How do you calculate resistance in series?

For series resistors, simply add them: Rtotal = R1 + R2 + ... + Rn. The total series resistance is always greater than any individual resistor. Current is the same through all resistors.

How is voltage distributed in series resistors?

Voltage divides proportionally to resistance: Vn = Vin × Rn / Rtotal. A larger resistor gets a larger share of the voltage. This is the principle behind the voltage divider.

What are series resistors used for?

Series resistors are used to limit current (LED current limiting, base resistors), create voltage dividers, reduce voltage levels, and protect inputs from overvoltage. They are also used to achieve non-standard resistance values.

Can I combine series and parallel resistors?

Yes. Complex resistor networks combine series and parallel sections. Simplify by finding the equivalent resistance of each parallel group, then add the series resistances. Use this series calculator for the series portions and the parallel calculator for parallel groups.

Series Resistor Calculator

Calculate total resistance for up to 5 series resistors with voltage drop across each.

Component Values

R3 (optional)

R4 (optional)

R5 (optional)

Supply Voltage (Vin) (optional)

Results

Total Resistance (Rtotal)47.00 kΩ

Rtotal = R1 + R2 + ... + Rn

Series Resistors Explained

In a series circuit, current flows through each resistor in sequence. The total resistance is simply the sum: Rtotal = R1 + R2 + ... + Rn. The same current flows through all resistors, and each one drops a share of the supply voltage proportional to its resistance.

The voltage divider is the most important application of series resistors. Vout = Vin × R2/(R1+R2) lets you create any fraction of the input voltage. Series resistors also appear as current limiters (for LEDs and inputs), protective series resistors for op-amp inputs, and impedance-matching elements.

Power must be considered when sizing resistors. Each resistor dissipates P = I² × R watts. Make sure the total power across all resistors in a chain is within the power ratings. Higher-wattage resistors (¼W, ½W, 1W) are required for higher currents.

Series Resistance

Rtotal = R1 + R2 + ... + Rn

Voltage Drop

Vn = Vin × Rn / Rtotal

Key Points

Rtotal = R1 + R2 + ... + Rn — always larger than any single resistor
Same current flows through all: I = Vin / Rtotal
Voltage divides proportionally: Vn = Vin × Rn / Rtotal
Power: P = I² × R — check each resistor rating

Applications

Current limiting for LEDs and sensor inputs
Voltage divider for ADC input scaling
RC filter component design
Pull-up and pull-down resistor networks

Practical Examples

Three resistors, 5 V supply

100 Ω + 220 Ω + 470 Ω in series from a 5 V source. Find total resistance, current, and voltage across each resistor.

Rtotal = 790 Ω · I = 6.33 mA · V470 = 2.97 V · V220 = 1.39 V · V100 = 0.63 V

Non-standard resistance value

Need 1.3 kΩ but it is not a standard E24 value. Combine 1 kΩ + 270 Ω + 33 Ω in series.

R = 1000 + 270 + 33 = 1303 Ω (within 0.2% of 1.3 kΩ target)

Formula Reference

Series resistance formulas

Total resistance:  Rt = R1 + R2 + R3 + ...
Voltage drop:      Vn = Vin × Rn / Rt
Current (same):    I = Vin / Rt

Design Examples

LED string (12V, 3 LEDs in series)

3× white LEDs (Vf = 3.2V each) + current limiting resistor on a 12V rail. Target current 20mA.

V_resistor = 12 – 3×3.2 = 2.4V · R = 2.4/0.02 = 120Ω (use 150Ω for safety)

Precision voltage divider

5V → 2.5V reference using two equal resistors in series. Add 0.1µF bypass cap for noise rejection.

R1=10kΩ + R2=10kΩ → V_out = 5V × 10k/20k = 2.5 V

Current limiting for a relay coil

12V relay coil (resistance = 200Ω, rated 50mA). Add a series resistor to limit inrush.

R_total needed = 12V/50mA = 240Ω → add 39Ω in series (200 + 39 = 239Ω)

Design tip

In series circuits, the largest resistor dissipates the most power.
Check each resistor power rating: P = I² × R.
Standard ratings: ¼W (hobbyist), ½W (general), 1W, 2W (power applications).

Did you know? Resistors in series simply add. This is useful for building high-precision values: adding a 1 kΩ 0.1% and a 10 Ω 0.1% gives 1010 Ω ±1 Ω. High-voltage divider chains also use series resistors to distribute voltage stress across multiple lower-rated components.