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.
How the LED Resistor Formula Works
An LED is a diode, so once it reaches its forward voltage it behaves almost like a short circuit and the current runs away until the LED burns out. A current-limiting resistor in series fixes this by absorbing the difference between the supply voltage and the LED's forward voltage. The design formula is R = (Vsupply − VLED) / ILED, a direct application of Ohm's Law.
Resistors only come in fixed steps, so the calculator rounds up to the nearest E24 standard value, which keeps the current at or just below your target rather than over it. It also works out the resistor's power dissipation (P = VR × I) so you can pick a safe wattage; if you want to double-check that figure, use the resistor power dissipation calculator.
The key quantities are voltage in the volt (V), current in the milliamp (mA) (20 mA is typical for a standard 5 mm LED) and resistance in the ohm (Ω).
How to Use This Calculator
- Enter the supply voltage (Vs), or tap a 5 V, 12 V or 3.3 V preset.
- Enter the LED forward voltage (Vf) from the data sheet, or click a colour for a typical value (red 2.0 V, blue 3.2 V, white 3.3 V, and so on).
- Enter the forward current (If), usually 20 mA for a standard indicator LED.
- Set series and parallel counts if you are driving more than one LED, the number in series per string and the number of parallel strings.
- Read the results, exact resistance, nearest E24 value, voltage across the resistor, power and minimum watt rating, each with step-by-step working.
Worked Examples
Red LED on 12 V. Supply 12 V, Vf = 2.0 V, If = 20 mA. Voltage across the resistor VR = 12 − 2.0 = 10 V, so R = 10 / 0.02 = 500 Ω, rounding up to the E24 value 510 Ω. Power P = VR × I = 10 × 0.0196 = 0.2 W, so use a 1/4 W resistor.
White LED on 5 V. Supply 5 V, Vf = 3.3 V, If = 20 mA. VR = 1.7 V, R = 1.7 / 0.02 = 85 Ω → 91 Ω (E24). Power is only 0.03 W, so a 1/8 W resistor is fine.
Three LEDs in series on 12 V. Three red LEDs total 3 × 2.0 = 6 V. VR = 12 − 6 = 6 V, R = 6 / 0.02 = 300 Ω. Putting LEDs in series shares one resistor and wastes less power than driving them separately.
Series vs Parallel LEDs
Series LEDs share the same current, so one resistor sets the current for the whole string. This is the most efficient option, as long as the supply voltage is comfortably higher than the combined forward voltage.
Parallel strings each need their own resistor. Never put bare LEDs in parallel behind a single resistor: forward voltage varies slightly between parts, so the lowest-voltage LED hogs the current and fails early. For high-power LEDs, or when the supply margin is too small for a resistor to regulate well, switch to a constant-current LED driver instead. For more background, see the Wikipedia overview of the LED circuit.
LED Resistor Calculator FAQ
What happens if I connect an LED directly to a power supply without a resistor?
The LED draws unlimited current because a diode's resistance drops near zero above its forward voltage. The LED will burn out within seconds or faster. A series resistor enforces the safe current limit.
Can I use a higher-value resistor than calculated?
Yes. A higher value means less current and dimmer output, but the LED is safe. This is useful for reducing brightness on indicator LEDs. The LED may not light visibly below about 2 mA.
What if my supply voltage equals the LED forward voltage?
The voltage drop across the resistor would be near zero, meaning it cannot regulate current. You need a supply voltage at least 1 to 2 volts above the total LED forward voltage. Below that margin, use a constant-current LED driver.
Can I use one resistor for multiple LEDs in parallel?
No. Forward voltage varies between individual LEDs due to manufacturing tolerances. The LED with the lowest forward voltage hogs the most current and may burn out. Always use one resistor per parallel string.
What resistor do I need for a 12V LED strip?
Most 12V LED strips have resistors built in. If building your own, calculate the resistor for each group of series LEDs. Typically three LEDs at about 3.3V each with a 100 ohm resistor per group on a 12V supply.
Does it matter which side of the LED the resistor goes on?
No. The resistor can go between the supply and the LED anode, or between the LED cathode and ground. The current through the series circuit is the same regardless of placement.
How do I calculate the resistor for a high-power LED?
Same formula: R = (Vs − Vf) / If. A 1W white LED typically has Vf of 3.3V and If of 350 mA. For high-power LED lighting above 1W, a constant-current driver is strongly recommended over a simple resistor.
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