Resistor Calculator.

Formula and practical context.

Formula, variables, and method

series: Rₜ = ΣR; parallel: Rₜ = 1 ÷ Σ(1/R). Enter 1–100 positive resistance values in ohms, separated by commas, spaces, or line breaks. The result combines a list of positive resistance values using either the series sum or the reciprocal parallel formula. Start by identifying what each entered number represents and whether it is a measured value, a specification, or an assumption. Convert units before combining quantities, preserve full precision through intermediate steps, and round only the displayed result. For series, add every resistance. For parallel, add each reciprocal and invert the total; the parallel result must be below the smallest branch. This sequence makes the output reproducible and helps expose a misplaced unit or decimal point before it affects a decision.

Worked example and independent check

Resistors of 100 Ω and 220 Ω total 320 Ω in series and approximately 68.75 Ω in parallel. To check the example independently, write the formula first, substitute the values with their units, and complete one operation at a time. Compare the order of magnitude with a familiar reference rather than accepting a formatted result automatically. For series, add every resistance. For parallel, add each reciprocal and invert the total; the parallel result must be below the smallest branch. If a second method or inverse calculation is available, use it to recover an original input. Small differences from hand calculation should come only from displayed rounding, not from a different definition.

Units and result interpretation

All entered values are ohms. Convert kilo-ohms by multiplying by 1,000 and megaohms by multiplying by 1,000,000 before entry. The result combines a list of positive resistance values using either the series sum or the reciprocal parallel formula. A unit is part of the quantity, not a label added after the arithmetic. Keep decimal and binary prefixes, temperature scales, elapsed-time units, and electrical prefixes explicit. When comparing two scenarios, change one assumption at a time and retain the same unit convention. The result describes only the relationship named by this calculator; it does not automatically establish capacity, safety, compatibility, quality, or compliance.

Validation and boundary cases

Every list item must be a finite positive number; empty items, zero-ohm branches, negative values, and more than 100 components are rejected. Blank values, malformed text, non-finite numbers, impossible ranges, zero divisors, and unsupported combinations receive an explicit message rather than a silent fallback. Boundary values are checked before the formula runs. If an input comes from a specification sheet, weather report, network plan, or measurement instrument, confirm its definition and precision. A syntactically valid value can still be unsuitable for the real situation, so inspect the assumptions alongside the result.

Assumptions, privacy, and practical limits

Nominal resistance does not model component tolerance, temperature coefficient, frequency-dependent impedance, power rating, or wiring resistance. All entered values are processed in the browser and are not sent to a calculation service. Use the result as a transparent estimate or transformation, keep the source values with any saved result, and consult the linked authoritative material for definitions and applicability. For safety-critical, health-adjacent, security, network, chemical, or electrical work, verify the result with current standards, official warnings, qualified professionals, and appropriate test equipment.

Check assumptions and sources.

Nominal resistance does not model component tolerance, temperature coefficient, frequency-dependent impedance, power rating, or wiring resistance.

NIST SI derived unit: ohm