Resistors in Series Calculator

The Resistors in Series Calculator finds the total resistance of any number of resistors connected end to end, and — when you add a supply voltage — it also solves the shared circuit current, the voltage dropped across each resistor (the voltage divider), and the power each resistor dissipates. An animated circuit diagram and a proportional voltage-division bar make it easy to see exactly what a series string does.

What Is a Series Resistor Circuit?

Resistors are in series when they are joined end to end so that there is only one continuous path for current to flow. The current that leaves the power source passes through the first resistor, then the second, then the third, and so on before returning to the source. Because there is a single path, the same current flows through every component, and the individual resistances add together to form a larger total resistance.

Resistors in Series Formula

The total resistance of resistors in series is just the sum of the individual values:

If a supply voltage \(V\) is connected across the string, Ohm's Law gives the single current shared by all the resistors:

Each resistor then drops a portion of the supply voltage in proportion to its resistance — this is the voltage divider rule:

And the power dissipated by each resistor, plus the total power delivered by the source:

Key Properties of a Series Circuit

• Resistance adds up: The total is always larger than the biggest single resistor.
• Current is shared: Exactly the same current flows through every resistor.
• Voltage divides: The supply voltage splits between the resistors in proportion to their resistance, and all the drops add back up to the source voltage.
• Power splits too: The largest resistor drops the most voltage and dissipates the most power.
• Order does not matter: Rearranging the resistors does not change the total resistance, current, or any voltage drop.

Series vs Parallel Resistors

Worked Example

Suppose three resistors — 470 Ω, 330 Ω, and 220 Ω — are connected in series across a 12 V supply.

• Total resistance: \(470 + 330 + 220 = 1020\ \Omega\)
• Current: \(I = 12 / 1020 = 0.01176\ \text{A}\) (about 11.76 mA)
• Voltage across the 470 Ω resistor: \(0.01176 \times 470 = 5.53\ \text{V}\)
• The three drops (5.53 V + 3.88 V + 2.59 V) add back up to 12 V.

Common Uses of Series Resistors

• Voltage dividers: Producing a smaller reference voltage from a larger supply.
• Current limiting: Adding resistance in series with an LED or sensor to keep current safe.
• Reaching non-standard values: Combining standard resistor values to hit a target resistance.
• Increasing power rating: Splitting the voltage across several resistors so each handles less power.

How to Use This Calculator

1. Enter each resistor value: Type the resistance of every resistor and choose its unit (Ω, kΩ, or MΩ). Use the ➕ button to add more resistors or the ✕ button to remove one.
2. Add a supply voltage (optional): Enter the supply voltage to also calculate the current, each resistor's voltage drop, and the power dissipated.
3. Click Calculate: The tool computes the total resistance and the full circuit analysis instantly.
4. Review the results: Explore the animated circuit diagram, the voltage-division bar, the per-resistor breakdown table, and the step-by-step formula walkthrough.

Frequently Asked Questions

How do I make a resistor value I do not have?

Put standard resistors in series: their values add. Enter the ones you already have plus the value you need as the Target total, and the calculator shows how much is still missing, the nearest single E12 or E24 value and the best pair of standard values that makes up the difference, with the error of each.

How much power can each resistor in series handle?

The same current flows through every resistor, so each one dissipates I squared times R. Enter the supply voltage and choose the rating of your parts (for example 1/4 W): each resistor is flagged when it goes above half of its rating, the usual derating rule, or above the rating itself.

How do you calculate resistors in series?

Resistors in series add together, so the total resistance is simply the sum of all the individual resistances: R_total = R₁ + R₂ + … + Rₙ. Because they add, the total is always larger than the biggest single resistor in the string.

Is the current the same for resistors in series?

Yes. A series circuit has only one path for charge to flow, so exactly the same current passes through every resistor. You find it with Ohm's Law: I = V divided by R_total, where V is the supply voltage.

How is the voltage divided across resistors in series?

Each resistor drops a share of the supply voltage in proportion to its resistance. The drop across resistor i is Vi = I × Ri = V × (Ri / R_total). All the individual voltage drops add back up to the total supply voltage, which is why a series string is called a voltage divider.

What is the difference between series and parallel resistors?

In series, resistors are connected end to end so their resistances add and the total increases. In parallel, resistors share the same two nodes so the total resistance decreases and is always smaller than the smallest resistor. Series circuits share current and divide voltage; parallel circuits share voltage and divide current.

How do I calculate the power dissipated by each resistor?

Once you know the current I, the power dissipated by resistor i is Pi = I² × Ri, and the total power delivered by the source is P = V × I. Use the total power to check that your resistors are rated for the heat they must handle.

Does the order of resistors in series matter?

No. The total resistance, the current, and each resistor's voltage drop and power depend only on the resistance values, not on the order they appear in the string. Rearranging series resistors does not change the circuit's behavior.

How does a load connected to a voltage divider change its output voltage?

A load connected across part of a divider acts in parallel with that resistor, lowering the effective resistance and changing the output voltage. For example, a 1 kΩ lower resistor in a divider becomes 500 Ω if a 1 kΩ load is placed across it. The calculator’s series-resistor division describes the unloaded string, so include the load’s effect in a fuller circuit calculation when it draws significant current.

What happens if one resistor in a series string fails open?

An open failure breaks the only current path, so current stops through the entire series string and the components downstream lose power. A shorted resistor has the opposite effect: it is bypassed, reducing the total resistance and changing voltage drops and current. If the circuit behaves unexpectedly, power it down before inspecting components and use a meter to check continuity and resistance.

How do I calculate current through 1 kΩ and 2 kΩ resistors in series on 12 V?

Add the resistances to get 3 kΩ, then divide the supply voltage by the total: 12 V ÷ 3,000 Ω = 0.004 A, or 4 mA. The same current flows through both resistors. Their voltage drops are 4 V across 1 kΩ and 8 V across 2 kΩ, which add to the 12 V supply. This assumes an ideal source and no other circuit load.

Additional Resources

• Series and Parallel Circuits - Wikipedia
• Voltage Divider - Wikipedia
• Ohm's Law - Wikipedia