🔌 Series Resistor Calculator

How to Use This Calculator

Enter the resistance values for each resistor in the series chain. Start with the first two fields and add more with the "Add Resistor" button if needed (up to 10 total). The total equivalent resistance appears as soon as you have at least two valid values.

Series Resistance Formula

In a series circuit, the same current flows through every component. Voltage drops across each resistor in proportion to its resistance. The largest resistor takes the largest share of the supply voltage. Adding more resistors in series always makes the total resistance larger and reduces the current.

Worked Examples

Where This Comes Up in Real Life

Series resistors are used to limit current through components. When connecting an LED to a 9 V supply, a series resistor prevents too much current. If the LED needs 20 mA and has a forward voltage of 2 V, the resistor must drop 7 V: R = 7 / 0.02 = 350 Ω. You would typically choose the nearest standard value of 330 Ω or 390 Ω from the E12 series.

Voltage dividers are two series resistors used to create an intermediate voltage. With R1 = 10 kΩ and R2 = 10 kΩ on a 5 V supply, the midpoint is 2.5 V. Change R2 to 22 kΩ and the midpoint becomes 5 × 22 / (10 + 22) = 3.44 V. This is how many sensors and microcontrollers set reference voltages. The total resistance also determines how much current the divider draws from the supply.