Resistor Combination Finder

Target resistance:

Use the desired nominal resistance; the result table ranks candidates by signed error from this target.

Stock source:

Choose standard values or a custom resistor list.

E-series:

E24 is common bench stock; E96 and E192 are useful for precision dividers and references.

Combination search:

Search single values, series pairs, parallel pairs, or a bounded three-part mixed network.

Stock range:

Use resistor notation such as 10, 4k7, 1M, or 0.47.

Rows to show: {{ boundedResultLimit }} rows

Show enough alternatives to pick a part you actually have.

Allow repeated resistor values

Leave on when you can use two or three parts with the same nominal value.

Custom stock values:

Accepted examples: 100, 220, 1k, 2k2, 4.7k, 10k.

Examples:

Use a preset to verify the ranking, then adjust the target or stock list.

Load sample

{{ error }}

Stock tolerance: {{ boundedTolerancePercent }}%

Adjust when your actual parts have a different tolerance than the selected series.

Applied voltage: {{ appliedVoltageDisplay }}

Used only for the audit and part-stress column; it does not change resistance ranking.

Part power rating:

The audit flags candidates whose hottest individual resistor exceeds this rating.

Mixed search depth: {{ boundedMixedDepth }} values

Increase only when mixed mode misses a target and the browser remains responsive.

Display precision:

Calculation uses full JavaScript precision; this only changes formatting.

Rank	Topology	Network	Nominal	Error	Tolerance	Part stress	Copy
{{ row.rank }}	{{ row.topologyLabel }}	{{ row.network }}	{{ row.nominalDisplay }}	{{ row.errorDisplay }}	{{ row.toleranceDisplay }}	{{ row.stressDisplay }}

Check	Value	Detail	Copy
{{ row.check }}	{{ row.value }}	{{ row.detail }}

Embed:

Customize

Include current inputs

Size

Advanced

Width

Height

Aspect ratio

Max height

Collapsible embed

Allow fullscreen

Referrer policy

Sandbox tokens

Choosing a resistance value is rarely just a matter of reading one number from a schematic. A prototype drawer may have E12 or E24 parts, a precision circuit may need an E96 or E192 value, and a repair bench may need to make a failed value from whatever is already in stock. Combining resistors gives a practical way to reach a target resistance without waiting for a special part, but the network becomes a small design choice rather than a single interchangeable component.

The first distinction is topology. Series resistors sit in one current path, so their resistance values add and every part carries the same current. Parallel resistors share the same two nodes, so their conductances add and the equivalent resistance becomes lower than any one branch. Mixed networks use both ideas, often producing a closer nominal match at the cost of more solder joints, more board space, and more ways for tolerance or heat to change the final result.

Preferred-value families explain why the available parts often cluster around familiar numbers such as 1.0, 1.2, 1.5, 2.2, 3.3, 4.7, and 6.8 within each decade. A sparse family is easy to stock and good enough for many pullups, LEDs, filters, and bias paths. Denser families give finer choices for references, dividers, trim networks, and measurement circuits, but they also increase search time and may not reflect the parts that are actually on the bench.

Nominal value: The selected resistance before real tolerance, temperature, aging, voltage stress, or measurement error is included.
Equivalent resistance: The single resistance that would draw the same current as the whole network under the same applied voltage.
Preferred values: Standard resistor families such as E6, E12, E24, E48, E96, and E192 that repeat across powers of ten.

Series, parallel, and mixed resistor networks compared against a target resistance before tolerance and power checks.

A close resistance match should be read as a candidate, not an automatic build approval. Tolerance can move every part away from its nominal value, heat can change resistance and damage the resistor body, and a high-voltage path may fail for voltage rating or spacing reasons even when the wattage looks acceptable. Precision dividers, references, filters, and current-setting circuits also need the surrounding circuit checked, because the useful answer may be a ratio, cutoff, or current rather than the resistance value alone.

The most useful combination is often not the row with the smallest decimal error. A slightly less exact single resistor can be easier to assemble and replace than a three-part network, while two equal parts in parallel can spread power and make a low value from common stock. Good resistor matching balances accuracy, inventory, topology, thermal margin, serviceability, and the tolerance budget of the real circuit.

How to Use This Tool:

Start with the resistance the circuit needs, then limit the search to parts and topologies you would actually build.

Enter Target resistance and choose ohm, kohm, or Mohm. A usable run changes the summary from Check inputs to either Exact nominal match or Closest bench match.
Choose Stock source. Standard E-series generates preferred values from the selected family, while Custom bench stock searches only the pasted list.
Set E-series or paste Custom stock values. The custom box accepts plain numbers plus resistor notation such as 4k7, 2k2, 1M, and 3R3.
Constrain Stock range before comparing rows. If the message says No stock resistor values are available in the selected range, fix unreadable custom values, check the range endpoints, or widen the range.
Pick Combination search. Use the broad single-plus-pairs mode for bench exploration, the series-only or parallel-only modes when the schematic is fixed, and mixed search only when a three-part network is acceptable.
Adjust Rows to show and Allow repeated resistor values. Turning repeated values off removes candidates that need two or three of the same nominal part.
Open Advanced for review settings. Stock tolerance feeds the tolerance audit and plot band, Applied voltage and Part power rating feed the stress check, Mixed search depth widens the three-part pool, and Display precision changes formatting only.
Confirm the result in Combination Ledger and Build Audit. A row is ready to consider only after the nominal error, tolerance note, and part-stress status all fit the real circuit.

