Resistance combination search
You need a resistance no single resistor gives you. Resistora searches the sixteen resistance circuits — two to four resistors wired in series and parallel — filling their slots either from the kit you own or from a whole IEC 60063 series, ranks everything it finds by error, and hands you the three that land closest.
Combination Search and E-Series Search each answer out of one set of resistors — a kit or a series — and the two are never mixed in one search. Circuit Calculator runs the other way: it is given the resistors and returns the number they make.
Resistora answers one question: you need a resistance you do not have, so what can you build to reach it? The target is a resistance between 0.1 Ω and 100 MΩ, and the answer is a circuit of two to four resistors — unless a single part already sits closer, which you will be told.
A resistor calculator runs forwards: hand it two resistors and it reports what they come to. This runs backwards. You state the value, and what comes back is the arrangement — which parts, in which slots, wired which way. Everything else follows from that direction, because a search running backwards has to know which parts are on the table.
Sixteen resistance circuits exist: two of them fill two slots, four fill three, and ten fill four. Whatever range of sizes you ask for, every circuit in it is searched on every run — none sampled, none skipped. Circuits that divide a source voltage down to an output voltage answer a different question and are ranked on a different quantity; they will be a tool of their own, and that tool is not built.
Soldering two resistors together is only worth it if the result beats the one part you could have used instead. Searching a kit, Resistora speaks up whenever a resistor you already own is at least as close as the best combination, names it, and sends you to that one. Searching a series, the nearest standard value and its error sit above the list on every answer, won or lost.
Both tools are free and both work signed out. The search itself runs inside your browser on background threads; there is no calculation server behind it. An account changes nothing about the search — it exists to keep your kit, so the drawer you typed in once follows you to your next device.