LED Resistor Calculator

Calculate resistor values and wattage ratings for series or parallel current-limiting LED circuits.

Circuit Configuration

Circuit Layout

Number of LEDs

Supply Voltage Vcc (V)

LED Forward Voltage Vf (V)

Target Current If (mA)

Apply 2x Safety Factor

Common LED Colors

Schematic Diagram

Resistor Resistance

Exact value:

150.0 Ω

Nearest E24 (5%):

150.0 Ω

Nearest E12 (10%):

150.0 Ω

Resistor Power Rating

Power Dissipated:60.0 mW

Min Resistor Wattage:1/8 W (0.125W)

Safety Margins: Resistors get hot during operation. Choosing a power rating that is at least 2x the actual dissipated power extends circuit lifespan and prevents fire hazards.

Current limiting resistors for electronics design

Light Emitting Diodes (LEDs) possess a non-linear voltage-current relationship. Without a series current-limiting resistor, the diode will draw excessive current from the power supply, resulting in immediate thermal failure.

The ohm index formula is: R = (Vcc - N × Vf) / If for series chains. Standard manufacturing classifications define the E12 and E24 logarithm series.

Built and maintained by Meet Shah · Last updated 1 August 2026

What this tool is used for

Sizing a series resistor for an LED at a known supply voltage.
Finding the nearest standard E12 or E24 value to the calculated one.
Checking the power the resistor will dissipate.
Working out the current an existing resistor allows.
Choosing a resistor for a string of LEDs in series.

Frequently Asked Questions

What determines the resistor value?: Ohm's law applied to the leftover voltage: R = (V_supply − V_forward) ÷ I. A 5 V supply with a 2 V red LED at 20 mA leaves 3 V across the resistor, so R = 3 ÷ 0.02 = 150 Ω. Everything else is choosing the nearest standard value upward.
Why does an LED need a resistor at all?: Because its current rises almost vertically past the forward voltage — it has no meaningful internal resistance to limit itself. Connect one directly across a supply and it draws whatever the supply can deliver until it burns out, usually in under a second.
What forward voltage should I use?: It varies by colour because it is set by the semiconductor's band gap: red around 1.8–2.2 V, yellow and green 2.0–2.4 V, blue and white 3.0–3.4 V. That is why a white LED will not light at all from a 3 V coin cell that runs a red one happily.
Should I round the resistor up or down?: Up, always, to the next standard E12 or E24 value. Rounding down raises the current above your target; rounding up costs a barely perceptible amount of brightness, since perceived brightness is roughly logarithmic in current.
Can I put several LEDs on one resistor?: In series, yes — add the forward voltages and use one resistor for the string, as long as the supply exceeds the total. In parallel, no: small manufacturing differences in forward voltage make one LED take most of the current, so parallel LEDs each need their own resistor.

Common errors and gotchas

Using the supply voltage rather than the difference across the resistor, which overstates the value needed.
Ignoring power dissipation, so the arithmetic is right and the resistor overheats.
Assuming a forward voltage, which varies by colour and by part — check the datasheet.
Wiring LEDs in parallel behind one resistor, where the brightest hogs the current.
Choosing the nearest standard value downward, which pushes the current above the LED's rating.