How to Calculate Resistors in Series
When resistors are connected end-to-end so there is only one path for current to flow, they are in series. Because charge has nowhere else to go, the same current passes through every resistor in the chain, and their resistances simply add together: R_total = R₁ + R₂ + R₃ + … + Rₙ. This calculator sums up to four resistor values and, if you provide a supply voltage, also finds the circuit current, the power dissipated, and how much voltage each resistor drops.
Why series resistances add directly
Resistance opposes current flow, and stacking resistors in series is like lengthening a single wire — each one adds its own opposition without giving current a shortcut around it. Applying Ohm's Law (V = IR) to the whole chain with a single shared current I gives V_total = I·R₁ + I·R₂ + I·R₃ + … = I·(R₁ + R₂ + R₃ + …), so the equivalent resistance seen by the source is simply the sum of the individual resistors. This is the opposite of the parallel case, where 1/R_total = 1/R₁ + 1/R₂ + …, and the combined resistance is always smaller than the smallest individual resistor.
Current, voltage divider, and power
Once R_total is known, Ohm's Law gives the single current that flows through every resistor: I = V ÷ R_total. That same current, multiplied by each resistor's value, gives its individual voltage drop: Vₙ = I × Rₙ — equivalently written as the voltage-divider rule, Vₙ = V × (Rₙ ÷ R_total). The drops across all resistors always add back up to the source voltage (Kirchhoff's Voltage Law). Power dissipated by the whole chain is P = V × I = I² × R_total, and each resistor individually dissipates I² × Rₙ as heat — check that figure against each component's power rating before building the circuit, and derate to roughly 50–70% of a resistor's rated wattage for reliable long-term operation.