Interpreting Results:

The top row is the closest nominal equivalent resistance found under the current stock source, range, repeat-value setting, and search mode. A positive Error is above the target, and a negative error is below it. Ranking uses absolute percent error first, then fewer parts, then topology priority, then nominal resistance when candidates remain tied.

Do not treat Inside stock tolerance as proof that the circuit will meet its requirement. That recommendation only compares the nominal miss with the selected tolerance percentage. Check Build Audit for search limits and stress warnings, and compare a few nearby rows when board space, repeated values, part size, or replacement simplicity matters.

How to interpret resistor combination outputs
Output	Useful reading	Verification cue
Summary	Closest nominal resistance, network expression, signed error, topology, stock tolerance, stress label, and stock count.	If it still says Check inputs, fix the first validation message before trusting tables or charts.
Combination Ledger	Ranked networks with topology, nominal value, signed error, tolerance estimate, and part stress.	Prefer a buildable network over a fragile decimal win when the difference is smaller than the circuit tolerance.
Build Audit	Stock count, search size, best nominal error, tolerance window, voltage, hottest part, and recommendation.	Set the applied voltage and per-part rating to the real design, or the stress row is only a placeholder.
Error Rank Plot	Signed percent error for leading candidates, with the selected stock tolerance shown as a comparison band.	Look for clusters. A compact cluster of nearby choices is often more useful than one isolated candidate.

Power labels are screening labels. Within rating means the hottest calculated part is at or below 70% of the entered rating, Near rating means above 70% and at or below 100%, and Over rating means above the entered rating before any derating. High-power, high-voltage, hot, low-noise, or precision circuits still need datasheet and layout review.

Technical Details:

Equivalent resistance is deterministic for positive resistor values. A series path adds resistance directly because the same current passes through every part. A parallel group adds reciprocal resistance because each branch sees the same voltage and contributes current according to its own conductance. A mixed network applies these same rules to a fixed three-part shape before the resulting equivalent resistance is compared with the target.

Preferred-value stock is generated across decade ranges. E6 through E96 use fixed preferred-number bases, while the dense E192 family is generated as a logarithmic series. Wider stock ranges and denser families improve the chance of a close match, but they also increase the number of candidate networks. The browser search blocks stock lists above 1,200 values so the combination ranking remains responsive.

Formula Core:

The core equations convert a selected topology into equivalent resistance, then express the miss from the target as signed percent error.

\begin{array}{rcl} R_{series} & = & \sum_{i = 1}^{n} R_{i} \\ R_{parallel} & = & {(\sum_{i = 1}^{n} \frac{1}{R_{i}})}^{- 1} \\ R_{mixed} & = & R_{1} + {(\frac{1}{R_{2}} + \frac{1}{R_{3}})}^{- 1} \\ E_{%} & = & \frac{R_{eq} - R_{target}}{R_{target}} \times 100 \end{array}

Resistor combination quantities and boundaries
Quantity	Meaning	Boundary or note
R_target	Target resistance after the selected unit is converted to ohms.	Must be greater than zero.
R_eq	Equivalent nominal resistance of one candidate network.	Single, series, parallel, and mixed topologies use different equations.
E%	Signed percent error from the target.	Ranking uses absolute error, but the sign is retained for interpretation.
Worst-case tolerance	The largest percent movement after all selected parts are recalculated at low and high tolerance limits.	It assumes all parts move together to the selected tolerance edge.
RSS estimate	A root-sum-square shortcut equal to selected tolerance divided by the square root of part count.	It is an independent-error estimate, not a promise about unmatched physical parts.

A 165 ohm target with two 330 ohm resistors in parallel gives 1 / (1 / 330 + 1 / 330), or 165 ohm, so the signed error is 0%. With 5% stock tolerance, the conservative worst-case tolerance remains 5%, while the RSS estimate for two independent parts is about 3.54%.

Search and Stress Rules:

Resistor search modes and stress rules
Rule area	Behavior	Limit or consequence
Single and pair search	Ranks single stock values plus every two-part series and parallel pair in the selected stock range.	The stock list must stay at or below 1,200 values.
Series pairs	Adds two selected values.	The equivalent resistance is greater than either selected part when both are positive.
Parallel pairs	Uses reciprocal conductance for two values connected across the same nodes.	The equivalent resistance is lower than the lowest branch value.
Mixed search	Includes singles and pairs, then tests R1 + (R2 parallel R3) and (R1 + R2) parallel R3 shapes.	The three-part search uses the nearest 24 to 90 stock values on a logarithmic scale, based on Mixed search depth.
Stress check	Calculates total current, total power, hottest individual part, and utilization against the entered per-part wattage.	At or below 70% is Within rating, above 70% to 100% is Near rating, and above 100% is Over rating.

