How this instrument works
A light-emitting diode is not a resistor, and treating it like one is the single most common wiring mistake in beginner electronics. Its current-versus-voltage curve is exponential rather than linear: below a threshold almost nothing conducts, and just past that threshold the current climbs so steeply that a fraction of a volt separates a dim glow from a dead chip. What a datasheet calls forward voltage, Vf, is really just where that curve happens to sit at one particular current — not a fixed property of the part, but a snapshot taken at the current you actually care about.
Wire the supply, the LED, and a resistor into one loop and Kirchhoff's voltage law says their drops must sum to the supply voltage: Vs = Vf + I·R. Because Vf changes only slowly with current near the LED's working point, it behaves as though it were fixed for the purpose of this calculation, which leaves the resistor to absorb whatever voltage the LED does not — Vs minus Vf — while holding the current to the value you actually chose. Rearranging that single subtraction and division is the entire formula: R = (Vs − Vf) ⁄ If. None of the diode's nonlinearity survives the rearrangement; only the resistor does real Ohm's-law work here.
Two things keep this from being exact. Forward voltage drifts on its own: it falls by roughly two millivolts per degree Celsius as an LED warms, and it climbs again at higher current because the die carries a small internal series resistance of its own. Vf also depends heavily on which semiconductor made the LED, not just its size — a red or amber indicator built from aluminium gallium arsenide typically sits near 1.8 to 2.2 V, while a blue or white LED built from indium gallium nitride needs closer to 2.8 to 3.4 V for the same current. Plugging a red LED's forward voltage into a calculation meant for a blue one is a frequent source of resistors that are quietly the wrong value.
- Enter Supply voltage — the battery or rail feeding the whole circuit, in volts.
- Enter LED forward voltage — read it off the datasheet at your target current, or use a typical figure for the LED's color.
- Set LED forward current — the current you want the LED to run at; 20 mA is the standard default for a 5 mm indicator LED.
- Read Current-limiting resistor, then round up to the next standard resistor value your parts bin actually has.
- If the page reports an error instead, Supply voltage is below LED forward voltage — that LED cannot light from this supply at all.
Worked example — a red LED on a 9 V battery
A 9 V PP3 battery is about to drive a diffused red LED rated for a forward voltage of 2.0 V at its intended operating current of 20 mA. Enter 9 into Supply voltage, 2 into LED forward voltage, and 0.02 (or 20 mA on the unit menu) into LED forward current. The resistor has to absorb whatever the LED does not: 9 − 2 = 7 V. Dividing that by the 0.02 A target gives R = 7 ⁄ 0.02 = 350 Ω, the value Current-limiting resistor returns.
350 Ω is not a standard resistor value, so the nearest E24 part, 360 Ω, is what actually goes on the board; wired in, it draws 7 ⁄ 360 ≈ 19.4 mA, close enough to the 20 mA target that no eye could tell the difference. That resistor also has to survive the power it dissipates: P = If²·R = 0.02² × 350 ≈ 0.14 W, comfortably inside a common quarter-watt part's rating. Ask the same circuit for a dimmer 10 mA instead and the required resistor doubles to 700 Ω, because the same 7 V now has to be spread across half the current.
Questions
Why can't I just wire the LED straight to the battery?
Because an LED's current rises exponentially with voltage rather than linearly, so nothing limits it once the diode turns on. A 9 V supply across a 2 V red LED with no resistor is limited only by wiring and battery resistance, which is nowhere near enough — current spikes to whatever the source can deliver, and the die overheats and fails within a fraction of a second.
Which forward voltage figure should I use for my LED?
Whatever the datasheet lists at your target current, since Vf depends on that current too. Without a datasheet, use typical figures by color: red, amber, and yellow LEDs (AlGaAs or GaAsP chips) sit near 1.8–2.2 V; green varies more, roughly 2.0–3.2 V depending on chemistry; blue, white, and UV LEDs (InGaN chips) run closer to 2.8–3.4 V.
What current should LED forward current actually be set to?
20 mA is the long-standing default for a standard 5 mm indicator LED and sits safely inside most datasheets' 20–30 mA continuous rating. High-brightness LEDs are often visibly bright at 2–5 mA and last longer run there; small SMD packages like 0603 or 0805 usually want less current too, so check the part's own datasheet rather than assuming 20 mA fits everything.
I don't have the exact resistor value the calculator returned — does that matter?
Rarely by much. Rounding up to the next standard value lowers current slightly and dims the LED a little; rounding down raises it slightly. For the 350 Ω example above, a 330 Ω resistor pushes current to about 21.2 mA and a 390 Ω resistor drops it to about 17.9 mA, both fine for a general-purpose indicator LED.
Why does the calculator refuse a result when supply voltage is too low?
Because Supply voltage must exceed LED forward voltage, or the subtraction Vs − Vf reaches zero or negative, which has no physical resistor to match — the LED simply cannot turn on from that supply. A 2 V supply cannot light a 2 V red LED at any current; you need headroom for both the diode's own drop and the resistor's.
Does the resistor's power rating matter, or just its ohms?
Both. The resistor dissipates P = If² × R as heat, and exceeding its rated wattage cooks it regardless of how correct the ohm value is. In the 350 Ω, 20 mA example that's about 0.14 W, well inside a common quarter-watt part; push the same resistor to 100 mA instead and it dissipates 3.5 W, enough to damage a small resistor almost immediately.