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LED Current Limiting

Electronics & PhysicsCircuits and Components

From the LED datasheet: red is about 1.8–2.2 V, white and blue about 2.8–3.4 V.

Exact resistance (Ω) —
Next E12 resistor up (Ω)
—
Current with that resistor (mA)
—
Resistor dissipation (W)
—
Suggested power rating
—
Power reaching the LEDs
—

An LED current limiting resistor is sized as R = (supply voltage − LED forward voltage × number of LEDs) ÷ current. For a 5 V supply, a 2 V red LED and 20 mA, that is 3 V ÷ 0.02 A = 150 Ω, dissipating 0.06 W. Pick the next standard value up so the current lands at or below the target.

About this tool

An LED has almost no resistance once it starts conducting, so wiring it straight to a supply lets the current climb until the die fails. A series resistor drops the surplus voltage and fixes the current. This calculator takes the supply voltage, the LED forward voltage from its datasheet, the current you want and how many LEDs share the string. It returns the exact resistance, the next E12 resistor up, the current that resistor really gives, how much heat it sheds and a power rating with headroom. The working is shown line by line. One limit: forward voltage varies between parts and with temperature, so the real current can differ from the figure here by a few milliamps; for high-power LEDs above roughly 350 mA a constant-current driver is the better choice.

How to use it

  1. Enter the supply

    Type the voltage feeding the circuit, such as 5 V USB, 9 V battery or 12 V adapter.

  2. Add the LED data

    Enter the forward voltage and the current from the datasheet; 20 mA suits most 5 mm indicator LEDs.

  3. Set the string length

    Use the − and + buttons to set how many LEDs sit in series on the one resistor.

  4. Read the part to buy

    Use the E12 value and the suggested wattage; check the actual current stays within the LED rating.

Examples

5 V supply, one 2 V LED, 20 mA

Supply voltage
5
LED forward voltage
2
LED current
20
LEDs in series
1

Result Exact resistance (Ω): 150
Next E12 resistor up (Ω): 150
Current with that resistor (mA): 20
Resistor dissipation (W): 0.06
Suggested power rating: 1/8 W
Power reaching the LEDs: 40%

  1. LED string drop = 1 × 2 V = 2 V
  2. Voltage across resistor = 5 − 2 = 3 V
  3. R = 3 V ÷ 0.02 A = 150 Ω
  4. Next E12 value up = 150 Ω, giving 20 mA
  5. Resistor power = 3 V × 0.02 A = 0.06 W

Three volts are left for the resistor, so 150 Ω sets 20 mA. It burns 0.06 W, inside a 1/8 W part.

9 V battery, one 2 V LED

Supply voltage
9
LED forward voltage
2
LED current
20
LEDs in series
1

Result Exact resistance (Ω): 350
Next E12 resistor up (Ω): 390
Current with that resistor (mA): 17.95
Resistor dissipation (W): 0.126
Suggested power rating: 1/2 W
Power reaching the LEDs: 22.2%

  1. LED string drop = 1 × 2 V = 2 V
  2. Voltage across resistor = 9 − 2 = 7 V
  3. R = 7 V ÷ 0.02 A = 350 Ω
  4. Next E12 value up = 390 Ω, giving 17.95 mA
  5. Resistor power = 7 V × 0.01795 A = 0.126 W

350 Ω is not a stock value, so the next E12 part, 390 Ω, is used and the LED runs at about 18 mA.

12 V, three 3.2 V white LEDs

Supply voltage
12
LED forward voltage
3.2
LED current
20
LEDs in series
3

Result Exact resistance (Ω): 120
Next E12 resistor up (Ω): 120
Current with that resistor (mA): 20
Resistor dissipation (W): 0.048
Suggested power rating: 1/8 W
Power reaching the LEDs: 80%

  1. LED string drop = 3 × 3.2 V = 9.6 V
  2. Voltage across resistor = 12 − 9.6 = 2.4 V
  3. R = 2.4 V ÷ 0.02 A = 120 Ω
  4. Next E12 value up = 120 Ω, giving 20 mA
  5. Resistor power = 2.4 V × 0.02 A = 0.048 W

The string drops 9.6 V, leaving 2.4 V for a 120 Ω resistor; 80% of the supply power reaches the LEDs.

How it is calculated

R = (Vs − n × Vf) ÷ I; P = (Vs − n × Vf) × I

R
series resistance in ohms
Vs
supply voltage in volts
Vf
forward voltage of one LED
n
number of LEDs in series
I
target current in amperes (mA ÷ 1000)
P
power dissipated in the resistor, watts

Kirchhoff's voltage law says the supply equals the sum of the drops around the loop, so whatever the LEDs do not drop appears across the resistor. Ohm's law then gives the resistance for the chosen current. The tool rounds up to the next E12 value (1.0, 1.2, 1.5 … 8.2 per decade) because rounding up can only lower the current, then recalculates current and power with that part. The power rating suggested is at least double the dissipation.

When not to use it

  • High-power LEDs of 1 W and above, which need a constant-current driver.
  • LED strips with built-in resistors, which already take 5 V or 12 V directly.
  • Parallel LEDs sharing one resistor, since mismatched forward voltages split the current unevenly.

Common mistakes

  • Using a typical forward voltage of 2 V for a blue or white LED that actually drops about 3 V.
  • Rounding the resistor down to the nearest stock value, which pushes the current above target.
  • Forgetting the resistor's wattage, so a small part overheats on a 24 V supply.
  • Entering the current in amps instead of milliamps.

Frequently asked questions

Why does an LED need a resistor at all?

Above its forward voltage an LED's current rises very steeply with small voltage increases. Without something to limit it, the current is set only by the supply's capability and wiring resistance, which usually means hundreds of milliamps and a dead LED. The resistor makes the current depend mostly on its own value.

Where do I find the forward voltage?

The datasheet lists it as VF at a test current, often 20 mA. Typical values are 1.8–2.2 V for red and yellow, 2.0–2.4 V for green from older chemistries, and 2.8–3.4 V for blue, white and true green InGaN parts.

Can several LEDs share one resistor?

In series, yes: add up their forward voltages, which this calculator does with the LED count. In parallel it is poor practice, because the LED with the lowest forward voltage takes most of the current. Give each parallel branch its own resistor.

What current should I choose?

Standard 5 mm indicator LEDs are rated around 20 mA continuous, but modern high-efficiency parts are bright at 5–10 mA. Running below the maximum extends life and cuts the resistor's heat. Never exceed the absolute maximum current in the datasheet.

Why round up to the E12 series?

Resistors are stocked in preferred values. E12 has twelve per decade with 10% spacing, the range most hobby kits carry. Choosing the next value up keeps the current at or under your target; choosing down would exceed it.

How hot will the resistor get?

It dissipates the leftover voltage times the current. At 3 V and 20 mA that is 0.06 W, trivial for a 1/4 W resistor. At 24 V with one LED it reaches about 0.44 W, which needs a 1 W part, and a long series string would waste less.

Cite this tool

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APA

Tools Directory Editorial Team. (2026, October 18). LED Current Limiting. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/electronics-physics/circuits-and-components/led-current-limiting/

MLA

Tools Directory Editorial Team. “LED Current Limiting.” Tools Directory, 18 Oct. 2026, https://tools.milunir.com/electronics-physics/circuits-and-components/led-current-limiting/. Accessed 27 Oct. 2026.

Chicago

Tools Directory Editorial Team. “LED Current Limiting.” Tools Directory. Last modified October 18, 2026. Accessed October 27, 2026. https://tools.milunir.com/electronics-physics/circuits-and-components/led-current-limiting/.

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