CALTRX

LED Resistor Calculator

Calculate the current-limiting resistor for any LED. Enter supply voltage, LED forward voltage, and desired current — get the resistance value and power rating needed.

V
V
mA

Enter your values above to see the results.

Tips & Notes

  • ✓LED forward voltage (V_f) varies by color: Red/Yellow ~2.0 V, Green ~2.1 V, Blue/White/UV ~3.0-3.5 V. Always check the datasheet — running at rated current without correct V_f estimate risks over-driving the LED.
  • ✓Typical LED current is 10-20 mA for standard indicator LEDs. High-brightness LEDs designed for illumination can handle 350 mA to 1 A or more but require heatsinking. Use 10 mA for battery-powered circuits to extend life.
  • ✓Multiple LEDs in series: subtract all V_f values from supply voltage before dividing by current. Two red LEDs in series from 5 V: V_resistor = 5 − 2×2.0 = 1.0 V; R = 1.0 / 0.020 = 50 Ω.
  • ✓LED brightness is not linear with current — doubling current does not double perceived brightness (due to the logarithmic nature of human vision). For dimming, use PWM (pulse-width modulation) at constant current rather than reducing current.
  • ✓The resistor power dissipation P = (V_supply − V_f) × I_LED — all the supply voltage not used by the LED is wasted as heat in the resistor. For efficiency, use a constant current LED driver IC instead of a resistor when power matters.

Common Mistakes

  • ✗Connecting an LED directly to a 5 V or 3.3 V supply without a current-limiting resistor — LEDs are current devices with very low dynamic resistance. Without a resistor, current rises exponentially until the LED burns out within seconds.
  • ✗Using the wrong LED forward voltage — assume 2.0 V for red LEDs and 3.2 V for blue/white LEDs as starting points, but verify with the datasheet. Using 2.0 V for a white LED (V_f = 3.2 V) gives 60% too much current.
  • ✗Forgetting to check the resistor power rating — a 5 V supply with a 2.1 V green LED at 20 mA: V_resistor = 2.9 V, P = 2.9 × 0.020 = 58 mW. A 1/8 W (125 mW) resistor is marginal — use 1/4 W for safety.
  • ✗Using a single resistor for multiple parallel LEDs — individual LED forward voltages vary slightly even within the same batch. One resistor for parallel LEDs lets current crowd into the LED with the lowest V_f, burning it out. Use individual resistors for each LED.
  • ✗Not accounting for supply voltage variation — a 9 V battery fresh is 9 V but drops to 6 V as it discharges. If R = (9−2) / 0.020 = 350 Ω, at 6 V: I = (6−2) / 350 = 11.4 mA (acceptable). Verify LED still works at minimum supply voltage.

LED Resistor Calculator Overview

The LED current-limiting resistor calculation is the most common practical application of Ohm's Law in electronics. Every LED in a circuit needs one, and getting it wrong causes either a burnt-out LED (no resistor or too small) or a dim, inefficient display (too large a resistor).

LED resistor formula:

R = (V_supply − V_f) / I_LED | P_resistor = (V_supply − V_f) × I_LED
EX: 9 V battery, white LED V_f = 3.2 V, target I = 15 mA → R = (9 − 3.2) / 0.015 = 5.8 / 0.015 = 387 Ω → use 390 Ω standard → P = 5.8 × 0.015 = 87 mW → use 1/4 W resistor
Multiple LEDs — series vs. parallel:
Series: R = (V_supply − n×V_f) / I | Parallel: use individual resistors per LED branch
EX: 12 V supply, three red LEDs (V_f=2.0 V each) in series at 15 mA → R = (12 − 3×2.0) / 0.015 = 6.0 / 0.015 = 400 Ω → use 390 Ω → P = 6.0 × 0.015 = 90 mW
LED forward voltage by color — quick reference:
LED ColorTypical V_fResistor (5 V, 20 mA)Resistor (3.3 V, 10 mA)
Infrared (850-940 nm)1.2–1.5 V175–190 Ω180–210 Ω
Red (620-750 nm)1.8–2.2 V140–160 Ω110–150 Ω
Orange (590-620 nm)2.0–2.2 V140–150 Ω110–130 Ω
Yellow (570-590 nm)2.0–2.2 V140–150 Ω110–130 Ω
Green (495-570 nm)2.0–3.5 V75–150 ΩSee note*
Blue (450-495 nm)3.0–3.5 V75–100 Ω*Not recommended
White (broadband)3.0–3.5 V75–100 Ω*Not recommended
UV (390-420 nm)3.2–3.8 V60–90 Ω*Marginal
Supply voltage vs. LED efficiency (resistor method):
Supply VLED V_fV_resistorEfficiency (V_f/V_s)Resistor Waste
3.3 V3.2 V (white)0.1 V97%Very low
5 V3.2 V (white)1.8 V64%Moderate
5 V2.0 V (red)3.0 V40%High
12 V3.2 V (white)8.8 V27%Very high — use driver
12 V3×3.2 V series2.4 V80%Low
The asterisk in the table (*Not recommended) indicates that a 3.3 V supply is too close to the blue/white LED forward voltage for safe resistor operation — a small forward voltage variation changes current dramatically. A 3.3 V supply powering a 3.2 V V_f LED through a 10 Ω resistor: I = (3.3−3.2)/10 = 10 mA. But if V_f drops to 3.0 V (temperature or unit variation): I = 0.3/10 = 30 mA — three times the target. Use a constant-current driver IC for white/blue LEDs from 3.3 V supplies.

