Resistor Combination Finder
Find series, parallel, or mixed resistor matches for a target value, ranked by nominal error with tolerance and power-stress checks.{{ summaryHeading }}
| Rank | Topology | Network | Nominal | Error | Tolerance | Part stress | Copy |
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| Check | Value | Detail | Copy |
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Resistor combination matching is useful when the exact value in a schematic is not in the drawer, on the reel, or in the preferred value series you plan to buy. A series connection raises total resistance by adding parts together, while a parallel connection lowers total resistance by adding conductance. Mixing those two ideas can often land closer to a target than a single stocked value.
Nominal matching is only one part of a build decision. A resistor marked 10 kohm with 5% tolerance is allowed to be above or below that value, and two nominally identical parts may not move in the same direction. The useful question is usually not just which network is closest on paper, but whether the miss is small enough compared with part tolerance, inventory, circuit sensitivity, power dissipation, and the space available on the board.
Combination matching is especially helpful for prototypes, repair work, LED current limiters, voltage dividers, feedback networks, calibration fixtures, and classroom circuit checks. It can also keep a bill of materials smaller by reusing stocked parts instead of adding one unusual value for a small correction.
The result should be treated as a shortlist for engineering review, not as a parts approval. After the nominal value looks acceptable, check the real resistor tolerance, power rating, voltage rating, temperature coefficient, package size, and supplier availability before committing the network to hardware.
How to Use This Tool:
Start with the resistance you need, then choose whether the candidates should come from a standard preferred-value series or from the actual parts on your bench.
- Enter Target resistance and choose ohm, kohm, or Mohm. The summary compares every candidate's nominal equivalent resistance with this target.
- Choose Stock source. Standard E-series generates preferred values inside the stock range, while Custom bench stock uses the pasted values you already have.
- Set E-series or paste Custom stock values. Custom entries can use normal numbers and common resistor notation such as 4k7, 2k2, 1M, or 3R3.
- Limit Stock range so the search stays relevant. If the range is too broad or no values are available, fix the named validation message before reading the result tables.
- Choose Combination search. Use single plus pair mode for a broad first pass, series only when you need a higher value, parallel only when you need a lower value, or mixed mode when a three-part network is acceptable.
- Set Rows to show and Allow repeated resistor values. Repeated values are normal when you can use two identical parts, but turn them off when your stock count is limited.
- Open Advanced when the tolerance, applied voltage, part power rating, mixed search depth, or display precision matters. The tolerance value affects the audit rows and the green tolerance band on the error plot; voltage and rating drive the part-stress check; mixed depth only changes the bounded three-part search.
Read the summary first, then compare Combination Ledger, Build Audit, and Error Rank Plot. A close top row is useful only if the network is practical to build and the physical parts can safely carry the circuit current.
Interpreting Results:
The headline value is the closest ranked nominal match for the current stock list and search mode. A positive error means the network is above the target; a negative error means it is below. The ranking uses absolute percent error first, then prefers fewer parts, then a stable topology order when candidates are otherwise close.
| Output | What it tells you | What to check next |
|---|---|---|
| Summary | The closest nominal equivalent resistance, its network expression, signed error, topology, selected tolerance, and stock count. | If the summary says inputs need review, fix that message before trusting any table or chart. |
| Combination Ledger | Ranked candidate rows with topology, network expression, nominal resistance, signed error, and tolerance estimates. | Do not choose purely by rank. Compare part count, repeated values, layout simplicity, and availability. |
| Build Audit | A compact setup check covering target, stock library, search space, top nominal error, tolerance window, source voltage, part stress, and recommendation. | Set stock tolerance, applied voltage, and part rating to match the actual build before using the recommendation. |
| Error Rank Plot | A signed percent-error chart for the visible leading candidates, with the selected tolerance shown as a target band. | Look for a cluster of practical alternatives, not only the first bar. A slightly larger error can be easier to build. |
| JSON | A structured snapshot of the current inputs, stock summary, power model, top candidate, tables, and chart rows. | Use it for notes or review, but avoid treating the exported data as a substitute for schematic checks. |
Parallel candidates often look attractive because two common values can make a value below either branch. That same topology also changes current sharing and physical layout. Series candidates are easier to reason about, but the total value is always above each part. Mixed candidates can produce tighter nominal matches, at the cost of more parts and a search that intentionally examines a limited pool of nearby stock values.
A row marked inside stock tolerance means the nominal miss is smaller than the selected tolerance percentage. It does not mean the assembled circuit will meet a voltage, current, gain, timing, thermal, or calibration requirement. Use the result to pick candidates, then verify the surrounding circuit with the actual resistor tolerances and ratings.
Technical Details:
Positive-resistance networks made only from series and parallel groups are monotonic: increasing any individual resistor increases the equivalent resistance of the network. That makes the nominal calculation deterministic and also makes the low and high tolerance limits easy to model by moving all selected parts to their low or high limits.
