Current Divider Calculator
When current flows into two parallel resistors, it divides between them in inverse proportion to their resistances. The current divider rule states that the current through R1 is I1 = I_total * R2 / (R1 + R2), and through R2 is I2 = I_total * R1 / (R1 + R2). Notice that R2 appears in the numerator for I1 (and vice versa), so the larger resistor always carries less current. This is the fundamental dual of the voltage divider rule and is used in circuit analysis, current sensing, and parallel load design.
Current divider formulas
I1 = I_total * R2 / (R1 + R2)
I2 = I_total * R1 / (R1 + R2)
V = I1 * R1 = I2 * R2
The voltage across both parallel resistors is the same and equals I1 * R1 or equivalently I2 * R2. Verify by checking I1 + I2 = I_total.
Current divider applications
- Current sensing: a small shunt resistor in parallel with a load carries a known fraction of the total current, allowing measurement with a voltmeter.
- Biasing transistor circuits: current mirrors and bias networks rely on current divider principles to set operating points.
- Protective parallel resistors: a fuse or circuit breaker can be paralleled with a low-value resistor to set the trip current threshold.
- Parallel loads in power systems: knowing the current split helps verify that each branch stays within its rated current capacity.
Frequently asked questions
What is the current divider rule?
For two resistors in parallel, the current through each is inversely proportional to its resistance. I1 = I_total * R2 / (R1 + R2) and I2 = I_total * R1 / (R1 + R2). The smaller resistor carries more current.
Why does more current flow through the smaller resistor?
Both parallel resistors have the same voltage across them. By Ohm's law (I = V/R), a smaller resistance means more current flows through it. The current divides inversely with resistance.
Does the current divider rule apply to more than two resistors?
Yes. For any branch resistance Ri in a parallel network, Ii = I_total * (R_parallel / Ri), where R_parallel is the total equivalent parallel resistance. For two resistors, R_parallel = R1*R2/(R1+R2).
Does the current divider apply to AC circuits?
Yes, the current divider rule applies to impedances in parallel: I1 = I_total * Z2 / (Z1 + Z2), where Z1 and Z2 are complex impedances. The formula works identically when reactances are involved.
How do I verify the result?
The sum of branch currents must equal the total current: I1 + I2 = I_total. Also, both branches must have the same voltage: V = I1 * R1 = I2 * R2. Check both conditions to confirm the result.
Official sources
- NIST: NIST SP 811, Guide for SI Units.
- OpenStax University Physics: Resistors in Series and Parallel, Vol. 2 Ch. 10.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.