Quick-fill a common supply voltage or type your own below
Select LED Colour for a typical forward voltage (Vf) or enter your own value below
LED Resistor Circuit
A current-limiting resistor is connected in series with the LED to control the current flow. Without it, the LED draws excessive current and burns out. The resistor drops the difference between the supply voltage and the LED’s forward voltage.
R = (Vsupply – VLED) / ILED — The resistor value labels update when you enter values in the calculator.
LED Resistor Calculator
LEDs are diodes — they have a fixed forward voltage drop and will draw as much current as the circuit allows. Connecting an LED directly to the power supply will burn it out almost instantly. A series resistor absorbs the voltage difference between the supply voltage and the LED’s forward voltage, limiting the current to a safe level. This calculator helps you determine the resistor value for a single LED, LEDs in series, or LEDs in parallel. Enter the required values and the calculator returns the exact resistance, the nearest standard E24 value, and the power dissipated by the resistor.
The LED Resistor Formula
Apply Ohm’s law to calculate the resistance required:
R = resistor value (Ω)
Vs = supply voltage (V)
Vf = LED forward voltage (V)
If = LED forward current (A)
The supply voltage must be greater than the LED voltage. The voltage drop across the resistor equals Vs − Vf. For one or more series-connected LEDs sharing one resistor, replace Vf with the sum of all forward voltage drops.
Worked Examples
Single LED — Red LED on 5 V
A red LED has a forward voltage of ~2.0 V. Desired current: 20 mA (a safe value for standard 5 mm LEDs).
Power = (5 − 2.0) × 0.020 = 60 mW
Nearest E24: 150 Ω — a ¼ W (250 mW) resistor is fine.
Single LED — White LED on 3.3 V
A white LED has a forward voltage of ~3.3 V. On a 3.3 V supply there is almost no voltage headroom — the voltage drop across the resistor would be near zero. You need a supply voltage greater than the LED forward voltage. Use 5 V instead:
Nearest E24 (round up): 91 Ω
Actual current = 1.7 / 91 = 18.7 mA — slightly under 20 mA, safer for the LED.
Three LEDs in Series on 12 V
Three red LEDs in series, each with a 2.0 V forward voltage. Total forward voltage: 3 × 2.0 = 6.0 V.
Nearest E24: 300 Ω — Power = 6.0 × 0.020 = 120 mW
All three LEDs share the same current path, so one resistor handles the entire string. The number of LEDs you can connect in series is limited by the supply voltage — the total forward voltage must stay well below Vs, leaving enough headroom for the resistor to regulate current.
LEDs in Parallel
Never use a single resistor for LEDs in parallel. Each LED has slight manufacturing variations in forward voltage, so the current splits unevenly — one LED may hog most of the current and burn out while others barely light. Instead, give each LED (or each series string) its own resistor. The calculator shows the resistor needed per string and the total current drawn from the supply.
Example: four parallel strings, each with one LED and one resistor, on a 9 V supply with red LEDs (2.0 V, 20 mA). Each string needs R = (9 − 2.0) / 0.020 = 350 Ω → 360 Ω (E24). Total supply current = 4 × ~19.4 mA ≈ 78 mA.
LED Forward Voltage by Colour
The forward voltage varies by LED colour because different semiconductor materials emit different wavelengths. These are typical values — always check your LED’s data sheet for the exact number.
| LED Colour | Forward Voltage (typical) | Wavelength |
|---|---|---|
| Infrared | ~1.2 V | 850–940 nm |
| Red | ~2.0 V | 620–645 nm |
| Yellow | ~2.1 V | 585–595 nm |
| Green | ~2.2 V | 520–535 nm |
| Blue | ~3.2 V | 460–475 nm |
| White | ~3.3 V | Broadband |
The leads of an LED identify polarity: the longer lead is the anode (+), the shorter is the cathode (−). You can also look inside the LED — the larger internal element is the cathode. Current flows from anode to cathode, and the resistor can go on either side.
Choosing the Resistor
Resistance Value
The calculator gives the exact calculated resistance and the nearest E24 standard value, always rounded up. Rounding up means the LED gets slightly less than its rated maximum current — this is safer and barely affects brightness. If you need to decode the value from a physical resistor’s colour bands, use our Resistor Colour Code Calculator.
Power Rating
The power dissipated by the resistor is P = (Vs − Vf(total)) × If. For most single LED circuits at 20 mA, this is under 250 mW — a standard ¼ W resistor works. For high-power LEDs at 350 mA or above, the resistor may need ½ W or 1 W. The calculator shows the exact dissipation and recommends a minimum watt rating. For detailed power analysis, see our Electrical Power Calculator.
Current Limiting vs. Constant Current
A resistor provides simple current limiting — cheap and effective for fixed-voltage circuits. But if the supply voltage fluctuates, the LED current changes too. For LED lighting applications where consistent brightness matters, a constant-current LED driver is better. For indicator LEDs, hobby projects, and fixed-voltage circuits, a resistor is the standard approach.
Series or Parallel — Which to Use
Use LEDs in series when your supply voltage is high enough to cover the total forward voltage plus resistor headroom. Series is more efficient — one resistor, one current path, no current-sharing problems. Use parallel strings when you have more LEDs than the supply voltage can handle in a single series chain, or when you need redundancy (one LED failing does not kill the whole string). Each parallel string must have its own resistor.
Identifying LED Polarity
LEDs are diodes — they only conduct in one direction. Connect them backwards and nothing happens (no light, no damage at normal voltages). The anode (+) connects toward the supply voltage through the resistor; the cathode (−) connects toward ground. Three ways to identify polarity: the longer lead is the anode, the flat edge on the LED’s base marks the cathode, and the smaller internal element visible inside the LED is the anode. An LED that does not light up is almost always installed backwards — flip it before assuming it is faulty.
Frequently Asked Questions
Last updated: March 2026