This page explains the rules behind every CalcFind result, so you know exactly what you're getting and can compare it with other tools.
Where calculations happen
All math runs in your web browser using code we've written and tested. Nothing you enter is sent to our server, so results appear instantly and your figures stay private.
Rounding and precision
- Computers store most decimals as close approximations. To stop errors like 0.1 + 0.2 = 0.30000000000000004, we round every result to 12 significant digits. That is far more precise than any everyday use needs.
- Answers are displayed with up to 6 decimal places (percentages up to 4), with trailing zeros removed. The full value is used in any following step.
- Very large or very small numbers are shown in scientific notation, for example 1.5e+16.
Percentages
- Percent change divides by the starting value: (new − old) ÷ |old| × 100. Using the absolute value means a move from −50 to −25 counts as an increase.
- Percentage difference divides by the average of the two absolute values, so the order of the numbers doesn't matter.
- The % key on the basic calculator follows desk-calculator convention: after + or − it means "percent of the running total", elsewhere it divides by 100.
- When a calculation is undefined, such as a percentage of zero, we explain why instead of showing "Infinity" or "NaN".
Dates and ages
- Dates are treated as calendar days with no time zone, so daylight-saving changes can never add or remove a day.
- Ages count whole years, then whole months, then remaining days. If the birth day doesn't exist in a month (for example the 31st), the last day of that month is used.
- People born on February 29 have their birthday on February 28 in common years. Legal rules vary by country and state.
- Day counts measure the days between two dates and don't include both ends.
Testing
Each formula has automated tests built from hand-worked examples and edge cases: zero, negative numbers, leap years, month ends and invalid input. The tests run before every update is published, and an update that fails any test doesn't go live.
Rates, limits and tax rules
Future money calculators that depend on official figures, such as 401(k) contribution limits or tax brackets, will show the year they apply to and link to the official source. We review those figures every January and whenever the agency publishes new numbers.
Corrections
If you think a result is wrong, email contact@calcfind.com with the values you used. Confirmed errors are fixed and the page's "last checked" date is updated.