Series and parallel resistor calculator
Work out the equivalent resistance of two, three or four resistors wired in series, in parallel, or both. Pick one of the sixteen resistance circuits, put a resistance in every slot, and Resistora shows what that circuit produces — written out and drawn — then takes the value straight to a search of your kit or of a series.
Pick the circuit
Choose the shape you are wiring, from two resistors up to four.
Fill every slot
Type a resistance for each one, anything from 0.1 Ω to 100 MΩ. The answer follows as you type.
Take it to a search
Not the value you were after? Carry it into your kit or a series and find what lands closer.
What it does
You pick one of the sixteen resistance circuits and put a resistance in every slot. Resistora works out what that circuit produces, writes it the way an engineer says it out loud — (1 kΩ + 2.2 kΩ) ∥ 4.7 kΩ — and draws the circuit with every slot labelled by the resistance filling it. There is no search to run, so there is nothing to press: the answer follows the input. The same value may fill any number of slots, because nothing here is owned. Switch to another circuit and the slots the two shapes share keep what you typed.
How series and parallel resistance is worked out
Resistors in series add: R = R1 + R2 + R3. In parallel their reciprocals add: 1/R = 1/R1 + 1/R2 + 1/R3, which for two resistors is R = R1 × R2 / (R1 + R2), always less than the smaller of the two. A mixed circuit is worked from the inside out, one series or parallel group at a time. So 1 kΩ and 2.2 kΩ make 3.2 kΩ in series and 687.5 Ω in parallel, and (1 kΩ + 2.2 kΩ) ∥ 4.7 kΩ makes 1.904 kΩ.
The one tool here that runs forwards
A resistor calculator is given a circuit and returns its resistance. Every other tool here runs the opposite way: it is given a resistance and returns the circuits that reach it. This page is the one that does the arithmetic, and it exists because the two questions are the same job a few seconds apart — once you know what three resistors make, the next thing you ask is what would make the value you actually wanted.
Every circuit it offers
There are sixteen resistance circuits: two built from two resistors, four from three, and ten from four. Every one of them can be chosen and filled here — the same fixed set both searches work through. Each is offered as its shape rather than by a name, because the shape is what you are holding.
Where the answer goes
Under every answer stand two ways out, equal in weight: search this resistance against the kit you own, or against a whole IEC 60063 series. Each carries the value with it as the target, so nothing is retyped. Neither is the recommended one; which tool answers depends only on where your resistors come from.
What it does not do
This tool has no target, so it measures no error and names no nearest standard value. It reads and shows no kit and no series. It does not model tolerance, because Resistora records it nowhere. Divider circuits — an output voltage from a source voltage — answer a different question and will be a tool of their own. Nothing is kept, either: reload the page and the slots are empty again.
Free, and it never leaves your browser
Circuit Calculator is free and works signed out. There is nothing to sign in for: the arithmetic runs in your browser, and nothing you type is sent anywhere or kept.