Resistor Calculator
Adjust the inputs below. Results update as you type.
How it works
4-band resistor: first two bands = digits, third = multiplier, fourth = tolerance. Color order: Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Violet=7, Grey=8, White=9. Multiplier: same colors ×1 through ×10⁹. Series: R_total = R₁ + R₂ + ... Parallel: 1/R_total = 1/R₁ + 1/R₂ + ... For two parallel resistors: R = R₁R₂/(R₁+R₂).
Input guidance
- Confirm date/time/unit settings before comparing outputs.
- If a task has optional fields, run both with and without them to understand impact.
- Save scenario variants when making practical decisions.
The formula
Resistor color bands encode resistance: the first bands give significant digits, the next a multiplier (power of ten), and the last the tolerance.
Worked example
Brown-black-red-gold = 1, 0, ×100, ±5% = 1,000 ohms (1 kΩ) at 5% tolerance.
More examples to test
- Quick-pass example: run default assumptions for a first estimate.
- Refined example: update one assumption at a time to isolate impact.
How to interpret results
Use outputs as operational estimates and pair them with local constraints, business rules, or provider requirements.
When this can be inaccurate
Utilities can miss local policy details, special-case rules, and environment-specific constraints.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
Find productivity tools for everyday workflows
General recommendation, not professional advice.
Frequently asked questions
How do I read resistor color codes?+
Each color maps to a digit. Read the significant-figure bands, apply the multiplier band, and the final band gives tolerance.
What is tolerance?+
How far the actual resistance may vary from the labeled value. A 5% tolerance on 1 kΩ means it falls between 950 and 1,050 ohms.
How do resistors combine?+
In series, resistances add. In parallel, the reciprocals add: 1/R_total = 1/R₁ + 1/R₂ + …
Why are outputs slightly different from another tool?+
Different tools can use different rounding rules, default assumptions, and treatment of edge cases.
How can I improve estimate reliability?+
Use verified inputs from real records and re-run calculations after major assumption changes.