Enter resistor values
Add every resistor in the series path and select the correct ohm, kilo-ohm, or mega-ohm unit.
Electrical Circuit Tool
Series Resistor Calculator
Add any number of resistors in ohms, kilo-ohms, or mega-ohms. Calculate equivalent resistance, circuit current, voltage drop, and power dissipation for every component in a series circuit.
Unlimited resistor rows
Mixed resistance units
Current, voltage, and power results
Circuit inputs
Build your series resistor circuit
Add resistor values below. Supply voltage is optional, but it is required for current, voltage-drop, and power results.
Individual resistor results
Voltage drop, power dissipation, and suggested minimum wattage for each resistor.
| Resistor | Resistance | Current | Voltage drop | Power | Suggested rating |
|---|
Series total
Adds all resistors
Voltage drops
Per-resistor results
Current calculation
Uses Ohm’s law
Power checking
Suggested wattage
How it works
Calculate a series resistor circuit in four steps
The calculator adds resistance values first, then applies Ohm’s law when a supply voltage is available.
Add the resistance
All values are converted to ohms and added to obtain the equivalent series resistance.
Apply the supply voltage
Current is calculated by dividing the source voltage by total resistance.
Review voltage and power
See voltage drop, power dissipation, and suggested wattage for every resistor.
How resistors behave in a series circuit
Resistors are in series when they are connected end-to-end along a single current path. Because there is only one path, the same current flows through every resistor. The equivalent resistance is the arithmetic sum of all component resistance values.
Increasing the number or value of series resistors raises total resistance. For a fixed supply voltage, a higher total resistance reduces circuit current. Each resistor receives a portion of the source voltage proportional to its resistance.
Series resistor formulas
Total resistance
Rtotal = R1 + R2 + R3 + ...
Series current
I = Vsupply ÷ Rtotal
Voltage drop
Vresistor = I × Rresistor
Resistor power
Presistor = I² × Rresistor
Choosing a safe resistor power rating
A resistor must be rated above its expected power dissipation. Operating continuously at the exact rating can produce high temperature and reduce reliability. Use the selected design multiplier as a starting point, then confirm ambient-temperature derating, enclosure ventilation, pulse loading, and manufacturer data.
Practical applications
When this series resistor calculator is useful
Use the tool for circuit planning, component selection, troubleshooting, and electrical learning.
LED current limiting
Check resistance and resistor power when series components are used to limit current.
Voltage-divider chains
Review current and voltage drops through multiple resistors connected in one path.
Bias and sensing networks
Estimate total resistance and dissipation in simple analogue and measurement circuits.
Electronics education
Demonstrate how resistance, current, voltage, and power interact in a series circuit.
Component substitution
Combine available resistor values to approximate a required total resistance.
Circuit troubleshooting
Compare expected voltage drops and current with measurements from an operating circuit.
Frequently asked questions
Series Resistor Calculator FAQs
Answers to common questions about equivalent resistance, circuit current, voltage drop, and resistor power.
How do I calculate resistors connected in series?
Add every resistor value together. For resistors R1, R2, R3, and so on, the equivalent resistance is R total equals R1 plus R2 plus R3. The same current flows through every resistor in a series circuit.
Does the order of resistors change total resistance?
No. Changing the physical order of resistors in a simple series circuit does not change the equivalent resistance, circuit current, or total power. Individual voltage drops still depend on each resistor value.
How is current calculated in a series resistor circuit?
When a supply voltage is entered, circuit current is calculated with Ohm’s law: current equals supply voltage divided by total series resistance. The resulting current is the same through every resistor.
How do I calculate the voltage drop across each resistor?
Multiply the series current by the resistor value. A larger resistor receives a larger share of the supply voltage. The individual voltage drops should add up to approximately the source voltage.
How is resistor power calculated?
Power for each resistor can be calculated as current squared multiplied by resistance. It can also be calculated as voltage drop multiplied by current. Select a resistor wattage rating above the calculated dissipation.
Can I mix ohms, kilo-ohms, and mega-ohms?
Yes. The calculator converts every entered value to ohms before adding the resistors. You can use different units in the same series circuit.
What resistor wattage rating should I choose?
The resistor rating must exceed the calculated power. A common design approach is to use at least twice the expected dissipation when space, temperature, ventilation, and reliability allow. Follow component and project requirements.
Why is my calculated current extremely small?
Current decreases as total resistance increases. Check whether a resistor was entered in mega-ohms instead of kilo-ohms or ohms, and confirm that the supply voltage and units are correct.
Electrical safety and component notice
This calculator provides theoretical estimates for
ideal series resistors. Real circuits can be affected by
resistor tolerance, self-heating, temperature
coefficient, source resistance, pulse energy, component
voltage limits, and measurement uncertainty. De-energise
circuits before changing components.