Why does the lower resistance branch get more current?

Parallel branches share the same voltage. Since I = V/R, the branch with lower resistance draws higher current.

Can the current divider rule be used for AC circuits?

Yes, but resistance must be replaced with impedance. For reactive AC circuits, phase angle and complex impedance should be considered.

Current Divider Calculator — Parallel Branch Current & Resistor Split

Q: What is the current divider formula?

For two parallel resistors, I1 = Itotal × R2 / (R1 + R2) and I2 = Itotal × R1 / (R1 + R2). Current divides inversely to resistance.

Current Divider Calculator

Engineer-grade tool — branch currents, power dissipation warnings, load analysis, reverse design, AC mode & shareable results.

⚡ Key Insight: Current splits inversely to resistance — the lower-resistance branch always draws more current because both branches share the same voltage (Vp). Use the Reverse tab to design resistor values for a target split ratio.

📐 Circuit Reference Diagram

Total Current (Itotal)

Resistance R1 (Ω)

Resistance R2 (Ω)

Enable Load Resistance (RL)— adds third parallel branch

AC AC Mode — use impedance Z instead of R

In AC mode, enter impedance Z (Ω) values. The current divider formula is identical: I1 = Itotal × Z2/(Z1+Z2). Phase angles are not calculated — for phasor analysis use complex impedance tools.

Presets: 10mA · 1Ω‖2Ω 100mA · 470Ω‖1kΩ 5A · 0.5Ω‖2Ω

⚡ Live Circuit Diagram

Branch Current I1

—

I2 (Branch 2)

—

Itotal − I1

Voltage (Vp)

—

I1 × R1

Req Parallel

—

R1‖R2

R2/R1 = — I1/I2 = —

⚡ Power Dissipation — ¼W limit highlighted in orange, >1W in red

P in R1

—

P in R2

—

P Total

—

📊 Load Effect Analysis (RL varies)

RL	I1 (A)	I2 (A)	IRL (A)	Vp (V)	Ptotal (mW)

Total Current (Itotal)

Resistance R1 (Ω)

Resistance R2 (Ω)

Enable Load Resistance (RL)— adds third parallel branch

AC AC Mode — use impedance Z instead of R

In AC mode, enter impedance Z (Ω). Formula: I2 = Itotal × Z1/(Z1+Z2). Phase angles not calculated here.

Presets: 10mA · 1Ω‖2Ω 100mA · 470Ω‖1kΩ 5A · 0.5Ω‖2Ω

⚡ Live Circuit Diagram

Branch Current I2

—

I1 (Branch 1)

—

Itotal − I2

Voltage (Vp)

—

I2 × R2

Req Parallel

—

R1‖R2

R2/R1 = — I1/I2 = —

⚡ Power Dissipation — ¼W limit highlighted in orange, >1W in red

P in R1

—

P in R2

—

P Total

—

📊 Load Effect Analysis (RL varies)

RL	I1 (A)	I2 (A)	IRL (A)	Vp (V)	Ptotal (mW)

Branch Current I1

Resistance R1 (Ω)

Resistance R2 (Ω)

Presets: I1=3mA · 2Ω‖4Ω I1=50mA · 100Ω‖200Ω

⚡ Live Circuit Diagram

Total Current Itotal

—

I2 (Branch 2)

—

I1 × R1/R2

Voltage (Vp)

—

I1 × R1

Req Parallel

—

R1‖R2

R2/R1 = — I1/I2 = —

⚡ Power Dissipation

P in R1

—

P in R2

—

P Total

—

↺ Reverse Calculator — desired split → resistor ratio

Enter the current ratio you need (I1 : I2) and optionally one resistor value to get the other.

I1 : I2 = :

R1 = (optional — leave blank to get ratio only)

Itotal = (optional — for full solution)

How to use: Enter the current split ratio you want (e.g. I1:I2 = 2:1 means branch 1 gets double the current of branch 2). Optionally enter R1 to calculate the exact R2 needed, and Itotal for full branch current values.
Key insight: Current splits inversely to resistance — I1/I2 = R2/R1. So if you want I1 twice as large as I2, you need R1 half of R2.

