Align assumptions before comparing decimal places
If two calculators disagree, first match the mode, units, reference year and timing conventions. Then examine rounding. More displayed digits cannot repair a different assumption, and two equal rounded outputs may still have different underlying values.
Visible assumptions
Each tool identifies its supported calculation modes and units. Formula descriptions and source links are available as page text, alongside illustrative inputs and results. A financial model, a biological estimate, and exact arithmetic carry different uncertainty.
Calculation and precision
Most engines use JavaScript numerical arithmetic. Fractions, large integers, address ranges, and selected decimal operations use appropriate exact or higher-precision representations. Display rounding is separate from internal arithmetic where practical.
Inputs are checked for missing values, invalid units, impossible dates, division by zero, and model-specific restrictions. Extreme cases can exceed a method’s supported numerical range; those should return a useful error rather than a plausible-looking invalid answer.
Verification and coverage
We check representative calculations against independently derived examples and source references, and test edge cases such as invalid dates, boundary values, and unit conversions. These checks reduce errors but cannot prove that every possible input is correct.
A calculator’s page describes the implemented methods and their limitations. A matching name does not mean it reproduces every feature of another service. Methods that rely on unavailable data or proprietary rules are identified where relevant.
Privacy by design
Most calculations run in your browser without an account. Road mileage sends selected locations to map services. Reference-data modes retrieve public snapshots without sending your entered amount or selected pair. The privacy policy explains local files, sharing, and optional browser-assistant tools.