The bar mitzvah ceremony for boys or bat mitzvah for girls when they turn 13 -- although some Jewish girls celebrate a bat mitzvah at 12 -- is a coming-of-age ceremony that formally marks the age when Jewish children become responsible for observing the commandments. Guests at a bar mitzvah party typically bring gifts, such as cash, to celebrate the milestone. Although no guidelines exist on the set amount you should give for a bar mitzvah, being familiar with Jewish customs can help you determine an appropriate gift.

Cash gifts for bar mitzvahs are typically given in multiples of 18, such as $18, $36 or $54. Eighteen is the numerical value of the hay and yud, which make up the word "chai," Hebrew for "life." Gifts in increments of 18 -- a long-held tradition -- express your hope that the recipient enjoys a long and prosperous life, according to Chabad.org. However, you won't find a specific increment of $18 that is required or customary.

Consider your relationship with the recipient and your resources when you're deciding how much to give. For example, you'll likely want to give a larger gift to a close family member than to a school acquaintance of your son or daughter. You can also consider how much you'd give the person as a birthday gift; a bar mitzvah is more special than a birthday, but that amount can serve as a baseline estimate. "The Everything Etiquette Book" recommends using $50 as a starting point or $100 for close friends and family. Average gift amounts vary widely by community, however, so check with a friend or the rabbi if you're still uncertain.

You don't have to give cash for a bar mitzvah; you can alternately give Jewish-themed gifts, such as biographies of Jewish people or a Jewish prayer book, or other presents related to the recipient's hobbies or interest. However, remember that a bar mitzvah is a spiritual event, not just a birthday party, so try to give a gift with meaning, such as planting trees in Israel, which are available in increments of -- what else -- $18 per tree.

Bar mitzvah gifts are typically given at the party rather than at the service, since in most cases, not everyone attending the service will go to the party -- if there is one. However, some branches of Judaism, such as Orthodox Judaism, prohibit carrying gifts in the Synagogue or at all during Shabbat. If the bar mitzvah occurs on Shabbat -- Friday night or Saturday before sundown -- you may want to send your gift before or after that day. If you aren't sure what to do, ask the child's parents or a rabbi for guidance.