LED Resistor Calculator
Calculate the series resistor for an LED from the supply voltage, the LED's forward voltage and the current you want. You can also find the current for a resistor you have.
Electrical safety
- This is an estimate for low-voltage DC circuits. Mains power (wall outlets), large currents and amplifier outputs are outside its scope.
- Always check the LED's maximum ratings (current and voltage) in its datasheet. There is a risk of electric shock, heat and fire.
- The power rating of the resistor is not calculated. Choose a resistor rated well above the power shown here.
- The result does not guarantee that a real circuit is safe or will work.
The formula
An LED passes too much current and is damaged unless its current is limited. When you connect an LED to a supply, put a resistor in series to set the current.
Resistance R = (supply voltage − number of LEDs in series × LED forward voltage) ÷ current
- The forward voltage (V_f) is the voltage across the LED while it is lit. It depends on the LED type and color, so enter the value from the datasheet of your LED. This page puts no default value in the field.
- With n LEDs in series, n forward voltages are subtracted from the supply voltage.
- Power dissipated in the resistor P = current² × R (the power lost as heat).
How the voltage, current, resistance and power are related
The voltage left across the resistor is the supply voltage minus the forward voltages of the LEDs. By Ohm's law, the current through the resistor is that voltage divided by its resistance, and the same current flows through the LEDs, because they are in series. The power the resistor turns into heat is its voltage times the current. If you pick a resistor with a different value, the current changes: that is what "Find the current for a resistor" calculates. For the general relation between V, I, R and P, use the Ohm's Law Calculator.
Example
From a reference article (Wikipedia, "LED circuit"; checked October 30, 2026): a supply of 12 V, a forward voltage of 1.8 V and a current of 5 mA give R = (12 − 1.8) ÷ 0.005 = 2,040 Ω. The power in the resistor is 0.005² × 2,040 = 0.051 W (51 mW).
In practice you pick an available resistor close to the calculated value. Use "Find the current for a resistor" with the value you picked to check the current that will actually flow. This calculator does not include a list of standard resistor values.
When the supply voltage is too low
If the supply voltage is no higher than the number of LEDs × the forward voltage, nothing can be calculated (the voltage across the resistor would be 0 or less, and the current would not be defined).
About the formulas on this page
The formula for the resistance is the same as the one in the reference article (Wikipedia, "LED circuit"), R = (V_supply − V_f) ÷ I, with one LED in series. The formula for several LEDs in series and the formula for the resistor's power are not written in that article. They are derived here from the sum of the voltage drops and from Ohm's law and the power formula.
What is not covered
Parallel connections (give each LED its own resistor), constant-current circuits, PWM dimming, LED brightness and lifetime, and filling in the forward voltage automatically are not covered. For the battery run time, see the Battery Capacity Calculator.
Frequently asked questions
Which forward voltage should I enter?
Enter the value in the datasheet of the LED you use. It depends on the color and the material. The reference article also explains that it differs by LED material: roughly 1.7 to 2.0 V for red and roughly 2.8 to 4.0 V for violet, for example.
What if the calculated resistor value does not exist?
Pick a nearby value and use "Find the current for a resistor" to check that the current does not exceed the LED's rating. It is common to choose the side with less current (the larger resistance).
What about LEDs in parallel?
Give each LED its own resistor. Sharing one resistor between LEDs is not calculated here.