LED Resistor Calculator: Series Resistor Value, Power and Nearest Standard Value

Find the resistor that protects an LED and the power it dissipates.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Enter the supply voltage, the forward voltage of one LED, the current you want and the number of LEDs in series.

The calculator finds the voltage the resistor must drop, the resistor R = V over I, and the power it dissipates.

It gives the nearest standard E12 and E24 values and the actual current with the E24 resistor.

Formulas used

  • R = \frac{V_s - nV_f}{I_F}
  • P = (V_s - nV_f)I_F

Worked example

With the default numbers in the calculator above:

  1. Voltage the resistor has to drop: 10.00 V
  2. Resistor value: 500.00 ohm
  3. Power dissipated in the resistor: 200.00 mW. Use a resistor rated at least 400.00 mW (twice the power)
  4. Nearest standard values: E12: 470.00 ohm; E24: 510.00 ohm
  5. Actual current with the E24 resistor: 19.61 mA

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

Why does an LED need a resistor?

An LED conducts almost freely above its forward voltage, so the current would rise until it burns out. The resistor sets the current.

What power rating should the resistor have?

At least twice the calculated power, so it runs cool.

Can I put LEDs in parallel on one resistor?

It is better to give each its own resistor, because the LEDs differ a little and one would take most of the current.

What are the E12 and E24 values?

The standard resistor sequences with 12 or 24 values in each decade, with a tolerance of 10 or 5 percent.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Horowitz and Hill, The Art of Electronics; LED data sheets (forward voltage and current).

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