LED Series Resistor Calculator
Calculate the series resistor needed to safely drive an LED. Enter your supply voltage, the LED's forward voltage, and the desired current to get the resistor value in ohms.
Resistor Value for LED Current Limiting
LED Series Resistor Formula
Where:
- \(R\) — series resistor value (Ω)
- \(V_s\) — supply voltage (V)
- \(V_f\) — forward voltage of the LED (V)
- \(I\) — desired current through the LED (A)
This is Ohm's law applied to the voltage the resistor must drop: the supply voltage minus the LED's forward voltage, divided by the current you want to flow through the circuit.
LED Series Resistor – Calculation Example
Example: you want to power a red LED with a forward voltage of 2 V from a 5 V supply at a current of 20 mA (0.02 A).
\[ R = \frac{V_s - V_f}{I} = \frac{5 - 2}{0.02} = \frac{3}{0.02} = 150~\Omega \]
Result: a series resistor of 150 Ω limits the current to 20 mA. The nearest standard value is 150 Ω (E12 series); if unavailable, use 160 Ω or 180 Ω to be safe.
How to Use the Calculator
- Enter your Supply voltage in volts — for example, 5 for a USB power source or 3.3 for a microcontroller GPIO pin.
- Enter the LED forward voltage in volts. Check the LED datasheet; typical values are about 2 V for red, 3.2 V for blue and white.
- Enter the LED current in amperes. A common choice is 0.02 A (20 mA); for indicator LEDs, 0.005–0.01 A is often enough.
- Read the Resistor Value in ohms, then pick the nearest standard resistor value (rounding up is safer than rounding down).
What Is a Series Resistor for an LED?
An LED is a diode, not a resistor. Once the voltage across it exceeds its forward voltage, its current rises extremely steeply — a small increase in voltage causes a huge increase in current. Without anything to limit the current, an LED connected directly to a power supply will draw far more current than it can handle and burn out within seconds.
A resistor placed in series with the LED solves this problem. It absorbs the excess voltage (the supply voltage minus the LED's forward voltage) and, by Ohm's law, sets the current to a safe, predictable value. This is the simplest and most common way to drive an LED, used everywhere from breadboard prototypes to production circuits.
The calculator's result tells you the exact resistance for your target current. A higher resistance gives a dimmer LED and lower power dissipation; a lower resistance gives a brighter LED but risks exceeding its maximum current rating, typically 20–30 mA for standard 5 mm LEDs.
If the calculator shows an error, check your inputs: the supply voltage must be greater than the LED's forward voltage, and the current must be positive. For example, a blue LED (\(V_f \approx 3.2\) V) cannot be driven from a single 1.5 V battery without a boost converter.
Practical Tips and Limitations
- Round up to a standard value. Resistors come in standard series (E12, E24). For a calculated 143 Ω, use 150 Ω rather than 120 Ω — slightly less current is always safer.
- Check the resistor's power rating. Power dissipated is \(P = (V_s - V_f) \times I\). In the example above, \(P = 3 \times 0.02 = 0.06\) W, well within a standard 0.25 W resistor. For high-current LEDs, use a resistor rated for the dissipation or a constant-current driver instead.
- Forward voltage varies. The values in datasheets are typical; actual forward voltage changes with current, temperature, and manufacturing spread. Expect some variation in brightness between individual LEDs.
- Multiple LEDs: LEDs in series share the same current, so subtract the sum of their forward voltages from the supply. LEDs in parallel each need their own series resistor — sharing one resistor causes uneven current distribution.
- High-power LEDs (1 W and above) should not be driven with a simple resistor; use a dedicated constant-current driver for efficiency and safety.
For current-limiting calculations with other components, see our Zener Diode Resistor Calculator. If you end up combining resistors to reach the needed value, our Parallel Resistance Calculator and Series Inductance Calculator page's series formulas will help, and the Wire Gauge Calculator is useful for sizing the wires that carry the current.
Frequently Asked Questions
What are the typical forward voltages for different LED colors?
Typical values are: red 1.8–2.2 V, yellow and amber 2.0–2.2 V, green 2.0–3.2 V, blue 3.0–3.4 V, white 3.0–3.4 V, and infrared 1.2–1.5 V. The exact value depends on the LED's chemistry and current, so always check the datasheet when accuracy matters. If you don't know the forward voltage, the middle of the range above gives a safe estimate for hobby use.
How do I convert milliamps to amps for the current field?
Divide milliamps by 1000. LEDs are usually rated in mA, but the formula needs amperes: 20 mA = 0.02 A, 10 mA = 0.01 A, 5 mA = 0.005 A. If you forget to convert and enter 20 instead of 0.02, the calculator will return a resistor value 1000 times too small, and the LED would burn out instantly. As a quick sanity check, typical results for small LEDs are between 100 Ω and 1 kΩ.
Can I connect an LED without a resistor if the supply equals the forward voltage?
It's strongly discouraged. A "3 V" supply (like two AA cells) is not regulated, and an LED's forward voltage drops as it heats up, which lets current climb — a runaway effect that can destroy the LED or the driver. The one common exception is a 3 V coin cell (CR2032), whose high internal resistance naturally limits the current. With any other source, include a resistor or use a constant-current driver.
Does it matter whether the resistor goes before or after the LED?
No. In a series circuit the same current flows through every component, so the resistor limits the LED equally well whether it is connected to the anode (positive) side or the cathode (negative) side. What does matter is the LED's polarity: the anode must face the positive supply and the cathode (the side with the flat spot or shorter lead) toward ground, or the LED will simply stay dark.
What wattage resistor do I need?
Calculate the dissipated power with \( P = (V_s - V_f) \times I \), then pick a rating at least twice that for a cool, reliable run. For a red LED on 5 V at 20 mA: \( P = 3 \times 0.02 = 0.06 \) W, so a standard 0.25 W resistor is plenty. The exception is driving LEDs from 12 V or 24 V: a single white LED from 24 V at 20 mA dissipates over 0.4 W in the resistor, requiring a 1 W part or better.
Can I use a larger resistor value than calculated?
Yes, and it's always safe. A higher resistance simply means less current: the LED will be dimmer but never damaged. Doubling the resistor halves the current. This is often desirable — many indicator LEDs look just as bright at 2–5 mA as at 20 mA, saving power. Going lower than the calculated value is what risks exceeding the LED's maximum current rating (typically 20–30 mA for standard 5 mm LEDs).
How do I calculate the resistor for several LEDs at once?
For LEDs in series, add all forward voltages together and use the sum: \( R = (V_s - n \times V_f) / I \). Three red LEDs (2 V each) on 12 V at 20 mA need \( R = (12 - 6)/0.02 = 300 \Omega \). Make sure the supply exceeds the total forward voltage. For LEDs in parallel, give each LED its own resistor — a single shared resistor causes current to bunch into one LED due to small forward-voltage differences.