Guides
Plain-English explanations of how age is counted, why the details matter, and how to work things out yourself. Every guide lists its sources.
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How to calculate age manually, step by step
Work out anyone’s exact age with just a calendar: the three-step method, the borrowing shortcut, and worked examples for the tricky cases.
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Leap day birthdays: how age works if you were born on 29 February
A true birthday only every four years, but you still age every year. Here’s when, and why software and laws don’t always agree.
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Why age calculators sometimes give different answers
Most disagreements come down to a handful of conventions. Here they are, with examples you can test any calculator against.
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How to calculate age in Excel and Google Sheets
Copy-ready formulas for age in years, in years, months and days, as a decimal and in days, plus the quirks that trip people up.
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How age is counted around the world
Most of the world counts age from zero at birth, but several East Asian systems started at one. Here’s how each works and where they’re still used.
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Chronological age vs biological age
Your birthday tells you how long you’ve lived. Biological age tries to estimate how fast you’re aging. Here’s what each means and how far the science has got.
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Legal ages around the world
When the law treats you as an adult, and when you can vote, drink and drive, in ten countries. Checked against official sources in October 2026.
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How old is a dog in human years? What the science says
The seven-year rule is easy to remember and wrong. Here’s what DNA research and veterinary guidance say instead, and why size matters so much.
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Age milestones worth celebrating
Your birthday comes once a year, but there are plenty of other milestones: your 10,000th day, your billionth second, even your first birthday on Mars.
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What generation am I?
Generation labels are everywhere, but where do the boundaries come from? Here are the birth years for each generation and why sources disagree.