Current Divider Formulas

I1 = Itotal × R2 / (R1 + R2) I2 = Itotal × R1 / (R1 + R2) Vp = I1 × R1 = I2 × R2 (same voltage across parallel branches) Req = (R1 × R2) / (R1 + R2) (parallel equivalent)

🧠 Engineering Notes

Current division feels “opposite” at first I1 uses R2 in the formula because both branches share the same voltage. A larger R2 blocks branch 2, so more of the total current is forced through R1.

Power rating can be the real limit The current split may be mathematically correct, but a resistor can still overheat. Always compare each branch power with the resistor wattage rating and leave practical margin.

Good for parallel resistive branches This calculator is most reliable when the branches are mainly resistive and connected in true parallel. For capacitors, inductors or motors, use impedance and phase-aware analysis.

Use reverse mode for design work When you already know the current ratio you want, the Reverse tab is faster than trial and error. Enter I1:I2 and it gives the needed resistor ratio.

📋 Quick Design Reference

Current Split Rule

R1 = R250% / 50%

R2 = 2 × R1I1 = 2 × I2

R2 = 10 × R1I1 ≈ 91%

Power Check

FormulaP = I²R

Common resistor0.25 W

Practical margin2× or more

Best Use Cases

Bias networks✓

Parallel shunts✓

Reactive AC loadsUse Z

How the Current Divider Works

In a parallel circuit, every branch shares the same voltage. Because voltage is constant, current distributes inversely proportional to resistance: the lower-resistance branch draws more current. The formula I1 = Itotal × R2/(R1+R2) appears backwards at first — I1 depends on R2 because a larger R2 forces more current through R1 to maintain the same shared voltage across both branches.

Example: Itotal = 12 mA, R1 = 2 kΩ, R2 = 4 kΩ
I1 = 12 × 4/(2+4) = 8 mA  (67% of total — lower R gets more)
I2 = 12 × 2/(2+4) = 4 mA  (33% of total)
Vp = 8 mA × 2 kΩ = 16 V  ✓ (verify: 4 mA × 4 kΩ = 16 V)

Where Current Dividers Are Useful

Current sensing and shunts: A known low-value resistor can carry a controlled part of the current so voltage drop can be measured safely. This is common in battery, charger, motor-driver and power-supply circuits.

Load sharing: Parallel conductors, fuses, resistors or PCB paths rarely share current equally unless their resistance is closely matched. This tool makes the imbalance visible before the circuit overheats.

Bias and analog circuits: Current divider thinking helps when checking transistor bias networks, input loading, sensor branches and leakage paths. It is especially useful when a load branch quietly changes the expected current split.

AC Circuits — Use Impedance Z Instead of R

The current divider rule applies equally to AC circuits — simply replace resistance R with complex impedance Z (Ω). For purely resistive AC networks the calculation is identical. For reactive loads such as capacitors, inductors and transformers, phase angle matters: I1 = Itotal × Z2/(Z1+Z2) should be handled as a complex-number calculation.

Engineering Decision Checklist

Before finalising resistor values, verify: (1) Power rating — each resistor must handle I²R watts with a safety margin. (2) Current balance — if one branch carries more than 90% of current, check thermal stress. (3) Trace and wire sizing — at higher current, the PCB trace, connector and wire can become the weakest link. (4) Tolerance — a 1% resistor pair shares current more predictably than a 5% pair.

❓ Frequently Asked Questions

What is the current divider formula?

For two parallel resistors, I1 = Itotal × R2/(R1+R2) and I2 = Itotal × R1/(R1+R2). The branch with lower resistance gets more current.

Why does I1 use R2 in the formula?

Because the two branches share the same voltage. If R2 becomes larger, less current can pass through branch 2, so more of the total current flows through R1.

Can I use this for three parallel branches?

Yes, use the load resistance option to add a third branch. Internally, the calculator finds the parallel equivalent resistance, then calculates the shared voltage and each branch current.

Is the current divider rule valid for AC?

Yes, but use impedance Z instead of resistance R. For reactive circuits, impedance has both magnitude and phase, so a simple resistance-only answer is only an approximation.

Why should I check resistor power?

A current divider can give the correct current split but still burn a resistor. Check P = I²R for every branch and choose a resistor wattage with margin.

When should I not use a simple current divider?

Avoid it for unstable loads, high-power paths, large tolerance-sensitive designs, reactive AC networks, or circuits where the load changes the divider ratio significantly.