Resistor Series & Parallel Calculator

Resistors in series carry the same current through each component in turn, so their resistances add directly. The total equivalent resistance is the simple arithmetic sum of all individual values. For example, a 100-ohm, 220-ohm, and 330-ohm resistor in series give 650 ohms. Series combinations are commonly used in voltage-divider circuits and to limit current to a specific value. Because the formula is additive, series strings always result in a higher resistance than any single component in the group.

When resistors are connected in parallel, each one provides an additional path for current to flow. Every extra path reduces the overall opposition to current, which is why the combined resistance always falls below the smallest individual value. Mathematically, you add the reciprocals of all resistances and take the reciprocal of that sum. For just two equal resistors the result is exactly half of either one. Parallel combinations are used in circuits where you want to increase current capacity, split current between branches, or fine-tune a resistance value that you cannot achieve with a single standard component.

Enter all resistor values in plain ohms. The calculator works in consistent ohm units throughout, so you need to convert before typing. A 4.7 kilohm resistor goes in as 4700; a 1 megohm resistor goes in as 1000000; a 0.1 ohm sense resistor goes in as 0.1. The output is also in ohms, which you can then convert to kilohms or megohms mentally by dividing by 1000 or 1000000 respectively. Keeping everything in the same unit avoids scaling errors that are easy to make when mixing kilohm and megohm values.

It computes equivalent resistance only, which is one important parameter in circuit design. A complete design also requires verifying that each resistor's power rating is not exceeded under the expected current, that voltage ratings are appropriate, and that tolerances on individual components stack up acceptably for your application. For mains-voltage circuits, high-energy circuits, or anything going into a product sold to consumers, the design should be reviewed by a licensed electrical engineer and comply with applicable safety standards such as IEC or UL requirements.