LED Series Resistor Calculator

One resistor per series string

This model is for low-voltage DC indicator LEDs, not mains lighting, addressable LED strips, or high-power constant-current drivers. Each parallel branch has its own resistor and the entered number of series LEDs. Sharing one resistor across bare parallel LEDs does not ensure equal current. More parallel strings increase supply demand but do not change the resistance needed in each branch.

Method and rounded component selection

For n LEDs, nominal voltage across the resistor is Vn = nominal supply - n x nominal LED voltage. Ideal resistance is Vn / I, with I in amperes (mA / 1,000). Vn must be positive. Worst modeled resistor voltage Vw = maximum supply - n x minimum LED voltage. For tolerance fraction t, require R at least Vw / [I x (1 - t)].

The calculator rounds upward through this explicit candidate list per decade: 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91. Values are multiplied by powers of ten; rounding upward is intentional. This familiar 24-value list is a component-selection convenience, not proof of stock availability or a certified standard implementation.

Nominal current = Vn / R. Worst modeled current = Vw / [R(1 - t)]. Worst resistor dissipation = Vw squared / [R(1 - t)]. Minimum requested power rating = dissipation x your multiplier. Select an available rating at or above that result and check the manufacturer's temperature derating. Nominal supply current is branch current x number of strings; supply power is that current x nominal supply.

Worked example: two strings of three red LEDs

At 12 V nominal, 12.6 V maximum, 2 V nominal and 1.8 V minimum per LED, with a 20 mA ceiling and 5% tolerance, the nominal ideal is 300 ohm. The tolerance-aware minimum is about 378.9474 ohm; the rounded candidate is 390 ohm per branch. Nominal current is 15.3846 mA, worst modeled current is 19.4332 mA, and worst dissipation is 0.1399 W per resistor. A 2x allowance asks for at least 0.2798 W, so a 0.25 W part does not meet that allowance. Two branches draw about 30.7692 mA and 0.3692 W nominally.

Interpret headroom rather than chasing brightness

The rounded resistor can produce appreciably less nominal current than the ceiling; that is the trade-off for voltage and tolerance protection. If the supply barely exceeds the LED stack voltage, small forward-voltage changes have a large effect. A resistor does not regulate current as well as a suitable current driver. Do not infer brightness, LED lifetime, or junction temperature from these outputs.

Sources, bounds, and limitations

Adafruit's All About LEDs guide explains current limiting with Ohm's law and Kirchhoff's voltage law (undated guide; use the actual part datasheet for ratings). This tool uses those circuit laws, not an empirical brightness model. Nominal/maximum supply are bounded to 0.1-48 V, forward voltages to 0.1-6 V, branch ceilings to 0.1-100 mA, series count to 1-20, and parallel branches to 1-100.

Assumptions: constant entered forward voltages, resistive DC behavior, and simultaneous worst limits. Temperature dependence, transients, PWM, resistor pulse ratings, and power-supply faults are excluded. Even low voltage can involve large fault currents. Verify datasheets and the circuit with appropriate current limiting; the result is not a wiring or safety certification.

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Last updated: 2026-10-09. Educational component estimate; confirm electrical and thermal ratings.