Preferred-value series place nominal resistor values roughly evenly on a logarithmic scale. Lower-density series such as E6 and E12 have larger gaps and are normally paired with looser tolerance families. Higher-density series such as E96 and E192 give more possible nominal values per decade, which is useful when the target is part of a divider, filter, bias point, or reference network.
Formula Core:
Series resistance adds directly. Parallel resistance adds by conductance, where conductance is the reciprocal of resistance. Candidate error is then measured as a signed percent difference from the target.
| Symbol or term | Meaning | How it affects the result |
|---|---|---|
| Rtarget | The requested resistance after the selected unit is converted to ohms. | Every error percentage is measured against this value, so a wrong unit changes the whole ranking. |
| Req | The equivalent nominal resistance of one candidate network. | Series candidates add values; parallel and mixed candidates use reciprocal calculations. |
| E% | The signed percent error of the candidate versus the target. | Positive is high, negative is low, and rank order starts with the absolute value. |
| Worst-case tolerance | The spread from recalculating the same network with all selected parts at the low and high tolerance limits. | It shows a conservative nominal-value window, not temperature drift, derating, or package heating. |
| Root-sum-square estimate | A simplified independent-error estimate that divides the selected stock tolerance by the square root of part count. | It is useful for intuition, but it is not a guarantee for unmatched parts from unknown lots. |
The search space changes by mode. Single and two-part pair searches are exhaustive within the selected stock list. Mixed mode includes singles and pairs, then searches three-part networks from the closest stock values on a logarithmic scale so the browser remains responsive.
| Mode | Network types considered | Important boundary |
|---|---|---|
| Single + series/parallel pairs | Single stock values, two values in series, and two values in parallel. | Pairs are ranked exhaustively for the selected stock range. |
| Series pairs only | Two selected values added together. | The result cannot be lower than the smaller selected value. |
| Parallel pairs only | Two selected values joined across the same nodes. | The result is lower than the lowest branch value when both parts are positive. |
| Mixed search | Single values, pairs, R1 plus R2 parallel R3, and R3 in parallel with R1 plus R2. | The three-part portion uses the selected mixed depth, so increasing depth can reveal more candidates at higher cost. |
The accepted value notation follows common schematic and bill-of-material habits. 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 custom tokens are ignored during parsing, so check the stock count in the summary when a pasted list returns fewer candidates than expected.
Limitations, Privacy, and Accuracy Notes:
The calculation uses nominal resistor values. It estimates simple part current and power from the applied voltage and per-part rating you enter, but it does not measure the parts, inspect supplier inventory, check package availability, model temperature rise, or replace power and voltage stress review in the surrounding circuit. For high-current, high-voltage, high-precision, low-noise, or safety-related designs, confirm the candidate with a circuit calculation and the actual resistor datasheets.
Stock tolerance is a useful screening value, not a full uncertainty model. Temperature coefficient, long-term drift, solder heat, moisture, voltage coefficient, self-heating, resistor matching, and common lot behavior can matter more than the printed tolerance in precision dividers and references.
The resistance search runs in the browser, so target values and pasted stock lists do not need to leave the page for the matching calculation. Exported tables or copied rows can still contain design details, so handle them like any other circuit note from a private project.
Worked Examples:
3.3 kohm from E24 stock. With E24 selected and a stock range that includes 3.3 kohm, the top row should be an exact nominal single-value match. The pair rows are still useful because they show alternatives if that value is missing from your drawer.
165 ohm LED bench target. A custom list with values such as 47, 68, 100, 120, 150, 180, 220, 330, and 470 ohm can produce pair or mixed candidates closer than the nearest single 150 or 180 ohm part. The chosen network still needs a separate LED current and resistor power check.
5.02 kohm precision reference. A dense preferred-value series with repeated values turned off gives a cleaner shortlist when you want distinct parts. The nominal error may be small, but the tolerance audit should still be compared with the accuracy requirement of the reference or divider.
FAQ:
The nominal value is the printed or selected value, not a measured guarantee. A 1% or 5% resistor can be above or below nominal, and power, temperature, and aging can move it further in real use.
Parallel pairs are useful when the desired resistance is below available single values or when two stocked parts make a closer nominal value. Check current sharing, layout, and power dissipation before using the pair as a substitute for a single part.
Check the stock range, unit, and custom value notation. If the range is valid but too narrow, widen it. If a custom list was pasted, confirm that separators and suffixes such as k, M, and R are readable.
Yes. Paste measured values into the custom stock list when you are trimming a one-off circuit. Keep notes about the actual parts, because measured bench values do not automatically apply to future replacements.
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 nominal values spaced by series such as E12, E24, E96, and E192 so manufacturers and designers can reuse common parts.
- Nominal error
- The difference between the calculated equivalent resistance and the target before physical tolerance, heat, and drift are considered.
- Stock tolerance
- The selected percentage spread used for audit estimates. It should match the actual resistor family or measured inventory whenever possible.