Calculate the current in a series circuit from the supply voltage and each resistor value, or find the required voltage or a missing series resistor.
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Series Current Formula
In an ideal resistive series circuit, all resistors share one current path and the same current. For ohmic resistors, the source voltage and total series resistance determine that current. Non-ohmic loads, source internal resistance and reactive AC impedance require an appropriate model beyond this calculator.
Where:
- Is = series current
- Vs = total source voltage across the circuit
- Rs = total series resistance
This is the direct series-circuit form of Ohm’s law. If voltage increases while resistance stays the same, current increases. If total resistance increases while voltage stays the same, current decreases.
Total Resistance in Series
Before finding current, add all resistances in the series path.
Once the total resistance is known, divide the source voltage by that total resistance to get the current through the entire circuit.
Rearranged Forms
If you know any two of the three variables, you can solve for the third.
How to Use the Series Current Calculator
- Enter the total source voltage for the circuit.
- Enter every known series resistance in the list, separated by commas, spaces or semicolons (k and M suffixes such as 4.7k are accepted); the calculator adds them. A comma separates entries, not thousands. Select the common list unit explicitly: changing it reinterprets the unchanged list text.
- Select the voltage/current units or desired output unit. Numeric voltage and current unit changes convert physical values; the resistance-list unit explicitly reinterprets all entries as ohms, kilo-ohms or mega-ohms.
- Calculate to find the series current.
In voltage mode, enter the common current and resistance list. In missing-resistor mode, enter supply voltage, positive target current and the known list: Radded = V/I − sum(Rknown). A zero-ohm known element is allowed, but the completed circuit must have positive total resistance. Zero voltage gives zero current in current mode; zero current gives zero voltage in voltage mode. The zero-voltage/zero-total-resistance case cannot establish a positive target current.
Quick Reference
| What You Know | Use This Relationship | Purpose |
|---|---|---|
| Total voltage and total resistance | Iₛ = Vₛ / Rₛ |
Find current |
| Current and total resistance | Vₛ = Iₛ × Rₛ |
Find source voltage |
| Voltage and current | Rₛ = Vₛ / Iₛ |
Find total resistance |
| Individual resistors in series | Rₛ = R₁ + R₂ + R₃ + … + Rₙ |
Find equivalent series resistance |
Examples
Example 1: A 24 V source is connected to two resistors in series: 4 Ω and 8 Ω.
The series current is 2 A, and that same 2 A flows through both resistors.
Example 2: A circuit has a total resistance of 30 Ω and a measured current of 0.5 A.
The required source voltage is 15 V.
Voltage Drop Across Each Series Component
Although the current is the same everywhere in a series circuit, the voltage drop across each component depends on that component’s resistance.
The individual voltage drops add up to the source voltage.
This means higher-resistance components consume a larger share of the total voltage when the current is the same.
Common Mistakes
- Not adding resistors first: In a series circuit, total resistance is the sum of all resistors.
- Mixing units: Convert millivolts to volts and kilo-ohms to ohms if needed before checking results manually.
- Confusing series with parallel: Current is the same in series circuits, while voltage is the same in parallel branches.
- Using zero resistance: A total resistance of zero makes the simple current equation invalid for practical calculation.
Why Series Current Matters
Knowing the series current supports voltage-drop and power calculations. Component ratings, fuse selection, tolerances, temperature rise and source limits require separate engineering checks; this ideal arithmetic result does not verify circuit safety.
Related Relationships
If you also need power in a series circuit, these identities are commonly used after current is known:
For each resistor, Pi = I²Ri and Vi = IRi; the calculator lists every component, including a solved added resistor. These power estimates must be compared with actual component ratings, derating, tolerances and operating conditions; they do not prove safe operation.
