How the Current Divider Rule Works (with Examples)

What Is the Current Divider?

When current flows into a node that connects to two or more parallel paths, it splits among those paths. The current divider rule tells you exactly how much current flows through each branch without needing to solve the full circuit using Kirchhoff's laws.

It is the dual of the voltage divider — where a voltage divider splits voltage between series resistors, a current divider splits current between parallel resistors.

The Two-Resistor Current Divider

For two resistors in parallel with a total current I entering the node:

      I
      │
 ┌────┴────┐
 R1        R2
 │         │
 I1        I2
 └────┬────┘
      │

The current through each branch is:

I1 = I × R2 / (R1 + R2)
I2 = I × R1 / (R1 + R2)

Key observation: More current flows through the lower resistance — the formula for each branch uses the other resistor in the numerator.

This makes intuitive sense: a lower resistance path offers less opposition, so more current takes it.

Worked Example 1: Equal Resistors

Two 100Ω resistors in parallel. Total current I = 10mA.

I1 = 10mA × 100 / (100 + 100) = 10mA × 0.5 = 5mA
I2 = 10mA × 100 / (100 + 100) = 5mA

Equal resistors share the current equally. This confirms the formula.

Worked Example 2: Unequal Resistors

R1 = 220Ω, R2 = 470Ω. Total current I = 50mA.

I1 = 50mA × 470 / (220 + 470) = 50mA × 470 / 690 = 34.06mA
I2 = 50mA × 220 / (220 + 470) = 50mA × 220 / 690 = 15.94mA

Check: 34.06 + 15.94 = 50mA ✓

The lower resistance branch (R1 = 220Ω) takes the majority of the current.

The General Form: N Parallel Resistors

For any number of parallel resistors, the current through branch k is:

Ik = I × (1/Rk) / Σ(1/Ri)
   = I × G_k / G_total

Where G = 1/R is conductance in siemens (S). The current divides in proportion to conductance.

Worked Example: Three Branches

R1 = 100Ω, R2 = 200Ω, R3 = 300Ω. Total current I = 30mA.

Step 1 — Calculate conductances:

G1 = 1/100 = 10 mS
G2 = 1/200 = 5 mS
G3 = 1/300 = 3.33 mS
G_total = 10 + 5 + 3.33 = 18.33 mS

Step 2 — Calculate branch currents:

I1 = 30mA × (10 / 18.33) = 16.36mA
I2 = 30mA × (5 / 18.33) = 8.18mA
I3 = 30mA × (3.33 / 18.33) = 5.45mA

Check: 16.36 + 8.18 + 5.45 = 29.99mA ≈ 30mA ✓ (rounding)

Deriving the Rule from Ohm's Law

The current divider is a consequence of two facts:

Parallel resistors share the same voltage across them
Ohm's law: I = V/R

If the parallel combination has an equivalent resistance R_eq and total current I, the voltage across both branches is:

V = I × R_eq = I × (R1 × R2) / (R1 + R2)

Current through R1:

I1 = V / R1 = I × (R1 × R2) / ((R1 + R2) × R1) = I × R2 / (R1 + R2)

This is exactly the current divider formula — it is not a separate law, just Ohm's law applied to parallel circuits.

Practical Applications

Biasing a Transistor Base

A BJT transistor requires a stable base voltage to set the operating point. A voltage divider on the base is actually two parallel paths to ground (via the base-emitter junction) — the current divider tells you how much current each path draws.

Parallel LED Strings

When multiple LED strings are wired in parallel, they should ideally have matched forward voltages to share current equally. If they're mismatched, the current divider predicts the imbalance — and shows why individual current-limiting resistors per string are important.

Current Sensing

A shunt resistor in parallel with the main load is a classic current sensing circuit. The shunt has a very low resistance (e.g., 0.01Ω) so almost all current flows through the load, and a small measurable fraction flows through the shunt. The current divider formula lets you calculate the shunt voltage.

Load Sharing in Parallel Power Supplies

When two power supplies are connected in parallel to share load current, slight differences in output voltage mean they don't share equally. Current divider analysis — treating each supply's internal impedance as a parallel resistor — predicts the imbalance and informs the design of active load-sharing circuits.

Current Divider vs Voltage Divider: A Comparison

Property	Voltage Divider	Current Divider
Topology	Resistors in series	Resistors in parallel
What divides	Voltage	Current
Formula uses	The resistor itself in the numerator	The other resistor in the numerator
Higher resistance gets	More voltage	Less current
Lower resistance gets	Less voltage	More current

Use the Current Divider Calculator

The Current Divider Calculator on DevGizmo handles both two-resistor and multi-resistor parallel networks. Enter the total input current and resistor values to instantly calculate the current through each branch.