Frequently Asked Questions

Resistor R = (V_supply − V_f) / I_LED, where V_supply is your power supply voltage, V_f is the LED forward voltage from the datasheet, and I_LED is your desired current (typically 10-20 mA = 0.01-0.02 A). Example: 5 V supply, red LED V_f = 2.0 V, I = 20 mA → R = (5 − 2.0) / 0.020 = 3.0 / 0.020 = 150 Ω. Power dissipated in resistor: P = 3.0 × 0.020 = 60 mW → use a 1/4 W (250 mW) resistor for comfortable margin.

Forward voltage (V_f) is the voltage drop across the LED when conducting at its rated current — the voltage the LED consumes from your supply. Find V_f in the LED datasheet under "Forward Voltage" at the rated forward current. If you do not have the datasheet, use these typical values as starting points: IR/Red ~2.0 V; Orange/Yellow ~2.1 V; Green (standard) ~2.1 V; Green (high-brightness) ~3.0 V; Blue ~3.2 V; White ~3.2-3.5 V; UV ~3.5 V. Then measure the actual V_f across the running LED with a voltmeter to verify.

LEDs in series: subtract each V_f from supply voltage; total current is the same through all. V_resistor = V_supply − (V_f1 + V_f2 + ...); R = V_resistor / I_LED. Example: two white LEDs (V_f = 3.2 V each) from 12 V at 20 mA → V_resistor = 12 − 6.4 = 5.6 V → R = 5.6 / 0.020 = 280 Ω → use 270 Ω. LEDs in parallel: never use a single resistor. Individual V_f differences cause unequal current sharing. Use one resistor per LED branch.

Without a current-limiting resistor, an LED is directly connected to a voltage source. LEDs have an exponential V-I characteristic — a small increase in voltage causes a large increase in current. From a 3.3 V supply with a blue LED (V_f = 3.2 V), the 0.1 V excess drives enormous current, limited only by the supply impedance and LED bond wire resistance. The LED temperature rises rapidly, V_f decreases (LEDs have negative temperature coefficient), current increases further, and within seconds the LED junction burns out. Always use a current-limiting resistor or constant-current driver.

Analog dimming: reduce current by increasing resistor value. Simple but LEDs change color temperature at very low currents (warm white LEDs shift warmer at lower currents). PWM dimming: switch the LED on and off at a high frequency (1 kHz+) at full current, varying the on-time (duty cycle). At 50% duty cycle, the LED appears half as bright. PWM preserves color accuracy and efficiency. The human eye cannot detect flicker above about 60-100 Hz, but some people (and cameras) are sensitive up to 200+ Hz. For camera-friendly PWM, use 1 kHz minimum. Most LED driver ICs offer PWM dimming inputs.

A current-limiting resistor is simple and cheap but inefficient — the excess supply voltage (V_supply − V_f) is wasted as heat in the resistor. Efficiency = V_f / V_supply. For a red LED at 2 V from a 5 V supply: efficiency = 2/5 = 40%. A constant-current LED driver IC maintains a set current regardless of supply voltage changes or LED V_f variation, and can be much more efficient (switching drivers achieve 85-95% efficiency). Use resistors for simple indicator LEDs; use constant-current drivers for illumination LEDs, multiple LEDs in parallel, or battery-powered applications where efficiency matters.