LED Resistor Calculator

How it works

An LED drops a nearly fixed forward voltage Vf once it conducts, and its current rises steeply with any extra voltage, so it needs something to set the current. A series resistor does that: it takes the voltage left over, Vs − n × Vf for n LEDs in series, and by Ohm's law passes I = (Vs − n × Vf) ÷ R.

Solving for R at the LED current wanted gives the calculated resistance. Resistors are sold in preferred values; the calculator picks the next E12 value up (IEC 60063), so the actual current comes out at or a little below the current entered, and shows that current.

The resistor dissipates the leftover voltage times the current, P = (Vs − n × Vf) × If, worked out at the current entered (the E12 resistor, passing a little less current, dissipates no more). The rating margin multiplies that power, and the smallest common wattage rating at least that large is shown.

Formula

R = (Vs − n × Vf) ÷ If
R(E12) = smallest of 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 × 10^k that is ≥ R
I(actual) = (Vs − n × Vf) ÷ R(E12)
P = (Vs − n × Vf) × If;   minimum rating = P × margin

Example

One LED with a 2 V forward voltage at 20 mA from a 9 V supply needs (9 − 2) ÷ 0.02 = 350 Ω. The next E12 value is 390 Ω, which passes 7 ÷ 390 = 17.95 mA. The resistor dissipates 7 × 0.02 = 140 mW; twice that is 280 mW, so the rating shown is 1/2 W.

Three LEDs of 1.2 V each from 5 V at 10 mA leave 5 − 3.6 = 1.4 V for the resistor: 140 Ω, rounded up to 150 Ω (9.33 mA).

Assumptions and limitations

• The forward voltage is treated as constant. A real LED's Vf varies with current, temperature and between parts of the same type, so the actual current differs somewhat from the calculated one.
• Typical forward voltages in the help text are from Electronics Tutorials' LED characteristics table (2026), at 20 mA; the LED's data sheet gives the value for a specific part.
• The supply is a steady DC voltage. A battery's voltage falls as it discharges, and the current falls with it.
• LEDs in series share one current. LEDs in parallel on one resistor are not modelled.
• Resistor tolerance (±10 % for E12 parts) shifts the current by about the same proportion.
• The listed wattage ratings and the 2× margin are common practice, not a standard. This is an estimate for circuit design and learning, not a substitute for the LED's and resistor's data sheets.

Frequently asked questions