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Current Divider Calculator

Find the current through two parallel resistors.

How it is calculated

For two resistors in parallel, the total current I (amperes) splits as I1 = I × R2 ÷ (R1 + R2) and I2 = I × R1 ÷ (R1 + R2). Note the crossover: the current through R1 uses R2 on top, so the smaller resistance always carries the larger share. Resistances can be in ohms, kilo-ohms or any unit as long as both use the same one, because only their ratio matters. Both resistances must be above zero. The current is not checked for sign, so a negative value gives negative branch currents. Results are shown in amperes to four decimals, and I1 + I2 always equals I.

Worked example

A total current of 2 A reaches two parallel branches, R1 = 100 Ω and R2 = 300 Ω. The sum is 400. Branch 1: 2 × 300 ÷ 400 = 1.5000 A. Branch 2: 2 × 100 ÷ 400 = 0.5000 A. The two add back to 2 A. The 100 Ω branch is three times lower in resistance, so it takes three times the current.

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Common mistakes

  • Using R1 in the numerator for the current through R1. That is the voltage-divider habit; for current it is the other resistor.
  • Entering the supply voltage in the current field. The input is the total current entering the parallel pair.
  • Applying the formula to three or more branches. This version covers exactly two; more branches need conductances.
  • Mixing ohms and kilo-ohms for the two resistors, which shifts the split by a factor of 1,000.
  • Forgetting that resistors in series are not a divider of current; the same current flows through both of them.

About this calculator

This calculator applies the current divider rule, I₁ = I·R₂/(R₁+R₂), which follows from the fact that two resistors in parallel share the same voltage across them, so the current through each branch is inversely proportional to its own resistance — the smaller resistor carries the larger share of current. It's the mirror image of the voltage divider rule, and both are direct consequences of Ohm's law and Kirchhoff's current law.

Electronics engineers use it to size shunt resistors for ammeters, to balance current sharing between parallel LED branches or battery cells, and to design bias networks in analog circuits. Students learning basic circuit analysis rely on it as one of the first tools for solving parallel resistor networks without redrawing the whole circuit.

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