Series Resistor Calculator
When resistors are connected in series, they form a single current path from one terminal of the supply to the other. Every component in the chain carries the same current, while the supply voltage is shared among them. The total resistance of a series combination is simply the arithmetic sum of all individual values: Rt = R1 + R2 + R3 + ... This makes series resistors the simplest way to increase total circuit resistance. Voltage drops across each resistor in proportion to its resistance relative to the total, a principle known as Kirchhoff's Voltage Law. This calculator handles up to six resistors and optionally takes a supply voltage to compute the circuit current, voltage drop across each resistor, and power dissipated in each resistor. Knowing the power in each component is important for selecting the correct wattage rating. Leave any unused resistor fields blank. Results update as you type. For two or more resistors in parallel instead of series, use the parallel resistor calculator.
Series resistance formula
Rt = R1 + R2 + R3 + ...
I = V / Rt
V_n = I × R_n (voltage across each resistor)
P_n = I² × R_n (power in each resistor)
Worked example
R1 = 100 Ω, R2 = 220 Ω in series, supply = 9 V:
- Rt = 100 + 220 = 320 Ω
- I = 9 / 320 = 0.028125 A = 28.13 mA
- V across R1 = 0.028125 * 100 = 2.81 V
- V across R2 = 0.028125 * 220 = 6.19 V (total = 9 V, checks out)
Frequently asked questions
What is the formula for resistors in series?
For resistors in series, the total resistance is simply the sum of all individual resistances: Rt = R1 + R2 + R3 + ... The total is always greater than the largest single resistor in the group.
How does current behave in a series circuit?
In a series circuit, the same current flows through every resistor. There is only one current path, so the current is identical throughout. Voltage, however, is divided across the resistors in proportion to their individual resistance values.
How is voltage divided in a series circuit?
The voltage across each resistor is proportional to its resistance: Vn = V * Rn / Rt. This is the voltage divider principle. A larger resistor drops more voltage; a smaller resistor drops less. The sum of all voltage drops equals the supply voltage (Kirchhoff's Voltage Law).
When would I put resistors in series?
Typical uses include: achieving a resistance value unavailable as a single standard component, distributing power dissipation across multiple resistors, creating a voltage divider chain, adding a current-limiting resistor in series with an LED or other component, and building precision resistor networks.
Does the order of resistors in series matter?
No. The total resistance and current are the same regardless of the physical order of resistors in the series chain. However, the voltage drop across each individual resistor depends on its own value relative to the total, not its position.
Sources
- NIST: SI Units: Electric Current.
Reviewed by the CalculatorHub team, edited by James Graham. 14 June 2026.