What it does

You name a target resistance and how many resistors a combination may use. Resistora fills every slot of every resistance circuit that size allows with the resistors you have in play, keeps the twenty best and shows you the three closest. A resistor’s quantity is a ceiling on how many slots it may fill, so a kit holding one 1 kΩ is never answered with two of them. Solutions are ranked by error — how far the resistance a combination produces lands from the target you asked for — with ties going to the combination that uses fewer resistors, and after that to a fixed order over the circuits, so the same kit and the same target always give you the same list in the same order.

How it differs from a resistor calculator

A resistor calculator is given resistors and returns a value: you type 1 kΩ and 2.2 kΩ, it tells you they make 3.2 kΩ in series. Resistora runs the other way. You give it the value and it returns the resistors, out of your own kit. That inversion is the whole difference, and everything below follows from it.

Every circuit it searches

There are sixteen resistance circuits: two built from two resistors, four from three, and ten from four. You choose the fewest and the most resistors a combination may use, and every circuit in that range is searched on every run, exhaustively — nothing is sampled and nothing is skipped. It starts at two to four, which is all sixteen of them. While a search is running the page counts off how many of the circuits it is searching are done.

When a resistor you own is already closer

If a single resistor in play sits at least as close to your target as the best combination does, Resistora says so above the list, names that resistor and its error, and tells you to reach for it instead of building anything. Sending you to solder two resistors together when your kit already holds something better is the one answer this tool will not give.

When a target is out of reach

A kit reaches from its smallest parallel combination to its largest series one, and nothing beyond either end. Asking for fewer resistors brings both ends in. Ask for something under the floor or over the ceiling and Resistora names the limit and says which side of it you are on — then still shows the closest it found, with its error, rather than an empty list. The kit Resistora starts you with reaches from 25 Ω to 4 MΩ.

What it does not do

Divider circuits are not here. They answer a different question — an output voltage from a source voltage — and are ranked on a different quantity, so they will be a tool of their own rather than a setting on this one. Tolerance is not here either: Resistora does not record the tolerance of your resistors, so it does not model it. The percentage shown against a combination is error, how far the resistance it produces lands from the target you asked for, and it should not be read as a tolerance band.

Free, and it never leaves your browser

Combination Search is free and works signed out. The search itself runs in your browser, across several background threads, and your kit is never sent anywhere to be searched. Signing in does one thing: it keeps your kit, so it is still there next time.

A worked example

Take that starting kit — 100 Ω, 220 Ω, 330 Ω, 470 Ω, 1 kΩ, 2.2 kΩ, 4.7 kΩ, 10 kΩ, 22 kΩ, 47 kΩ, 100 kΩ and 1 MΩ, ten of each — and ask it for 5.6 kΩ. The nearest single resistor in it is the 4.7 kΩ, 16.1% off. What comes back is 100 Ω + 330 Ω + 470 Ω + 4.7 kΩ, four in series, landing on 5.6 kΩ exactly. Behind it: (1 kΩ ∥ 10 kΩ ∥ 100 kΩ) + 4.7 kΩ at 0.0161% off, then ((470 Ω + 470 Ω) ∥ 22 kΩ) + 4.7 kΩ at 0.0265% off.

Resistance	How many	In play	Actions
100 Ω	10
220 Ω	10
330 Ω	10
470 Ω	10
1 kΩ	10
2.2 kΩ	10
4.7 kΩ	10
10 kΩ	10
22 kΩ	10
47 kΩ	10
100 kΩ	10
1 MΩ	10

Find the resistance you need in the resistors you own