Series Resistance Calculator
This series resistance calculator adds resistors in series to find the total resistance, using the formula for series resistance. Enter up to four resistor values and the series resistance calculator returns the total in ohms and kilohms. Because resistors in series simply add, this series resistor calculator handles any string of resistors in one step.
Results
The Series Resistance Calculator is part of our DC/AC Circuits collection. Browse more dc/ac circuits to size, convert, and check your work alongside this tool.
What the formula for series resistance describes
When resistors sit end to end in one path, the current flows through each in turn, so the resistances add up. The formula for series resistance is simply the sum of the values, and this series resistance calculator adds up to four resistors for you. Adding a resistor in series always raises the total, which is why a series string resists more than any single resistor in it.
The formula for series resistance
The formula for series resistance adds every resistor in the path:
There is no reciprocal or product step, unlike a parallel circuit; series resistances add straight. This is the series circuit formula for resistance, and it holds because the same current passes through each resistor, so their voltage drops and their resistances stack. The series resistance calculator applies it to the values you enter.
Series resistance calculator example
A 100 ohm resistor in series with a 220 ohm resistor. The series resistance calculator adds them:
So the pair has a total resistance of 320 ohms, or 0.32 kilohms. Add a third resistor and the series resistor calculator keeps adding to the total.
How to use the series resistance calculator
Enter each resistor value in ohms, up to four, and the series resistance calculator returns the total resistance in ohms and kilohms along with the number of resistors used. Leave unused fields at zero, since the tool ignores them. For a longer string, add the extra resistors to the total by hand, since the series resistor calculator just sums whatever you enter.
Reading the series resistor calculator result
The total resistance is what the source sees across the whole string, and it is always larger than the biggest single resistor. That total sets the current through the circuit by Ohm law, since the current is the supply voltage divided by the total resistance. Each resistor then takes a share of the voltage in proportion to its value, which is how a voltage divider works. The series resistor calculator gives the total; use it with the supply voltage to find the current.
When to use the series resistance calculator
- Combining resistors with the series resistance calculator
- Building a voltage divider from a resistor string
- Sizing a dropping resistor with a led series resistance calculator
- Checking a resistors in series calculation by hand
Common series resistance calculator mistakes
- Using the parallel method. Series resistances add; do not use reciprocals, which are for parallel resistors.
- Mixing series and parallel. Reduce series groups first, then parallel ones, in a mixed network.
- Leaving in unused values. Set unused resistor fields to zero so they do not add to the total.
Tips for series circuits
- Add resistor values straight across for the total.
- Use the total with Ohm law to find the circuit current.
- For long strings, sum in groups to avoid slips.
Resistors in series calculation
A resistors in series calculation is the simplest combination in electronics, but the reason it works is worth seeing. In a series path there is only one route for the current, so the same current flows through every resistor. Each resistor drops a voltage equal to that current times its own resistance, and those drops must add up to the supply voltage. Rearranging shows the total resistance is just the sum of the parts, which is the total resistance formula series circuits obey. This is why a long string of small resistors can add up to a large resistance, and why removing one resistor lowers the total by exactly its value. To calculate ohms in series, you never need more than addition.
LED series resistance calculator use
The most common real use of series resistance is protecting an LED, and a led series resistance calculator handles it. An LED needs a set current, but on its own it would draw too much and burn out, so a resistor goes in series to drop the extra voltage. The resistor value is the supply voltage minus the LED forward voltage, divided by the desired current. For example, a 5 volt supply, a 2 volt LED, and a 20 milliamp target need a 150 ohm resistor. The LED resistor calculator sizes that dropping resistor directly, and this series resistance calculator confirms the total when several resistors share the job.
Series resistance calculator values
Total resistance for common resistor pairs, from the series resistance calculator:
| Resistor 1 | Resistor 2 | Total |
|---|---|---|
| 100 Ω | 100 Ω | 200 Ω |
| 100 Ω | 220 Ω | 320 Ω |
| 330 Ω | 470 Ω | 800 Ω |
| 1000 Ω | 2200 Ω | 3200 Ω |
Series Resistance Calculator FAQ
What is the formula for series resistance?
The formula for series resistance is R total = R1 + R2 + R3, the sum of the values. The series resistance calculator adds them to give the total in ohms.
Do series resistors add or divide?
They add. Series resistances simply sum, unlike parallel resistors, which combine by reciprocals. So a resistors in series calculation always raises the total.
How do I size an LED resistor in series?
Subtract the LED voltage from the supply, then divide by the LED current. A led series resistance calculator gives the dropping resistor that sets a safe current.
Is the total ever smaller than a single resistor?
No. In series the total is always larger than the biggest resistor, since every value adds to the path.