Series and Parallel Resistance Calculator
When you connect resistors together in a circuit, the total resistance depends on whether they are wired in series or in parallel. In series, the current travels through each resistor one after another, so the total resistance is simply the sum of all individual values. If you have three resistors of 10, 20, and 30 ohms in series, the total is 60 ohms. In parallel, the current can take any of the available paths simultaneously, so the total resistance is lower than the smallest individual resistor. The formula is the reciprocal of the sum of reciprocals: one over R total equals one over R1 plus one over R2 and so on. Two 100-ohm resistors in parallel give 50 ohms. This calculator handles up to six resistors in either configuration. Leave any unwanted inputs empty and the calculator treats them as not connected. You enter the resistor values in ohms and choose series or parallel using the radio buttons. The result shows the total resistance, the number of resistors included in the calculation, and the formula being applied. This is useful when you need a specific resistance value not available as a standard component, when designing voltage dividers, selecting pull-up or pull-down resistors, or checking circuit designs. Results are in ohms; for very large or small values, the output also shows kilohms or milliohms where appropriate.
How it works
Only fields with a valid positive number are included in the calculation. For series: R total = R1 + R2 + R3 ... For parallel: 1/R total = 1/R1 + 1/R2 + 1/R3 ... then R total = 1 divided by that sum. Any resistor field left blank or set to zero is excluded. The result is displayed in ohms to two decimal places.
Worked example
Two 100-ohm resistors in parallel: 1/R total = 1/100 plus 1/100 = 0.02, so R total = 1/0.02 = 50.00 ohms. These match the default values above. For a series example, enter 10, 20, and 30 in the first three fields with Series selected: total = 10 + 20 + 30 = 60.00 ohms.
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