General Calculators
Use practical everyday calculators for percentages, exact age between two dates, and the span between dates, each with a worked example and its own formula.
Overview
The general calculators cover the everyday numerical tasks that do not warrant a full spreadsheet: percentage math, exact age between two dates, and the gap between two calendar dates expressed in days, weeks, months, and years.
Choosing the right general calculator
Reach for the percentage calculator when you need to compute a percentage of a value, find what percent one value is of another, or measure a percent increase, decrease, or net change between two numbers. The tool exposes each mode explicitly so the formula matches the question you are actually asking. For example, use percent decrease for a discount, or percent change for a year-over-year comparison.
Reach for the age calculator when you need the exact age between a birth date and a comparison date, broken into years, months, and days using calendar-aware math rather than a flat 365-day approximation. Reach for the date difference calculator when you need elapsed time between two calendar dates, with both a calendar breakdown and total counts in days, weeks, and months.
Common mistakes to avoid
Percentage modes are not interchangeable: percent of value, percent increase, percent decrease, and percent change apply different formulas, and choosing the wrong mode changes the result. The single most common mistake is using percent change when the question is actually percent decrease, or vice versa. The sign of the answer flips and the interpretation differs.
Time-based calculators use UTC date boundaries and ignore time-of-day and timezone offsets. They are designed for calendar math, not stopwatch precision. The date difference calculator offers an include-end-date toggle that affects total day and week counts but leaves the calendar breakdown unchanged, so check the toggle state if a total looks off by one. For age, the comparison date must be on or after the birth date, and reversed dates do not produce a result.
Cross-tool tips
If you need an exact age but also want elapsed time totals, the age calculator gives both. If you only have two arbitrary dates and want a complete breakdown, the date difference calculator is the right one. For percent change questions involving negative values, double-check the sign and base value. The formulas are mathematically correct, but the interpretation depends on context.