-example inputs it is 560. Supply voltage (v) takes it from 470 to 560 and LED current (ma) takes it from 560 to 470.
Does molarity change if you heat a solution?
Yes, slightly: liquids expand as they warm, increasing the solution's volume without changing the amount of dissolved solute, which lowers the molarity. This is why precise chemistry work specifies the temperature a molarity was measured or prepared at.
What does half-life mean if something never fully disappears?
After each half-life, half of what remains decays, so the amount approaches zero without reaching it. In practice a sample counts as gone when too little is left to matter or to detect.
Sources and evidence
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