LED Resistor Calculator
Size the series resistor for any LED circuit: R = (Vs − Vf) / I with forward-voltage presets per LED color, series/parallel LED arrays, resistor power dissipation (P = I²R) with a 2×-margin wattage rating, and an E12/E24 picker that always rounds UP so LED current never exceeds your target. Everything runs in your browser — no data leaves this page.
1 · LED array
2 · Solve
Min supply Vs min = Vf total + I × R. Add your own safety headroom (about 1–2 V) on top for stable brightness.
How it works
Worked example. A red LED (Vf 2.0 V) on a 5 V supply at 20 mA: R = (5 − 2) / 0.02 = 150 Ω, which is exactly an E12 value. The resistor dissipates P = 0.02² × 150 = 0.06 W; 2× margin is 0.12 W, so the smallest standard rating is 1/8 W, and 1/4 W is the common safe choice.
Array example. Three red LEDs in series, two parallel strings, on 12 V at 20 mA per LED: Vf total = 6.0 V, I total = 40 mA → R = (12 − 6) / 0.04 = 150 Ω, P = 0.04² × 150 = 0.24 W → pick a 1/2 W resistor.
Key insight: the resistor never sets LED brightness by itself — it sets the current, and brightness follows the current. Rounding the E-value up is deliberate: rounding down would push the current above your target and shorten LED life.
Scope boundary
This tool: DC circuits with ideal LEDs (fixed Vf per color preset), 1–100 series LEDs per string and 1–100 parallel strings, standard E12/E24 values, resistive power with a 2× rating margin. Not this tool: constant-current LED drivers (no resistor needed), PWM dimming, AC circuits, temperature drift of Vf, or high-power LED thermal design.
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Frequently asked questions
What is the formula for an LED series resistor?
R = (Vs - Vf) / I, where Vs is the supply voltage, Vf is the total LED forward voltage, and I is the target LED current. Example: a red LED (Vf 2.0 V) on a 5 V supply at 20 mA needs R = (5 - 2) / 0.02 = 150 ohm. Resistor power is P = I squared x R = 0.06 W, so the smallest standard rating with a 2x margin is 1/8 W (0.125 W); 1/4 W is the common safe choice.
Why does an LED need a resistor at all?
An LED is non-ohmic: above its forward voltage its V-I curve is nearly vertical, so without a limiter the current rises until the LED burns out. The resistor absorbs the leftover voltage (Vs - Vf) and sets the current through Ohm's law, I = (Vs - Vf) / R. Without one, even a small supply overshoot can destroy the LED in seconds.
Do I need a resistor for an LED on a 3.3 V supply?
Yes, unless you use a constant-current driver instead. Take a white LED (Vf about 3.2 V) on 3.3 V at 20 mA: R = (3.3 - 3.2) / 0.02 = 5 ohm, and the nearest E12 value above it is 5.6 ohm. But with only 0.1 V of headroom the actual current swings a lot if Vf or the supply drifts by even tens of millivolts, so a lower current target or a constant-current driver is safer for 3.3 V logic supplies.
What are typical LED forward voltages by color?
Common datasheet ranges: red 1.8 to 2.2 V (typical 2.0 V), orange about 2.1 V, yellow 1.8 to 2.4 V (typical 2.1 V), standard green 2.0 to 2.4 V (typical 2.2 V), modern pure-green (InGaN) 3.0 to 3.4 V, blue 3.0 to 3.5 V (typical 3.2 V), white 3.0 to 3.6 V (typical 3.2 V), infrared 1.2 to 1.4 V, ultraviolet 3.4 to 3.8 V. Always confirm with your LED's datasheet before a final build.
How do I calculate the resistor for multiple LEDs in series and parallel?
In series, forward voltages add: 3 red LEDs give Vf total = 3 x 2.0 = 6.0 V, with the same current through all of them. In parallel strings, currents add: 2 strings at 20 mA each need 40 mA total. Compute R from the totals. Example: 3 red in series, 2 strings in parallel, on 12 V at 20 mA per LED: R = (12 - 6) / 0.04 = 150 ohm, P = 0.04 squared x 150 = 0.24 W, so with a 2x margin choose a 1/2 W resistor.
Why pick the next standard resistor value UP, not down?
Rounding down lowers the resistance and pushes the LED current above your target, which shortens LED life. Always round up to the nearest E12/E24 value. Example: a blue LED (Vf 3.2 V) on 12 V at 20 mA needs 440 ohm; the E12 value above is 470 ohm, giving (12 - 3.2) / 470 = 18.7 mA, safely at or below the 20 mA target.
What wattage rating should the resistor have?
Calculate the dissipated power P = I squared x R, then choose the smallest standard rating that is at least 2x that value for thermal margin. Example: a blue LED on 12 V at 20 mA with a 470 ohm resistor dissipates 0.188 W, and 2 x 0.188 = 0.376 W, so pick a 1/2 W resistor. A 1/4 W resistor (0.25 W) would run with almost no margin here.
Can I use a larger resistor than the calculator gives?
Yes, that is always safe: a larger resistor gives less current and a dimmer LED. Example: using 1 kOhm instead of 150 ohm for a red LED (Vf 2.0 V) on 5 V gives I = (5 - 2) / 1000 = 3 mA. Indicator LEDs are often run at 2 to 5 mA on purpose to save power while staying clearly visible.
What if my supply voltage is lower than the LED's forward voltage?
There is no valid resistor then: R = (Vs - Vf) / I would be zero or negative, which means no resistor can set the current. Options: use fewer LEDs in series, raise the supply voltage, or switch to a constant-current boost driver. As a rule of thumb, keep Vs at least 1 to 2 V above total Vf so the current stays stable.
Can I use one resistor for several LEDs in parallel?
Not recommended. Real LEDs have slightly different forward voltages, and the one with the lowest Vf hogs most of the current, which can cascade into failure. The correct approach is one resistor per series string: compute Vf total per string, then size R for that string's current. For general resistor-network math without LEDs, see our parallel resistor calculator.
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