What about LEDs in parallel?

Each parallel branch needs its OWN resistor — LEDs have manufacturing variation in Vf, and the one with the lowest Vf will hog all the current. Don't share a resistor across parallel LEDs.

Why snap to E12 values?

E12 (1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 × 10ⁿ) is the standard 5% tolerance resistor set sold by every electronics distributor. You almost certainly have these in your parts bin.

Why round UP, not to nearest?

Rounding UP slightly under-drives the LED (lower current), which is safe. Rounding DOWN exceeds target current, risking LED degradation. Always snap up.

Standard electronics theory; LED datasheets (Cree, Lumileds, Vishay); Horowitz & Hill "The Art of Electronics" 3rd ed §4.x. E12 series: IEC 60063.

What about LEDs in parallel?

Each parallel branch needs its OWN resistor — LEDs have manufacturing variation in Vf, and the one with the lowest Vf will hog all the current. Don't share a resistor across parallel LEDs.

Why snap to E12 values?

E12 (1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 × 10ⁿ) is the standard 5% tolerance resistor set sold by every electronics distributor. You almost certainly have these in your parts bin.

Why round UP, not to nearest?

Rounding UP slightly under-drives the LED (lower current), which is safe. Rounding DOWN exceeds target current, risking LED degradation. Always snap up.

Standard electronics theory; LED datasheets (Cree, Lumileds, Vishay); Horowitz & Hill "The Art of Electronics" 3rd ed §4.x. E12 series: IEC 60063.

LED Current-Limiting Resistor Calculator

R = (V_supply − V_LED) / I_LED. Required series-resistor value to safely drive an LED at a target current.

Result

Required resistor

150.0 Ω (use 1000 Ω E12)

Drop 3.00 V at 20.0 mA. Dissipation 60.0 mW → 1/8 W (125 mW) package.

Supply voltage5.00 V
Total LED forward voltage2.00 V (1 × 2.00 V)
Voltage drop across R3.000 V
Target LED current20.0 mA
Exact R value150.00 Ω
Nearest E12 (≥ exact)1000 Ω
Actual current with E12 R3.00 mA
Resistor dissipation60.00 mW (P = V_drop · I)
Recommended package1/8 W (125 mW)

Step-by-step

Voltage drop across R = V_supply − 1·V_LED = 5 − 2 = 3.000 V.
R = V_drop / I = 3.000 / 0.0200 = 150.00 Ω.
Snap UP to E12 standard: 1000 Ω (ensures I ≤ target).
Power = V_drop · I = 3.000 · 0.0200 = 60.00 mW; choose package with ≥ 2× headroom: 1/8 W (125 mW).

How to use this calculator

Enter supply voltage (battery / regulator output).
Enter LED forward voltage from the datasheet (or use the typical-color reference in the hint).
Set target current — 20 mA for indicator LEDs is the safe default.
For LEDs in series (e.g. 3 white LEDs from a 12 V supply), set "Number of LEDs in SERIES" — voltages add.

About this calculator

LEDs are current-controlled devices: at their forward voltage Vf they'll happily draw infinite current and self-destruct, so a series resistor is needed to limit current. The formula R = (V_supply − V_LED) / I_LED comes straight from Ohm's law on the resistor leg of the circuit. Typical Vf values: red 1.8 V, yellow 2.0 V, green 2.2 V, blue/white 3.0-3.4 V, IR 1.2 V — measure your specific part for precision. Indicator LEDs run at 10-20 mA; high-brightness 30-50 mA; power LEDs need a constant-current driver, not just a resistor. The calculator snaps up to the nearest E12 standard resistor value (5% tolerance — the universal hobbyist set) and recommends a power-rating package with 2× headroom over actual dissipation.

Frequently asked

Only if your supply IS a constant-current source (e.g. dedicated LED driver IC, 3V coin cell whose internal resistance happens to limit current). Otherwise the LED will draw destructive current and burn out in milliseconds.