Custom stock values are parsed as positive resistances. Commas, spaces, semicolons, ohm words, and common symbols are tolerated, and a letter can replace the decimal point: 4k7 means 4.7 kohm, 2k2 means 2.2 kohm, 1M means 1 Mohm, and 3R3 means 3.3 ohm. Invalid tokens are ignored, so an unexpectedly low stock count usually means one or more pasted values did not parse.

Limitations, Privacy, and Accuracy Notes:

The calculation uses nominal resistance values and a steady-state power estimate. It does not measure physical parts, verify supplier stock, model temperature rise, account for airflow or copper area, check voltage rating, or calculate creepage and clearance.

Use manufacturer datasheets for high-power, high-voltage, precision, low-noise, pulse, or safety-related circuits.
Set Applied voltage to 0 V when resistance matching is all you need. The stress rows then report Not checked.
The matching calculation runs in the browser. Copied rows, exports, and shared URLs can still reveal design details, so handle them like circuit notes.

Advanced Tips:

Start with the sparsest E-series that matches your inventory, then move to E96 or E192 only when the leading rows miss the tolerance target.
Turn off Allow repeated resistor values when the build must use distinct stocked parts or when repeated values would be hard to track during assembly.
Use Mixed search depth as a cost control. Pair searches are exhaustive, but the three-part mixed search deliberately samples nearby values.
When Near rating appears, treat the row as a derating prompt. Part size, board copper, ambient temperature, and airflow can make the entered wattage too optimistic.
For ratio-sensitive dividers, compare candidate networks by the ratio they create in the surrounding circuit, not only by equivalent resistance.
Paste measured values into Custom bench stock for one-off trims, but keep a note that future replacements may not match those measured parts.

Worked Examples:

These examples show how the same target can be solved differently depending on stock, topology, and the warning rows.

Bench value already stocked

With a 3.3 kohm target, Standard E-series set to E24, a 10 ohm to 1M range, and the broad single-plus-pairs search, the leading Combination Ledger row is a single 3.3 kohm resistor with 0% error. At 5 V and 0.25 W per part, the stress check is within rating, with the hottest part using about 3.03% of the entered rating.

LED stock target from repeated parts

A 165 ohm target with custom stock values such as 47, 68, 100, 120, 150, 180, 220, 330, and 470 ohm can produce an exact nominal parallel match from 330 ohm || 330 ohm. At 5 V and 0.25 W per part, each 330 ohm branch dissipates about 75.8 mW, or 30.3% of the entered rating, before any LED forward-voltage context is considered.

Precision reference trim

A 5.02 kohm target with E192 stock, repeated values off, and 0.5% tolerance can rank 150 ohm + 4.87 kohm as an exact nominal series pair. The Tolerance cell still shows a 0.5% worst-case window and about 0.35% RSS estimate, so an exact nominal row is not the same as a measured precision network.

Empty custom stock list

If pasted text contains unreadable values or the Stock range excludes every parsed value, the summary stays at Check inputs and the validation message reports that no stock resistor values are available. Fix notation such as missing k or M suffixes, then recheck the stock-count badge before reading the plot.

FAQ:

Why can a 0% nominal error still fail in the real circuit?

The error value compares ideal nominal resistance only. Real parts can shift with tolerance, heat, voltage stress, aging, and measurement conditions, so check Tolerance, Part stress, and the circuit requirement before building.

When is a parallel pair a good substitute?

A parallel pair is useful when the target is lower than convenient single values or when two stocked parts produce a closer Nominal row. Confirm current sharing, layout, and per-part power because both branches see the same applied voltage.

Why did my custom stock list lose values?

Only positive values that parse as resistance are kept. Check separators and notation such as 4k7, 2k2, 1M, and 3R3, then compare the summary stock-count badge with the number of values you expected.

Can I use measured resistor values?

Yes. Use Custom bench stock when measured values matter for a one-off trim. Keep measurement notes separately because those values may not apply to future replacements from the same nominal family.

What does mixed search depth change?

Mixed search depth changes how many nearby stock values are considered for the three-part mixed network. Pair searches remain exhaustive, but larger mixed depth can reveal more candidates at a higher browser cost.

Glossary:

Equivalent resistance: The single resistance value that would draw the same current as the whole network under the same applied voltage.
Preferred values: Standard resistor values in E-series families, spaced across each decade for manufacturing and stocking.
Nominal error: The difference between calculated equivalent resistance and target resistance before real tolerance and drift are included.
Conductance: The reciprocal of resistance. Parallel calculations add conductance before converting back to resistance.
Power stress: The estimated load on the hottest selected resistor compared with the entered per-part wattage rating.
RSS estimate: A root-sum-square tolerance shortcut used as an independent-error estimate for a multi-part network.

References:

IEC 60063:2015, Preferred number series for resistors and capacitors, IEC, 2015-03-27.
IEC 60062:2016, Marking codes for resistors and capacitors, IEC, 2016-07-12.
Resistor Power Rating, Derating, and Temperature Coefficient, DigiKey, 2015-12-15.
Vishay resistor FAQ on derating and thermal limits, Vishay.