Led Resistor Calculator
Size the series resistor for an LED from supply voltage, LED forward voltage, and desired current, and get the resistor's power dissipation.
Led Resistor Calculator
Calculator
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Formula: R = (V_supply - V_LED) / I_LED
Worked example — 150Ω, 1/8W
Formula
R = (V_supply - V_LED) / I_LED
The resistor drops the excess voltage. Use Ohm's law with the voltage difference across the resistor and desired LED current. Power dissipation P = I²R.
Worked Examples
Example 1: 5V with red LED
Problem:Vs=5V, Vled=2.0V, I=20mA
Solution:R = (5-2)/0.02 = 150Ω, P = 0.02²×150 = 60mW
Result:150Ω, 1/8W
Frequently Asked Questions
What happens without a resistor?
Without a current-limiting resistor, excessive current flows through the LED, quickly destroying it. LEDs have very low resistance when forward-biased.
What are the forward voltages for different LED colors?
Forward voltage depends on the LED color: Red/Orange: 1.8–2.2V, Yellow/Green: 2.0–2.4V, Blue/White/UV: 3.0–3.6V, Infrared: 1.2–1.6V. Always check the datasheet for your specific LED. Using the wrong forward voltage will result in an incorrect resistor value and may overdrive or underdrive the LED.
Can I connect multiple LEDs in series with one resistor?
Yes. For series LEDs, add up all the forward voltages and subtract from the supply voltage before dividing by the current. For example, three red LEDs (2V each) in series on 12V: R = (12 - 6) / 0.02 = 300 ohms. Series wiring requires only one resistor but needs a higher supply voltage. Parallel wiring needs a separate resistor for each LED for reliability.
Why should I choose a standard resistor value instead of the exact calculated value?
Resistors come in standard E12 or E24 series values (e.g., 100, 120, 150, 180, 220 ohms). Choosing the nearest standard value above your calculated resistance slightly reduces current, which protects the LED and is totally acceptable. Exact calculated values are not available and would have to be custom-ordered, which is impractical.
How do you choose the resistor's power rating?
Compute the dissipation with P = I-squared x R, or equivalently (V_supply - V_LED) x I. Driving a 2.0 V red LED at 20 mA from 5 V needs 150 ohms dissipating 0.02-squared x 150 = 0.06 W, or 60 mW. A 1/8 W (125 mW) resistor gives roughly double the headroom, which is the usual rule of thumb. Undersized resistors run hot, drift upward in value, and eventually fail open.
Why should each LED in a parallel arrangement get its own resistor?
Forward voltage varies from part to part even within one batch. Share a single resistor across parallel LEDs and the one with the lowest forward voltage takes most of the current, glowing brighter and heating up. Because forward voltage falls as the junction warms, that LED draws even more current - a runaway that dims the others and shortens its own life. Individual resistors set each branch's current independently.
What happens if the supply voltage is below the LED forward voltage?
Essentially no current flows and the LED stays dark, because the diode only conducts once the supply exceeds its forward drop. A 3.2 V blue or white LED simply will not light from a 3 V coin cell under load, no matter what resistor you fit. The calculator requires V_supply to exceed V_LED for this reason; if it does not, you need a higher rail or a boost converter.
How does running an LED below its rated current affect brightness and life?
Light output tracks forward current closely over the normal operating range, so driving 10 mA instead of 20 mA gives roughly half the brightness while producing less heat and extending life. Because the diode's current-voltage curve is exponential, a few tenths of a volt swing the current dramatically - which is precisely why a series resistor, rather than a fixed voltage, is used to set the operating point.
When should you use a constant-current driver instead of a resistor?
A resistor burns (V_supply - V_LED) x I as heat, so it becomes wasteful with a high supply rail. Feeding a 2 V LED at 20 mA from 12 V dissipates 10 x 0.02 = 0.2 W in the resistor against only 0.04 W in the LED. A constant-current driver regulates the current directly, keeping brightness steady as the supply sags, and is effectively mandatory for high-power LEDs running at 350 mA and above.
What is the difference between forward voltage and supply voltage in this equation?
Forward voltage V_LED is a property of the diode itself, set by the semiconductor bandgap and therefore by the colour, and it stays roughly fixed as current varies. Supply voltage V_supply is whatever rail you build the circuit on. The resistor exists only to absorb the difference between them, which is why R = (V_supply - V_LED)/I depends on the gap rather than on either voltage alone.
Reviewed for accuracy by Manoj Kumar, Mathematics Educator · Editorial policy
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