Reach a resistance no standard value offers
You need a resistance that no series stocks — 168.7 Ω, 3.2 kΩ — and you have nothing in hand yet. Name the target and the series you buy from, and Resistora works out which standard values, wired in series and parallel, land on it — then hands you the three combinations that come closest.
Name your target
Type the resistance you need. Anything from 0.1 Ω to 100 MΩ.
Pick your series
Choose the series you buy from, and how many resistors a combination may use.
Find combinations
Press Find combinations and the three that land closest come back, ranked by error.
What it does
You name a target resistance, the series you buy from and how many resistors a combination may use. Resistora fills every slot of every resistance circuit that size allows with the values of that series, keeps the twenty best and shows you the three closest. Nothing caps how often one value appears: the same value may fill every slot, because these resistors are bought rather than found. Combinations 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 question always gives 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 a catalogue you can actually order from. The constraint is not what a formula allows; it is what a distributor stocks.
Why the series decides how many resistors you get
A series across the decades is a far larger set of values than any drawer: 43 values in E6 between 1 Ω and 10 MΩ, 169 in E24, 1345 in E192. The search is exhaustive, and its cost is the number of values raised to the number of slots, so the two cannot both be generous. The limit was measured rather than argued: two resistors for E48, E96 and E192, three for E12 and E24, four for E6 alone. Pick a series and both counts move with it, so nothing you can choose is a search that will not finish. The limit is the same for everyone; there is no plan that lifts 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, within what your series allows, and every circuit in that range is searched on every run, exhaustively — nothing is sampled and nothing is skipped. While a search is running the page counts off how many of the circuits it is searching are done.
The single value you are trying to beat
Every answer names the nearest single value in your series and how far off it is, whether or not it wins. That number is the whole reason to solder two resistors together, so it belongs above the list rather than hidden behind it. When that one value is already at least as close as the best combination, Resistora says so plainly and tells you to buy it. Sending you to build a circuit that beats nothing is the one answer this tool will not give.
When a target is out of reach
A series reaches from its smallest parallel combination to its largest series one, and nothing beyond either end. E24 with two or three resistors runs from 0.3333 Ω to 30 MΩ. 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. Allowing more resistors brings both ends out, and a coarser series is what allows more.
What it does not do
Your kit is not here. This tool answers out of a catalogue, and Combination Search answers out of the resistors you own; the two are never mixed in one search. Divider circuits are not here either — 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. Neither is tolerance: Resistora does not record it anywhere, 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
E-Series Search is free and works signed out. There is nothing to sign in for: the tool reads a published standard, not anything of yours, and the search itself runs in your browser across several background threads.
A worked example
Ask E24 for 3.2 kΩ with two or three resistors. The nearest single value in that series is 3.3 kΩ, 3.13% off — the error you would live with by buying one part. What comes back lands exactly: 1 kΩ + 2.2 kΩ in series, then 1.2 kΩ + 2 kΩ, then 1.6 kΩ + 1.6 kΩ. That last one is two of the same value, which is allowed here and is not allowed in Combination Search, because a catalogue never runs out.