About the numbers
Earth's age is taken as 4.543 billion years, a figure scientists arrived at primarily through radiometric dating of the oldest known meteorites and Earth materials — it's one of the most well-established numbers in modern geology. Your age is calculated precisely: the tool finds the exact number of days between your birth date and today, then divides by the average length of a year (accounting for leap years) to produce a fractional-year age rather than a rounded whole number. Dividing that age by Earth's age, and multiplying by 100, gives the percentage of Earth's history your lifetime represents — a number so small that it's shown in scientific notation alongside an easier "1 in roughly X" comparison, since strings of leading zeros are hard to parse at a glance. The visual timeline places your lifetime, the extinction of the non-avian dinosaurs roughly 66 million years ago, and the emergence of Homo sapiens roughly 300,000 years ago, all on the same scale as Earth's full history — and even those two much older, much more consequential events turn out to sit right at the very end of the timeline too. Nothing you enter is saved: your birth date and the calculation both stay entirely in your browser.
4.543 billion years
Earth's widely accepted age, from radiometric dating.
Fractional-year precision
Your age is calculated to the day, not just the year.
Readable framing
Scientific notation plus a plain "1 in X" comparison.