LED Series Resistor Calculator
Calculate the correct series resistor for any LED from supply voltage, forward voltage, and target current. Shows the nearest E-series value.
- Exact resistance
- 150.0 Ω
- Nearest E24
- 150 Ω
- Power dissipation
- 60.0 mW
- Actual LED current
- 20.0 mA
R = (Vsupply − Vf) / I. Power = (Vsupply − Vf) × I.
Typical LED forward voltages: Red 1.8–2.2 V, Yellow 2.0 V, Green 2.1–3.0 V, Blue/White 3.0–3.4 V. Typical drive current: 10–20 mA for indicator LEDs. Snap the calculated value to the nearest E24 series resistor.
What resistor do I need for an LED at 12V?
For a standard red LED (Vf = 2 V) at 20 mA from 12 V: R = (12 − 2) / 0.020 = 500 Ω. Nearest E24 is 510 Ω (rated ≥ 250 mW to be safe — the resistor dissipates about 200 mW).
How much resistance for an LED?
Depends entirely on your supply voltage, LED forward voltage, and desired brightness. There is no single answer — this calculator computes it from the three inputs. For a 5 V supply and typical 2 V red LED at 20 mA, you need 150 Ω.
Power dissipation matters
The calculator reports the resistor's wattage. Standard through-hole resistors are 1/4 W; 0603 SMD are typically 1/10 W. If the calculated dissipation is above ~60% of the part's rating, pick a larger package or a higher-wattage part.
Frequently asked
- What resistor do I need for an LED at 12V?
- For a red LED (Vf ≈ 2 V) at 20 mA: R = (12−2)/0.020 = 500 Ω. Nearest E24 is 510 Ω. Verify power dissipation (~200 mW) against the resistor's rating.
- How much resistance for an LED?
- It depends on your supply voltage, the LED's forward voltage, and the target current. Use the formula R = (Vsupply − Vf) / I, then snap to the nearest E24 value.
- What size resistor for LED at 12V?
- About 500 Ω for red at 20 mA, 470 Ω for a green (Vf ~2.5 V) at 20 mA, 430 Ω for blue/white (Vf ~3.2 V) at 20 mA. Use at least 1/4 W resistors at 12 V.
- How to calculate what size resistor is needed?
- Subtract the LED forward voltage from the supply voltage, then divide by the target current in amperes. Result is in ohms.
An LED and its resistor make a good first board. Describe it, for example a status LED on a 5 V rail, and get the schematic and a routed PCB.
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