Resistor Combination Finder

Target resistance:

Changing the unit converts the displayed value and preserves the same target.

Stock source:

Preferred-value series:

Custom stock values:

Examples: 100, 220, 1k, 2k2, 4.7k, 10k.

Combination search:

Available stock range:

Enter minimum and maximum values such as 10 and 1M.

Allow repeated resistor values:

Turn this off when only one part of each nominal value is available.

Stock tolerance:

Zero is neutral. Use the tolerance printed on the actual parts when screening a build.

Applied voltage:

Zero leaves the power screen off and does not change nominal ranking.

Per-part power rating:

Use the nominal rating of each resistor; apply datasheet derating separately.

Ranked rows:

This changes the displayed shortlist, not the evaluated search space.

Display precision:

Formatting changes only; computation retains full precision.

Rank	Topology	Network	Nominal	Error	Tolerance estimate	Power screen	Copy
{{ row.rank }}	{{ row.topology }}	{{ row.network }}	{{ row.nominal }}	{{ row.error }}	{{ row.tolerance }}	{{ row.power }}

Check	Value	Interpretation	Copy
{{ row.label }}	{{ row.value }}	{{ row.note }}

A schematic may call for a resistance that is missing from the parts drawer or absent from a preferred-value series. Two or three available resistors can often reach the target closely enough, but the smallest nominal error is only the beginning of the design decision.

Topology determines the equivalent resistance and how current and heat are shared. Series resistors carry the same current and their values add. Parallel branches share the same voltage and their conductances add, producing an equivalent value below every branch resistance. Mixed networks can fit a difficult target more closely, at the cost of more parts, solder joints, board area, and tolerance interactions.

Practical differences among resistor network choices
Choice	Electrical effect	Practical tradeoff
Single resistor	Uses its nominal resistance directly.	Fewest parts and simplest service, but may miss the target.
Series pair	Raises resistance by adding values.	Same current in both parts; voltage and power split by resistance.
Parallel pair	Lowers resistance through added conductance.	Same voltage on both parts; power can be shared unevenly.
Three-part mixed network	Combines series and parallel behavior.	More possible matches, more construction and tolerance complexity.

Preferred-value E-series repeat a set of nominal values in every decade. Sparse series such as E6 and E12 are practical for general stock. Denser E96 and E192 sets offer finer nominal choices, but they do not guarantee that a particular value, tolerance, voltage rating, package, or power rating is actually available.

Real components move away from their labels. Manufacturing tolerance, temperature coefficient, aging, soldering, measurement method, and applied voltage all affect the built circuit. A nominally exact network can therefore be worse than a slightly imperfect one built from tighter, cooler, or better-characterized parts.

The surrounding circuit also decides what “best” means. A divider may care about the ratio between two legs, a filter about impedance and cutoff, a current-setting path about device limits, and a high-voltage path about working voltage and spacing. Equivalent resistance alone cannot approve those designs.

Treat ranked combinations as build candidates. Confirm inventory, measure the assembled network when accuracy matters, and apply the component manufacturer's derating and voltage guidance before energizing the circuit.

How to Use This Tool:

Define the electrical target and the stock you truly have, then add tolerance and voltage only when those screens reflect the planned build.

Enter Target resistance and choose Ω, kΩ, or MΩ. Changing the unit preserves the physical target rather than reinterpreting the same digits.
Choose IEC preferred-value series and an E-series, or choose Custom bench stock and paste the actual resistor values available. Limit the stock range so irrelevant decades do not dominate the search.
Select Combination search. Use pair-only modes for a simple build, or include three-part mixed networks when a closer fit justifies the extra parts. Turn on Allow repeated resistor values only when duplicate parts are available.
Open Advanced to enter stock tolerance, applied voltage, and per-part power rating. A zero applied voltage leaves the power screen off; a zero tolerance leaves the tolerance model off.
Review Ranked matches for topology, network expression, nominal value, signed error, tolerance estimate, and hottest-part power. Use Build audit to confirm the searched stock range and whether the three-part search was bounded.
If no rows appear, correct an empty custom list, reverse stock limits, an unavailable repeat-value policy, or a range that excludes every stock value.

Interpreting Results:

Rank 1 has the smallest absolute nominal error among the searched candidates. Ties favor fewer parts, then single, series, parallel, and the two mixed forms in that order; a lower nominal resistance breaks a remaining tie. This ranking does not include cost, package, temperature drift, voltage rating, noise, or board layout.

