Resistor Calculators
LED Resistor Calculator
Find the series resistor that sets your LED current safely, along with the power it must dissipate and the nearest standard value.
LED Series Resistor
Size the resistor that sets your LED current.
Why an LED Needs a Resistor
An LED is a diode, not a resistor. Below its forward voltage almost no current flows; above it, current rises very steeply for a small increase in voltage. Connecting one directly across a supply therefore lets current run away until something fails, usually the LED.
The standard fix is a series resistor that takes up the difference between the supply and the LED's forward voltage, turning the circuit into something Ohms Law can control.
Here Vs is the supply voltage, Vf the forward voltage of one LED, n the number of LEDs in series, and I the current you want flowing. Typical forward voltages are around 1.8 to 2.2 V for red, 3.0 to 3.4 V for blue, green and white, and 20 mA is a common design current for standard indicator LEDs.
How to Use the LED Resistor Calculator
Enter the supply voltage, the forward voltage from the LED datasheet, and the current you want. The current field takes microamps through amps, so 20 mA can be typed directly.
Set the number of LEDs in series. Their forward voltages add, so three white LEDs at 3.2 V need at least 9.6 V before any current flows at all. The parallel strings field is optional and describes how many identical series strings run from the same supply.
Select Calculate. The results give the exact resistance, the nearest standard E24 value rounded upward, the dissipation at both values, and a suggested power rating with margin. Rounding up rather than down keeps the current at or slightly below your target, which is the safe direction.
Worked Example: A 5 V Supply and a Red LED
A red LED with a 2 V forward voltage, run at 20 mA from a 5 V supply.
Sixty milliwatts is well inside a standard 0.25 W resistor, and 150 Ω happens to be a stock E24 value, so no rounding is needed.
Change the supply to 12 V and the resistor becomes (12 − 2) ÷ 0.02 = 500 Ω, dissipating 0.2 W. That is uncomfortably close to a quarter-watt part's limit, so a 0.5 W resistor is the better choice. Alternatively, putting four LEDs in series drops 8 V across the LEDs, leaving 4 V for a 200 Ω resistor dissipating only 80 mW, and producing four times the light for the same current.
Reading Your LED Resistor Results
The minimum resistor value is the exact figure from the formula. The nearest standard value is the next E24 value at or above it, since standard resistors come in preferred series rather than arbitrary values, and choosing the higher one keeps current below target rather than above.
Two dissipation figures are shown: one at the calculated resistance and one at the standard value you would actually fit. The recommended rating is roughly double the dissipation, giving margin for enclosure temperature and component tolerance.
The voltage across the resistor is worth watching. If it is under about a volt, small variations in supply or forward voltage cause large swings in current, because the resistor has too little headroom to regulate. A warning appears in that case.
Where LED Series Resistors Are Needed
Panel indicators are the classic case: one LED, one resistor, a fixed supply. The same arithmetic sizes the resistor on a microcontroller output pin, though there you must also check the pin's current limit, which is often 20 mA or less per pin and lower still in total across the package.
Series strings suit higher supply voltages. Running LEDs in series means they all carry the same current and therefore have matched brightness, while wasting far less power in the resistor. The limit is that the supply must comfortably exceed the sum of the forward voltages, with room left for the resistor.
For anything beyond indicators, particularly power LEDs, a constant-current driver replaces the resistor entirely. Drivers hold the current steady as the LED heats and its forward voltage falls, which a fixed resistor cannot do.
Limits of the LED Resistor Model
Parallel LEDs on one resistor is a common mistake. LEDs do not share current evenly, because tiny differences in forward voltage cause large differences in current. Give every parallel string its own resistor.
- Forward voltage varies between parts and falls as the LED warms, so the actual current will drift a little from the calculated figure.
- A resistor wastes the surplus voltage as heat. At high currents or large voltage differences, that loss dominates the circuit's consumption.
- The calculator refuses to produce a value when the supply does not exceed the total forward voltage, because no resistor can make that circuit conduct properly.
- Datasheet maximum current is an absolute limit, not a target. Running at or slightly below the recommended figure gives far better life.
Questions
LED Resistor FAQs
What if I do not know the LED's forward voltage?
Use a typical figure for the colour: about 2 V for red, amber and yellow, and about 3.2 V for green, blue and white. Then verify by measuring the voltage across the LED once the circuit is running.
Can several LEDs share one resistor in parallel?
It is possible but poor practice. Small differences in forward voltage make one LED take most of the current, so it runs hot and fails early. Use a separate resistor for each parallel string.
Why round the resistor value up rather than down?
Rounding up lowers the current slightly, which is the safe direction. Rounding down raises it above your target and shortens LED life.
What power rating should the resistor have?
At least twice the calculated dissipation. The calculator suggests the next standard rating above that margin, which covers ambient temperature and tolerance.
Check the Power Figure on the Ohms Law Wheel
Open the full Ohms Law Wheel calculator to solve voltage, current, resistance and power from any two known values.