Resistor Calculator

Resistors in series simply add together, while resistors in parallel combine through their reciprocals. In a series circuit, current flows through each resistor one after another, so every resistor adds opposition. In a parallel circuit, current splits across multiple paths, so the combined resistance is always lower than the smallest branch.

Series example: A 100Ω and a 220Ω resistor in series total 100 + 220 = 320Ω. The total is always larger than any individual resistor.

Parallel example: The same 100Ω and 220Ω resistors in parallel give (100 × 220) ÷ (100 + 220) = 22,000 ÷ 320 = 68.75Ω. Notice the result is smaller than 100Ω, the smallest resistor in the pair — that is always true in parallel.

A useful rule of thumb: two equal resistors in parallel give exactly half their value (two 100Ω resistors = 50Ω), three equal resistors give one third, and so on. Once you know the total resistance, you can find the circuit current with Ohm's Law(I = V ÷ R).

Every LED needs a current-limiting resistor sized with the formula R = (Vs − Vf) ÷ I, where Vs is the supply voltage, Vf is the LED forward voltage, and I is the desired LED current in amps. Without a resistor, an LED connected directly to a supply draws excessive current and burns out.

Worked example: Power a red LED (Vf = 2.0 V) from a 12 V supply at 20 mA (0.02 A). The resistor drops 12 − 2.0 = 10 volts, so R = 10 ÷ 0.02 = 500Ω. Since 500Ω is not a standard value, round up to the next E12 value: 560Ω. The resistor dissipates P = 10 × 0.02 = 0.2 W, so with a 2× safety margin you need a 1/2 W resistor— a common 1/4 W part would run too hot.

Always round upto the next standard value. A slightly larger resistor only makes the LED marginally dimmer, while a smaller one over-drives the LED and shortens its life. Typical forward voltages: red 1.8–2.2 V, green 2.0–2.2 V (standard) or ~3.2 V (high-brightness), blue and white 3.0–3.4 V. Check the datasheet when precision matters.

Resistors are manufactured in standard preferred values rather than every possible number. The most common is the E12 series(±10% tolerance), which has 12 values per decade:

Each value repeats in every decade — so you can buy 10Ω, 12Ω, 15Ω, … then 100Ω, 120Ω, 150Ω, and so on up through kilohms and megohms. When a calculated resistance falls between two standard values (like the 500Ω in the LED example above), round up to the next E12 value (560Ω) for current-limiting applications. Tighter series exist for precision work: E24 (±5%) and E96 (±1%) add intermediate values.