This LED resistor calculator sizes the series resistor that keeps an LED alive: R = (supply voltage − LED forward voltage) ÷ target current — with the resistor’s power dissipation and the standard E12 value to actually buy.

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One Resistor Between Your LED and the Smoke

An LED is not a tiny light bulb. A bulb's filament resists more as it heats, self-limiting its current; an LED is a diode whose current, once the forward voltage Vf is crossed, rises exponentially — a few tenths of a volt too many and current runs away until the junction cooks. The series resistor is the adult supervision: it drops the excess voltage and pins the current by Ohm's law, R = (Vs − Vf)/I. The defaults — 5 V supply, a red LED (Vf 2.0 V), 20 mA target — give R = 3/0.02 = 150 Ω, dissipating just 60 mW. Everything an LED circuit needs to know is in that one subtraction: the resistor only ever sees Vs − Vf; the LED's current is set entirely by that leftover voltage divided by R. This is also why LED brightness is controlled through current (or PWM), never by nudging voltage — the exponential curve makes voltage control a knife's edge, and why datasheets specify Vf as a RANGE (1.8–2.2 V for one red part) that honest designs must survive at both ends.

The working details are all corollaries. Series strings: n LEDs in series sum their Vf — three red LEDs on 12 V need R = (12 − 6)/0.02 = 300 Ω, and the string fails as one if the summed Vf approaches the supply (a car "12 V" rail that actually runs 13.8–14.4 V is the classic trap — design at the real rail voltage). Parallel LEDs each need their OWN resistor: shared-resistor parallels invite the lowest-Vf LED to hog current and cascade the failures. Power: the resistor burns (Vs − Vf) × I as heat — at 12 V driving one red LED that's 200 mW, a ¼ W resistor running hot at its limit; the 2× derating rule (choose rating ≥ 2× dissipation) is the difference between a warm part and a brown spot on the PCB. And efficiency: dropping 10 of 12 volts across a resistor wastes 83% of the power — fine for an indicator, inexcusable for illumination, where constant-current drivers replace the resistor entirely. For the humble indicator and hobby build, though, this calculator's formula plus an E12 standard value and the 2× power rule is the complete professional answer.

Worked Examples (Verified)

  • Defaults: 5 V, Vf 2.0, 20 mA → R = 150 Ω, resistor power 60 mW.
  • 12 V car rail: Vf 2.0, 20 mA → R = 500 Ω (E12: 470 or 560), power 200 mW — use ≥ ½ W.
  • White LED on 5 V: Vf 3.2, 20 mA → R = 90 Ω (E12: 91), power 36 mW.
  • Three in series on 12 V: 3 × Vf 2.0 = 6.0, 20 mA → R = 300 Ω, power 120 mW.
  • Gentle indicator: 9 V, Vf 2.0, 5 mA → R = 1,400 Ω — dimmer and nearly free to run.

LED Resistor Discipline

  • One resistor per LED or per series string — never one resistor feeding parallel LEDs.
  • Design at the extremes: highest supply voltage with lowest Vf sets peak current; check it stays under the LED's rating (typically 20–30 mA continuous for indicator LEDs).
  • Derate resistor power 2×; a part at its rating runs at ~150 °C+ surface temperature.
  • Vf rises slightly with current and falls with temperature — for precision current, use a current source/driver, not a resistor.
  • High-power LEDs (1 W+) do not belong on resistors: efficiency and thermal runaway both demand constant-current drivers.

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Frequently Asked Questions

How do you calculate the resistor for an LED?

R = (Vs − Vf) ÷ I: supply voltage minus the LED's forward voltage, divided by the target current in amps. A red LED (Vf 2.0 V) on 5 V at 20 mA: (5 − 2)/0.02 = 150 Ω. This calculator returns the exact value, the nearest standard E12 value, the resistor's power dissipation, and the actual current the standard value will deliver — the four numbers a build actually needs.

What happens if I connect an LED without a resistor?

Usually a brief, bright farewell. A diode's current rises exponentially above Vf: a 5 V supply against a 2 V LED doesn't push "a bit extra" current — it pushes whatever the supply can deliver through a near-short, destroying the junction in milliseconds to seconds (coin cells, with high internal resistance, are the forgiving exception — the cell itself acts as the resistor, which is why throwie LEDs survive on CR2032s). Any stiff supply — USB, bench, battery pack — requires the resistor this page sizes.

What resistor wattage do I need?

At least twice the dissipation P = (Vs − Vf) × I. The defaults dissipate 60 mW → any ¼ W resistor is comfortable. The 12 V single-red-LED case dissipates 200 mW → a ¼ W part would run at 80% of rating and hot; choose ½ W. The rule exists because resistor ratings assume free air at 70 °C ambient and a part at full rating runs extremely hot — derating 2× keeps temperatures sane, values stable and PCBs unscorched.

Can I run multiple LEDs from one resistor?

In SERIES, yes — that's one string, one resistor, with the Vf's summed (and the resistor sized for the leftover voltage). In PARALLEL, no: LEDs differ slightly in Vf, the lowest-Vf unit takes disproportionate current, heats, drops its Vf further and hogs more — thermal runaway that kills them in sequence. Parallel strings each get their own resistor. This is the most-violated rule in hobby LED work, and its failure signature (one LED brighter, then dead, then the next) is unmistakable.

What forward voltage should I use for my LED color?

Typical ranges at 20 mA: infrared ~1.2–1.5 V, red 1.8–2.2, orange/yellow 2.0–2.2, green 2.0–3.2 (chemistry-dependent), blue and white 3.0–3.4, UV 3.2–3.8. When in doubt use the middle of your part's datasheet range and verify current at the extremes; for a generic indicator with no datasheet, 2.0 V (red/yellow/green classics) or 3.2 V (blue/white) are the standard assumptions. On low-voltage rails the Vf choice dominates the design — a white LED on 3.3 V leaves only 0.1 V of headroom, making resistor control sloppy and a boost driver the real answer.

Why is my LED dimmer or brighter than expected?

Work the formula backwards: actual current = (Vs − Vf)/R with YOUR values. Usual culprits: the supply isn't what it claims (a "12 V" adapter at 12.6, a sagging 9 V battery at 7.5 under load), the LED's Vf sits at its range edge, or the resistor is a standard value off the computed one (150 → 180 Ω drops the defaults' current from 20 to 16.7 mA). Brightness tracks current almost linearly in indicator LEDs, so a 20% current error is a visible 20% brightness error — the calculator shows actual current with the E12 value for exactly this reason.

Is a resistor or a driver better for high-power LEDs?

A constant-current driver, decisively, above indicator duty. A resistor wastes the dropped voltage as heat (at 12 V in, 3.2 V LED, ~73% wasted) and its current drifts with supply and temperature; a switching constant-current driver regulates the LED current at 85–95% efficiency regardless. High-power LEDs (350 mA–3 A) on resistors also face thermal runaway as Vf falls with heat. The resistor's kingdom is indicators and small strings where simplicity wins and the wasted milliwatts don't matter.

Is this LED resistor calculator free?

Yes — free, instant, no sign-up. Enter supply voltage, LED forward voltage and target current for the resistor value, nearest E12 standard, power dissipation and resulting current.