Series vs Parallel Resistors: When to Use Each Configuration
Understand the difference between series and parallel resistor configurations. Learn the formulas, when to use each, and how to combine them for custom values.
Series Resistors
When resistors are connected end-to-end, the total resistance is simply the sum:
Key properties: same current flows through all resistors, voltage divides proportionally, total resistance is always greater than any individual resistor.
Parallel Resistors
When resistors share the same two nodes, their conductances add:
For two resistors this simplifies to:
Key properties: same voltage across all resistors, current divides inversely, total resistance is always less than the smallest individual resistor.
When to Use Series
- You need a non-standard resistance value (e.g., 13kΩ = 10kΩ + 3kΩ)
- You need to increase voltage rating (voltage divides across series resistors)
- Creating a voltage divider
- Distributing power dissipation across multiple components
When to Use Parallel
- You need a lower resistance than what you have
- You need to increase current capacity
- You need to increase power handling
- Creating a precision value from standard parts
Power Considerations
Series: Voltage Rating Distribution
Two 100Ω, 1/4W resistors in series can handle 10V total and 0.5W total. Each resistor dissipates power proportional to its resistance.
Parallel: Current Sharing
Two 100Ω, 1/4W resistors in parallel can handle 100mA total and 0.5W total. Warning: For parallel resistors of different values, the smaller resistor carries MORE current and dissipates MORE power. Check each one individually!
Common Mistakes
- Using parallel when series is simpler: If you need 9.9kΩ, use 10kΩ + 100Ω (2 parts series) rather than trying to find a parallel combination.
- Forgetting power derating: Two 1/4W resistors in parallel give 1/2W only if they share current equally (same resistance value).
- PCB layout issues: Parallel resistors need symmetrical traces to share current equally.
Use our Series/Parallel Resistor Calculator to compute total resistance with unlimited resistors.