How do you calculate total resistance in series?

In a series circuit, total resistance is simply the sum of all individual resistors: R_total = R1 + R2 + R3 + … The same current flows through each resistor.

How do you calculate total resistance in parallel?

In a parallel circuit, use the reciprocal formula: 1/R_total = 1/R1 + 1/R2 + 1/R3 + … For two resistors, the shortcut is R_total = (R1 × R2) / (R1 + R2). The total resistance is always less than the smallest individual resistor.

Is total resistance in parallel always less than the smallest resistor?

Yes. When resistors are connected in parallel, the total resistance is always lower than the smallest individual resistor. Adding more parallel paths gives current more routes to flow, reducing the overall resistance.

What is the formula for two resistors in parallel?

For exactly two resistors in parallel, the formula simplifies to: R_total = (R1 × R2) / (R1 + R2). This is sometimes called the "product over sum" formula.

When should I use series vs parallel resistors?

Use series resistors when you need to increase total resistance, limit current, or create a voltage divider. Use parallel resistors when you need to decrease total resistance, increase current capacity, or combine non-standard values to achieve a specific resistance.

How do I get a non-standard resistor value using common parts?

Combine two E24 series resistors in series or parallel. For example, 300Ω (not in E24) = 220Ω + 82Ω in series. 1kΩ with better precision = 1.5kΩ ∥ 3kΩ in parallel. For series: R_total = R1 + R2. For parallel: R_total = (R1 × R2) / (R1 + R2). This calculator finds the combined value so you can tune the individual values to hit your target.

Can I increase the wattage rating of a resistor by using multiple in parallel?

Yes. Two identical resistors in parallel share the current equally, so each dissipates half the total power. Their combined power rating is the sum of both ratings. For example, two 100Ω 0.25W resistors in parallel give 50Ω total with 0.5W combined rating. For this to work correctly, resistors in parallel should be equal in value so the load is shared evenly.

Series & Parallel Resistor Calculator

Calculate total resistance for series and parallel resistor combinations. Up to 6 resistors.

Resistor Values

Results

Total Resistance20 kΩ

Current at 3.3V165 µA

Current at 5V250 µA

Current at 12V600 µA

Series — R_total = R1 + R2 + …

Resistors in Series

When to use this: Use this when you need to hit a resistance value that doesn't exist in the E24 series, when you need to increase wattage capacity by paralleling resistors, or when analyzing how a multi-resistor network affects current flow at common supply voltages.

Resistors in series share the same current: every electron that flows through one must flow through all of them. The total resistance is simply the sum: R_total = R1 + R2 + R3 + … It always increases. The voltage across each resistor is proportional to its value, which is exactly why series resistors make voltage dividers.

Series combinations are handy when you need a value that doesn't exist in the E24 or E12 series. A 130Ω and a 220Ω in series give 350Ω, a value you won't find as a standard part. They're also used for precision attenuators and current-limiting networks where you need to split power across multiple components.

Series Formula

R_total = R1 + R2 + R3 + …

Key Points

Total R = R1 + R2 + R3 + … (always increases)
Same current flows through every resistor
Voltage divides proportionally across each resistor
If one resistor fails open, the whole circuit breaks

Applications

Voltage dividers
Current limiting (LED circuits)
Creating non-standard resistance values
Pull-up / pull-down resistor networks

Practical Examples

Get 300 Ω from E24 parts

300Ω doesn't exist in E24. Combine 220Ω + 82Ω in series — both standard E24 values.

220 + 82 = 302 Ω — within 1% of 300Ω

Double wattage with parallel resistors

Need a 50Ω 2W resistor but only have 100Ω 1W parts? Two 100Ω in parallel gives 50Ω with 2W combined rating.

100Ω ∥ 100Ω = 50 Ω, total power rating doubles

Precise 1 kΩ from two E24 values

1.5kΩ in parallel with 3kΩ gives exactly 1kΩ — useful when you need precision without ±1% resistors.

1.5k ∥ 3k = 1.00 kΩ exactly

Common Reference Combinations

Target R	Combination	Mode	Result	Error
300 Ω	220 Ω + 82 Ω	Series	302 Ω	+0.7%
500 Ω	470 Ω + 33 Ω	Series	503 Ω	+0.6%
2 kΩ	1.5 kΩ + 510 Ω	Series	2.01 kΩ	+0.5%
50 Ω	100 Ω ∥ 100 Ω	Parallel	50.0 Ω	0%
1 kΩ	1.5 kΩ ∥ 3 kΩ	Parallel	1.00 kΩ	0%
6.67 kΩ	10 kΩ ∥ 20 kΩ	Parallel	6.67 kΩ	0%

Series vs Parallel: which to choose?

Use series when you need to increase resistance or create a non-standard value higher than anything you have in your parts bin. Voltage drops across each resistor proportionally — useful for voltage dividers and current limiters. The downside is that power rating doesn't change: two 1/4W resistors in series are still rated 1/4W each.

Use parallel when you need to decrease resistance or increase power-handling capacity. The combined wattage rating is the sum of the individual ratings — two 1/4W resistors in parallel give 1/2W total. The total resistance is always less than the smallest individual resistor.

Property	Series	Parallel
Total resistance	Increases (sum)	Decreases (less than smallest)
Current	Same through all	Splits between branches
Voltage	Splits across each	Same across all
Power rating	Limited by weakest resistor	Adds up (shared load)
Use case	Non-standard values, voltage dividers	Higher wattage, lower resistance

Did you know? When you place resistors in parallel, the equivalent resistance is always lower than the smallest resistor in the network. This is useful for fine-tuning resistance values using standard E24 components.