Circuit Analysis & Fundamentals
Current Divider Calculator
Branch currents when a total current splits between two parallel resistors — with the inverse-ratio rule, the shared node voltage, and a labeled schematic.
About this calculator
When a current reaches a node and splits between two parallel resistors, each branch takes a share set by the OTHER branch's resistance: the easier path carries more current. For two resistors the rule is I1 = I · R2/(R1 + R2) and I2 = I · R1/(R1 + R2) — note the cross-over, R2 appears in the numerator for the current through R1. This is the mirror image of the voltage divider, where the resistor's own value appears on top.
Give the calculator the total current entering the pair and the two branch resistances, and it returns each branch current, the voltage common to both branches, and the equivalent parallel resistance. The schematic labels the split so the picture and the numbers agree.
The key intuition is inverse proportionality: the smaller resistor carries the larger current, because current follows the path of least resistance. Two equal resistors split the current evenly. A branch of nearly zero ohms hogs almost all of it; a branch of very high resistance carries almost none. This is exactly why a low-resistance fault path or an unintended parallel route can steal current from where it was meant to go, and why shunt resistors are sized deliberately to carry a known fraction of a measured current.
Design notes & common mistakes
- Cross-over rule: the current in R1 uses R2 on top — the OPPOSITE resistor. This is the mirror of the voltage divider, where a resistor's own value is on top.
- Least resistance wins: the smaller resistor carries more current. Equal resistors split the current evenly.
- A near-zero-ohm parallel path (a fault, or a much smaller shunt) steals almost all the current — the basis of both ammeter shunts and unintended sneak paths.
- The branch currents always add back to the total (KCL) — a quick check on your arithmetic.
Assumptions
- Two ideal resistors in parallel sharing a known total current; DC or low frequency.
- The source delivers the stated total current regardless of the load (ideal current source).
- Resistors are exact; tolerance and temperature coefficient are not applied.
When to use this calculator
Appropriate for
- Finding how a current splits between two parallel resistors
- Sizing an ammeter shunt or a deliberate current-sharing path
- Teaching current division and its inverse-ratio rule
Not suitable for
- More than two branches without using the conductance-ratio form
- Reactive branches or frequency-dependent current sharing
- Sources whose internal resistance is comparable to the branch resistances
What this calculator does not cover
- Exactly two parallel branches — three or more branches need the conductance-ratio form G_k/ΣG.
- Resistive and DC/low-frequency only; no reactive branches or frequency dependence.
- Assumes an ideal current source; a real source's internal resistance changes the split.
- Ideal resistors: no tolerance or temperature effects.
- As with every calculator on this site: results are preliminary and educational, are not verified for any specific installation, and must be reviewed against the applicable code edition and stamped by a licensed Professional Engineer before real-world use.
Frequently asked questions
What is the current divider formula?
For two parallel resistors, the current through R1 is I1 = I_total × R2/(R1 + R2), and through R2 is I2 = I_total × R1/(R1 + R2). Notice the cross-over: each branch current uses the OTHER resistor in the numerator, so the smaller resistor carries the larger current.
Why does the smaller resistor carry more current?
Because both branches share the same voltage, and by Ohm's law a smaller resistance passes more current at that voltage. Current follows the path of least resistance, dividing in inverse proportion to the branch resistances.
How is the current divider different from the voltage divider?
They are mirror images. In a voltage divider (series resistors) a resistor's OWN value appears in the numerator; in a current divider (parallel resistors) the OPPOSITE resistor's value appears in the numerator. Voltage divides in direct proportion to resistance; current divides in inverse proportion.
References
- Irwin, J. D., Basic Engineering Circuit Analysis, 11th ed. (current division)
- Nilsson, J. & Riedel, S., Electric Circuits, 11th ed. (parallel circuits, KCL)
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