LED Resistor Calculator
Enter your supply voltage, LED forward voltage, and desired current to find the correct current-limiting resistor for your circuit. Choose series or parallel wiring, set the number of LEDs, and the calculator returns the exact resistance, the nearest standard E12 and E24 value to buy, power dissipation across the resistor and each LED, and the minimum resistor wattage rating you need. All results update as you type.
Why LEDs need a current-limiting resistor
An LED is a diode, and like all diodes it has a very steep current-voltage curve: a small increase in voltage causes a large surge in current. Without a resistor in series, the only thing limiting current is the internal resistance of the LED and its wiring, which is tiny. The result is that the LED draws far more current than its rated maximum, generating heat that degrades the junction within seconds and causes permanent failure. A series resistor sets a predictable ceiling on current by converting the excess supply voltage into heat. It is the cheapest and most reliable protection for standard LEDs, and the calculation is straightforward Ohm's Law once you know three values: supply voltage, LED forward voltage, and target LED current.
The formula and how it works
The resistor value is R = (Vs - Vf x n) / If for a series string of n LEDs, or R = (Vs - Vf) / (n x If) for n parallel branches sharing one resistor. Vs is your supply voltage, Vf is the forward voltage of each LED, and If is the desired current through each LED in amps. The numerator is the voltage that must drop across the resistor (the leftover after the LEDs take their share), and the denominator is the current flowing through it. Power dissipated by the resistor is P = Vr x I, where Vr is the voltage across it and I is the current through it. Always buy a resistor rated for at least twice this calculated power to keep it from running hot.
Choosing a standard resistor value from E12 or E24
Resistors come in standardized values defined by the IEC 60063 preferred number series. E12 has 12 values per decade (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and their multiples) at 10% tolerance. E24 has 24 values per decade at 5% tolerance, filling the gaps between E12 values. Because LEDs are forgiving of small current variations, rounding to the nearest E12 value is almost always fine. If the calculated resistance falls between two standard values, always round up (choose the higher value) to keep current below, rather than above, the LED's rated maximum. Rounding down would push a higher current through the LED and shorten its life.
Series vs. parallel: which to use
In a series configuration, all LEDs sit in a single loop and carry exactly the same current, which is ideal because it guarantees even brightness. The downside is that the supply voltage must exceed the sum of all the LED forward voltages plus the resistor drop, so you need a higher voltage for longer chains. In a parallel configuration, each LED branch is connected between supply and ground, so the supply only needs to exceed one LED's forward voltage. However, parallel LEDs driven through a single shared resistor is risky because slight differences in forward voltage between LEDs cause uneven current sharing, some LEDs run too hot while others are dim. The professional solution is to give each parallel branch its own resistor, sizing it individually. This calculator shows the shared-resistor result for parallel as a rough starting point, but the best practice for three or more parallel LEDs is to use individual resistors or a constant-current driver.
Typical LED forward voltages by color
| LED Color | Typical Vf (V) | Typical If (mA) | Common uses |
|---|---|---|---|
| Infrared (IR) | 1.2 | 20-100 | Remote controls, sensors |
| Red | 1.8-2.2 | 10-30 | Indicators, displays |
| Orange | 2.0-2.2 | 10-20 | Indicators, traffic lights |
| Yellow | 2.0-2.4 | 10-20 | Indicators, status lights |
| Green (standard) | 2.0-2.4 | 10-20 | Indicators, go signals |
| Green (high-brightness) | 3.0-3.4 | 20-30 | Outdoor displays |
| Blue | 3.0-3.4 | 10-30 | Indicators, backlights |
| White | 3.0-3.4 | 20-30 | Lighting, backlights |
| Ultraviolet (UV) | 3.4-3.6 | 10-20 | Sterilization, curing |
These are representative values. Always check your specific LED datasheet for the actual Vf at your operating current.
Frequently asked questions
What forward voltage should I use if I do not have a datasheet?
Use the color-based presets as a starting point: about 1.8-2.0 V for red, orange, and yellow; 2.0-2.2 V for standard green; 3.0-3.4 V for blue, white, and high-brightness green; and 1.2 V for infrared. If the LED is unlabeled, 2.0 V is a safe guess for common 5 mm indicators running at 20 mA. The resistor will self-limit current even if your guess is slightly off.
What happens if I choose a higher resistor value than calculated?
A higher resistance means less current, so the LED will be dimmer but will run cooler and last longer. Rounding up to the next standard value is always the conservative choice. Avoid rounding down, which lets more current flow than the LED is rated for.
Can I connect multiple LEDs in parallel without individual resistors?
Technically yes, but it is not recommended. LEDs have slight manufacturing variations in forward voltage. With a shared resistor, the LED with the lowest Vf draws more current, runs hotter, and fails first. For two LEDs of the same type and color from the same batch, matched characteristics make this less of a problem. For three or more, or for LEDs of different colors, give each branch its own resistor.
What wattage resistor do I need?
Calculate the power the resistor dissipates as P = (Vs - total Vf) x I. Then choose a resistor rated for at least twice that value. Common through-hole resistors come in 0.125 W (1/8 W), 0.25 W (1/4 W), 0.5 W (1/2 W), 1 W, and 2 W ratings. For most indicator LED circuits with a 5 V or 9 V supply and 20 mA current, a 0.25 W resistor is sufficient. For LED strips or high-current applications, double-check the wattage rating.
Does it matter what type of resistor I use?
For most LED circuits, a standard carbon-film or metal-film resistor works perfectly. Metal-film resistors have tighter tolerance (1% vs 5-10%) and lower noise, but for current-limiting LEDs the difference is insignificant. Carbon composition resistors are fine too. Avoid using a wire-wound resistor in high-frequency PWM dimming circuits because their inductance can cause voltage spikes.
How do I calculate the resistor for an Arduino or 3.3 V microcontroller?
Use the I/O pin voltage (3.3 V or 5 V) as your supply voltage. For a 5 V Arduino pin driving a red LED (Vf 1.8 V) at 15 mA: R = (5 - 1.8) / 0.015 = 213 ohms. Round up to 220 ohms (E12). Arduino I/O pins are rated for 40 mA maximum per pin; keeping LED current at 10-20 mA leaves a safe margin and keeps the pin voltage from sagging.
What is the E12 series and why does it matter?
The E12 series is a set of 12 resistor values per decade defined by the IEC standard, spaced so that any value falls within 10% of a standard one. The values are 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, and 82 (then multiplied by 10, 100, 1000 and so on). Because resistors are manufactured and stocked in these values, specifying an E12 value means your part is almost always available. The E24 series doubles the count to 24 values at 5% steps, useful when you need a more precise resistance.