Why age matters in fitness calculations
Many fitness and nutrition formulas use age in whole years. Metabolic rate equations such as Mifflin–St Jeor, for example, subtract 5 kcal for every year of age, because resting energy use tends to fall gradually as we get older.
Always use your age in completed years — the number this calculator shows in large type — when a formula asks for age.
How your age is calculated
- Years are counted from your birth date to the chosen date, only counting a year once your birthday has passed.
- Remaining months and days are counted using real calendar months, so results are exact.
- If you were born on 29 February, your birthday is counted on 28 February in years that are not leap years.
- Everything runs in your browser — the dates you enter are not sent or stored anywhere.
Example
| Date of birth | Age on 30 September 2026 |
|---|---|
| 15 June 1990 | 36 years, 3 months, 15 days |
| 1 January 2000 | 26 years, 8 months, 29 days |
Frequently asked questions
How do I calculate my exact age?
Subtract your birth year from the current year, then subtract one if your birthday has not happened yet this year. The calculator also works out the remaining months and days for you.
Can I calculate my age on a future or past date?
Yes. Use the “Age on this date” field to find your age on any date after your date of birth.
Is my date of birth stored?
No. The calculation runs entirely in your browser and nothing you type is sent to FitNumbra.
Content reviewed: by the FitNumbra editorial team.