Why your Hijri age is higher
The Hijri calendar follows the moon. Its twelve months have 29 or 30 days each, so a Hijri year has 354 or 355 days, about 11 days fewer than a Gregorian year. Over a lifetime this adds up: every 33 Gregorian years are about 34 Hijri years. Someone who is 30 in the Gregorian calendar is already about 30 years and 11 months, or close to 31, in Hijri years.
Where the Hijri age matters
Saudi Arabia used the Hijri calendar for many official purposes for a long time, and older documents and records still show Hijri dates of birth. In Islamic practice, Hijri dates are used for religious matters such as Ramadan, Hajj and the zakat year. Some people also want to know their age in Hijri years out of interest. Rules that set an age for a legal or official purpose can use either calendar, so always check which one the authority uses.
How the calculator works
It converts your date of birth and the chosen date into the Umm al-Qura calendar, the official calendar of Saudi Arabia, using the calendar data built into your browser. It then counts completed Hijri years, months and days. If you enter a Hijri date of birth, it first finds the matching Gregorian date. The supported range is 1300 to 1500 AH, which is 1882 to 2077 CE.
Turning 18 Hijri years
Because Hijri years are shorter, you reach 18 Hijri years roughly six and a half months before your 18th Gregorian birthday. Type 18, or any other age, in the last box to see the exact date in both calendars and how many days are left. If the date has already passed, the result says so.
Moon sighting and one-day differences
Umm al-Qura is a calculated calendar. In some countries the start of each month is set by sighting the new moon, which can be a day earlier or later. For that reason a Hijri date of birth recorded in one country may be one day off from the calculated date. This does not change your age in years, but it can move a birthday by a day.