Error is signed. A positive percentage is above the target and a negative percentage is below it.
Worst tolerance moves every part to the same low or high tolerance edge. RSS is a planning estimate based on independent one-part changes, not a guaranteed production distribution.
Within nominal rating means utilization is at most 70%. Exactly 70% remains within; values above 70% through 100% are near the rating; values above 100% are over it.
A favorable power label still needs temperature derating, pulse behavior, working-voltage, and layout checks from the actual part datasheet.

Technical Details:

Equivalent resistance reduces a passive network to the single resistance that draws the same total current at the same applied voltage. Series networks add resistance directly. Parallel networks add reciprocal resistance, which is equivalent to adding conductance.

Formula Core:

The supported one-, two-, and three-part topologies use these exact equivalent-resistance identities.

R_{single} = R_{1} R_{series} = R_{1} + R_{2} R_{parallel} = \frac{R_{1} R_{2}}{R_{1} + R_{2}} R_{mixed A} = R_{1} + (R_{2} ∥ R_{3}) R_{mixed B} = (R_{1} + R_{2}) ∥ R_{3}

The signed nominal error compares equivalent resistance R with target resistance R_t.

e = 100 \times \frac{R - R_{t}}{R_{t}}

For a stock tolerance fraction t, the worst estimate evaluates the same topology once with every resistor multiplied by (1 − t) and once with every resistor multiplied by (1 + t). The larger deviation from nominal becomes the worst-case percentage. The root-sum-square estimate varies one resistor upward at a time and combines those percentage contributions quadratically.

e_{RSS} = \sqrt{\sum_{i = 1}^{n} e_{i}^{2}}

Power is evaluated from the applied voltage across the complete network. Series parts use the common current I = V / R and P_i = I²R_i. Parallel parts use P_i = V² / R_i. Mixed networks first find branch voltage or branch current from their topology. Utilization is the hottest part's estimated power divided by the entered per-part rating.

Search Rule Core:

Resistor network search coverage by selected mode
Search choice	Candidates evaluated	Coverage limit
Single values plus series and parallel pairs	Every stock value and every allowed two-part series and parallel pair.	Exhaustive within the selected stock and repeat policy.
Series pairs only	Every allowed two-part series pair.	Exhaustive within the selected stock.
Parallel pairs only	Every allowed two-part parallel pair.	Exhaustive within the selected stock.
Include three-part mixed networks	Singles, pairs, and both supported three-part mixed forms.	Three-part candidates use at most 24 stock values nearest the target on a logarithmic scale.

E6, E12, and E24 use fixed preferred bases. E48, E96, and E192 bases are generated as equal logarithmic steps across a decade and rounded to three significant digits, then repeated across powers of ten inside the requested range. A custom list accepts decimal values and resistor notation such as 4k7, removes duplicates, and ignores invalid or nonpositive entries.

The search accepts at most 1,200 stock values and returns 4 to 24 ranked rows. Display precision changes formatting from two to six significant digits; ranking and the underlying calculations retain their unrounded numeric values.

Limitations and Accuracy Notes:

The tolerance and power outputs are planning screens, not component qualification. They use nominal resistance, a shared tolerance percentage, a steady applied voltage, and one per-part power rating.

Temperature coefficient, ambient temperature, thermal resistance, pulse or surge energy, working-voltage limits, parasitic capacitance and inductance, noise, aging, and PCB heat spreading are not modeled.
The RSS value assumes independent deviations and should not replace worst-case analysis when every compliant unit must pass.
Three-part search is intentionally bounded, so a closer mixed combination may exist outside the selected 24-value pool.
All calculations stay in the browser; no stock list or circuit values are sent to a server by this search.

Worked Examples:

Exact 300 Ω series pair

For a 300 Ω target and custom stock containing 100 Ω and 200 Ω, a series-only search without repeated values returns 100 Ω + 200 Ω at 300 Ω. The nominal error is 0%. With tolerance and applied voltage left at zero, both additional screens correctly remain unmodeled.

Repeated parallel part

For a 150 Ω target with only a 300 Ω resistor in stock, allowing repeated values lets the parallel search use 300 Ω ∥ 300 Ω. The equivalent resistance is exactly 150 Ω. Before building, confirm that two matching parts are available and that each part's voltage and power limits suit the circuit.

References:

IEC 60063:2015, Preferred number series for resistors and capacitors, International Electrotechnical Commission, March 27, 2015.
Resistor and inductor application FAQ, Vishay.