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Age in Months Calculator

Created by Davide Borchia
Reviewed by Komal Rafay
Last updated: Jan 18, 2024

Our age in months calculator will tell you how many Moons (approximately) you've spent on this planet in the blink of an eye. Though it's not that common to count ages in months, from time to time (pun intended), it may be useful: we are here to help!

With this handy tool, you'll learn:

  • The history of the months;
  • How to calculate the chronological age in months; and
  • The fastest way to calculate your age in months.

Why do we use months?

The hour and the week are units of time entirely dependent on the choices of our ancestors: Babylonians and Egyptians laid down the basis of our time-keeping practices, the former by assigning a day to each of the visible "planets", thus creating a seven-day cycle, and the latter by dividing a day into two sets of 12 hours (Egyptians counted using a base-12 numerical system).

🙋 The minute and the second come from a division of the hour: that's why we didn't include them in this paragraph!

On the other hand, the day, the month, and the year all come from astronomical cycles:

  • The day is the time required by the Earth to complete a rotation;
  • The month approximates the duration of the Lunar revolution around the Earth; and
  • The year corresponds (more or less: leap years, we are watching you!) to the duration of an Earth's complete revolution around the Sun.

Our ancestors had Sun and Moon constantly in sight, thus using them to keep track of time. Let's take a closer look at the month.

History of the month

The month is as old as humanity: since we started looking up to the sky, the recurrence of Lunar phases suggested that the period of 29.5329.53 days was a good time unit: not too short (like a day), not too long (like a year).

A Lunar month's duration aligns with the duration of an average menstrual cycle: are you surprised to discover that the word menstruation shares the exact origin as the Latin word mens, month?

There is no connection between the Lunar month and the menstrual cycle: the synchronization of the two is a metropolitan legend. It may happen that your period syncs with the one of a close friend; however, this happens randomly, and after a while, everything ends up out of phase. We redirect you to a couple of related pages: confirmation bias and recall bias.

For a while, the Lunar month was enough. With the progress of science and the development of refined calendars, however, the significant offset between the duration of twelve Lunar months and a year brought us to the current calendar with twelve months of different lengths.

This calendar, with more or less significant variations, is now used worldwide. Let's take a look at it!

The months in the Julian calendar

The Gregorian calendar is the calendar we use in our everyday life. It has twelve months, and here they are:

  • January (31 days);
  • February (28-29 days);
  • March (31 days);
  • April (30 days);
  • May (31 days);
  • June (30 days);
  • July (31 days);
  • August (31 days);
  • September (30 days);
  • October (31 days);
  • November (30 days);
  • December (31 days).

🙋 If you ever wondered/got pissed because September, October, November, and December are not the months numbered 7, 8, 9, and 10, but from 9 to 12, you can thank Romans: they introduced (literally created from thin air) January and February and decided to place them at the beginning and not at the end of the year!

How to calculate your age in months

Whether you want to learn how to calculate your age in months or how to calculate your baby's age in months, we will teach you how to do it.

However, there's a huge catch: there is no easy mathematical formula for this: you can calculate the chronological age in months exactly only by counting the months between the starting and ending date.

Luck is on our side (kinda) since for every full year between the dates, we can add 1212 to the total. However, we need to count for the years of the starting and ending date!

Follow these steps to calculate the chronological age in months:

  • Subtract the ending and the starting years; add 11 to the result;
  • Multiply the result by 1212;
  • Sum the number of days in the initial and final months following these rules:
    • Sum the exact number of days of the final month;
    • Subtract the number of days of the initial months from 30.530.5 and sum to the previous result.
  • There can be two situations:
    • The number of days is greater than 30.430.4;
    • The number of days is smaller than 30.430.4.
    In both cases, take the integer part of the result of the division of the number of days by 30.430.4. It will be 00 in the second case. Sum the result to the number of months.
    The decimal part of the division is the fraction of "not completed" months between the dates.

🔎 Why 30.430.4? 30.430.4 is the average duration of a month. We could be more accurate than this, but the results would change only slightly.

The fastest way to calculate the age in months is to leave the task to a computer! That's why we created the age in months calculator. Choose the starting and ending dates, and let us do all the work!

Let's try an example anyway! Choose a date. It can be a birthday or any other date. The date of the Jimmy Carter rabbit incident is perfect: the 20th of April 1979. Choose another date, for example, the release of Monty Python and the Holy Grail, where another bunny shows an unexpected bloodthirst: the 25th of May 1975. How many months have passed since the murderous beast attacked the president and the release of the movie?

  1. Subtract the years and subtract 1: 197919751=31979-1975-1= 3.
  2. Multiply this number by 1212 to find the number of months in the full years: 3×12=363\times12=36.
  3. Sum the number of days: the rabbit attack happened on the 109th day of the year, while for the movie, we need to add 220 days. 109+220=329109+220=329.
  4. Divide the result by 30.430.4: 329/30.4=10.82329/30.4=10.82.
  5. Sum the two results to find the elapsed number of months: 36+10.8=46.836+10.8=46.8

Other chronological age calculators


How do I calculate my age in months?

To calculate your age in months, you can choose between different strategies:

  1. Count the months between the final and initial date;
  2. Approximately divide the number of days between the dates by 30.4;
  3. Ask a computer to do it for you!

Since months don't have a regular pattern, we can't refer to a simple equation to calculate the number of months between dates.

What's the age in months of a 2½ old baby?

30 months. Since there are 12 months in a year, we count twice times 12: 24. We then have to add the number of months in half a year: 6.

The result is 24+6=30.

In general, after this age, it's totally fine to start counting your baby's age in years. However, before that moment, the growth is so rapid you'd be justified to say 30 months old!

How many months does a US presidency last?

A presidential term lasts 4 years (at least, if nothing unexpected happens). This gives us an immediate result of 48 months: 12 each year.

The shortest term ever served is the one of William Henry Harrison, 31st president. Starting on the 4th of March 1841 and ending abruptly on the 4th of April of the same year, the term lasted only a month.
Franklin Delano Roosevelt was first inaugurated on the 4th of March 1933 and died in office on the 12th of April 1945: he is the longest-serving president with 145 full months.

Why are there 12 months in a year?

12 is the number of lunar months in a year. The duration of a lunar month (~29.5 days) doesn't fit perfectly with the length of a year: this is why months have different durations.

On the other hand, Lunisolar calendars use the lunar month's duration. This causes a discrepancy: every third year or so, the Chinese calendar adds a 13th month to compensate for it!

Davide Borchia
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