Resistor Calculators
Series Resistor Calculator
Total up any number of resistors in series, then add a supply voltage to see the loop current and how it divides across each one.
Series Resistors
Resistors end to end in a single current path.
How Series Resistance Adds
Resistors are in series when they form a single unbroken path, so the same current passes through every one of them. Their resistances simply add.
With a supply connected, Ohms Law gives the loop current from the total, and each resistor then drops a share of the supply voltage in proportion to its own value.
Those individual drops always sum back to the supply voltage, which is Kirchhoff's voltage law in its simplest form and a useful way to check your own arithmetic.
How to Use the Series Resistor Calculator
Enter a value in each resistor row and pick its unit. Add resistor creates another row, up to twelve in total, and the bin icon removes one. Two rows always remain, since a series combination needs at least two components.
Leave the supply voltage blank if you only want the total. Enter a voltage and the calculator adds the loop current, the total power, and a table listing the voltage drop, percentage share and power dissipation for every resistor in the string.
Select Calculate to solve. Empty or negative entries are marked inline against the specific row that needs correcting.
Worked Example: A Three-Resistor Series String
A 100 Ω resistor in series with a 220 Ω resistor:
Across a 9 V supply, the loop current is 9 V ÷ 320 Ω = 28.1 mA. The 100 Ω resistor drops 28.1 mA × 100 Ω = 2.81 V, and the 220 Ω resistor drops 6.19 V. Those add to 9 V, as they must.
The power figures follow: 79 mW in the first resistor and 174 mW in the second, so both fit comfortably within quarter-watt parts. Notice that the larger resistor takes the larger share of both the voltage and the heat, because the current through them is identical and P = I² × R.
Reading Your Series Network Results
The total resistance is the value the source actually sees, and it is what you would use in any further Ohms Law calculation for the circuit as a whole.
The loop current is the single current flowing through the entire string. This is the defining property of a series circuit and the reason the same current appears in every row of the results table.
The voltage drops show how the supply divides. The percentage column makes the proportion obvious at a glance, and the power column identifies which component runs hottest. In a long string it is common for one resistor to dissipate most of the heat, and that is the part whose rating needs checking.
Where Series Resistors Appear
Series resistors set current. Placing a resistor in series with an LED, a relay coil or a meter movement limits how much current the supply can push through it, which is the most common use in small electronics.
They also build values that are not available as single parts. Two 1 kΩ resistors in series give 2 kΩ, and adding a small resistor to a larger one is a standard way to trim a value into range without ordering a new component.
Series strings appear in measurement too. Multimeter voltage ranges use a chain of precision resistors to divide the input down to what the meter movement or converter can handle, with the taps between them selecting the range. Christmas light strings and some LED lamps work the same way, running many low-voltage devices from a single higher-voltage supply.
A series resistor is also a standard protective measure. Placed in the supply line to a prototype, it limits how much current a wiring mistake can draw, turning a short circuit into a warm resistor rather than a damaged board. The same idea appears as an inrush limiter at the input of a power supply, where a small resistance holds down the initial surge as the reservoir capacitor charges.
Limits of the Series Resistance Model
- One open component breaks the whole path. Series circuits fail completely rather than partially, which is helpful for safety and inconvenient for lighting.
- Tolerances accumulate. Ten 5% resistors in series produce a total whose spread you should check with a meter if the value matters.
- Voltage rating applies per component. A long string across a high voltage may exceed the rating of an individual part even though the total resistance looks correct.
- The model is purely resistive and assumes constant temperature. Parts that heat significantly will drift from these values in use.
Questions
Series Resistor FAQs
Does the order of resistors in a series circuit matter?
Electrically, no. The same current flows through all of them and the total is the same regardless of order. Physical layout can matter for heat dissipation and for where you can take measurements.
Why do the voltage drops add up to the supply voltage?
Because energy is conserved around the loop. Kirchhoff's voltage law states that the sum of the drops around a closed circuit equals the sum of the sources, which is a reliable check on your figures.
Which resistor gets hottest in a series string?
The largest one. Current is identical everywhere in the string, so P = I² × R makes dissipation proportional to resistance.
Can I mix units in the same calculation?
Yes. Each row has its own unit selector, so one resistor can be entered in ohms and another in kilohms. Everything is converted to ohms internally before the values are added.
Solve the String on the Ohms Law Wheel
Open the full Ohms Law Wheel calculator to solve voltage, current, resistance and power from any two known values.