Current Divider Calculator
Calculate current through each branch in a parallel resistor network.
What the current divider calculator does
The current divider calculator finds how a total current splits between two resistors connected in parallel. Enter the total current entering the pair and the two resistances, press Calculate, and the tool returns the equivalent parallel resistance and the current through each branch. It applies the current divider rule, the parallel-circuit counterpart of the voltage divider.
The formulas
- Equivalent resistance: Rt = (R1 × R2) / (R1 + R2)
- Current in R1: I1 = It × Rt / R1 = It × R2 / (R1 + R2)
- Current in R2: I2 = It × Rt / R2 = It × R1 / (R1 + R2)
The branch with the lower resistance carries the larger current, and the two branch currents always add up to the total. Both resistors see the same voltage, It × Rt.
Worked examples
| It | R1 | R2 | Rt | I1 | I2 | Voltage |
|---|---|---|---|---|---|---|
| 5 A | 10 Ω | 20 Ω | 6.667 Ω | 3.333 A | 1.667 A | 33.3 V |
| 100 mA | 1 kΩ | 1 kΩ | 500 Ω | 50 mA | 50 mA | 50 V |
| 2 A | 0.1 Ω | 10 Ω | 0.099 Ω | 1.980 A | 0.020 A | 0.198 V |
| 30 mA | 470 Ω | 2.2 kΩ | 387 Ω | 24.7 mA | 5.3 mA | 11.6 V |
The third row is a shunt: a small resistor in parallel with a meter or a load takes almost all the current, which is how ammeters measure large currents with a low-range movement.
Where the current divider appears
- Ammeter shunts. A meter movement rated 1 mA with 50 Ω is paralleled with a 0.05 Ω shunt to read 1 A full scale; 99.9% of the current bypasses the movement.
- Parallel LEDs or lamps. Unequal branch resistances lead to unequal currents and uneven brightness, which is why each LED gets its own series resistor.
- Power resistor arrays. Paralleling resistors shares dissipation; the calculator shows each part’s current so its power rating can be checked.
- Transistor biasing. Base current splits between the bias network and the base, and emitter current splits between the emitter resistor and any bypass path.
- Grounding and return paths. Return current divides between a ground plane and a parallel cable in proportion to conductance, which matters for noise and EMI.
More than two branches
For three or more parallel resistors, first find the total conductance G = 1/R1 + 1/R2 + 1/R3 + …, then each branch current is In = It × (1/Rn) / G. With the two-branch tool, combine any two resistors into their parallel equivalent and treat it as one branch, then split that branch again.
Power in each branch
Once the currents are known, P = I² × R gives each resistor’s dissipation. In the first example R1 dissipates 3.333² × 10 = 111 W and R2 1.667² × 20 = 55.6 W, so the smaller resistor handles more power. The power dissipation in resistor calculator checks whether a chosen part is rated for that.
Common mistakes
- Swapping the ratio. The current in R1 is proportional to R2, not R1. Lower resistance means higher current.
- Entering kilohms as ohms. 4.7 kΩ is 4700 Ω. Both fields expect ohms.
- Applying it to series circuits. Series elements share the same current; use the voltage divider calculator for the voltage split instead.
- Ignoring source resistance. A real source with internal resistance changes the total current when the load changes; the calculator assumes It is fixed.
Related tools
The current calculator finds I from voltage and resistance, and the watts to amps and amps to watts calculators convert between power and current for a given voltage. For AC circuits with capacitors, the capacitive reactance calculator supplies the reactance to use in place of resistance.
External references
Frequently asked questions
Why does the smaller resistor get more current?
Both branches see the same voltage. By Ohm’s law, the same voltage across a smaller resistance drives a larger current.
What if the two resistors are equal?
The current splits exactly in half and the equivalent resistance is half of either resistor.
Does the rule work with inductors and capacitors?
Yes, using impedances instead of resistances. For pure reactances the same ratios apply; with mixed components the calculation needs complex numbers.
Can I use it to size a shunt resistor?
Yes. Enter the meter’s resistance as R1, try values for R2, and adjust until I1 equals the meter’s full-scale current at the total current you want to measure.
What voltage appears across the parallel pair?
V = It × Rt. The calculator shows Rt; multiply by the total current.