Saying thank you with cash has always been a little tricky.
Although etiquette experts suggest tipping 15% to 20% for most services, some people are more generous than others and many aren't giving a gratuity at all.
Among all diners who leave tips, the average was 19%, according to a study by CreditCards.com.
Men also average 19%, the study found, a little more than women, who leave 18%, on average.
However, women were more likely than their male counterparts to always tip their hair stylists, waitstaff and food delivery, CreditCards.com found.
When broken down by generation, millennials tip 22%, on average, compared to just 17% for baby boomers. Boomers were the more consistent tippers, however.
The rise of mobile payments at coffee bars, food trucks and ride shares has made it harder for consumers to know when it's appropriate to tip and how much, according to Ted Rossman, industry analyst at CreditCards.com. "There's a whole big murky middle."
When presented with a variety of suggested tipping options, about 1 in 6 millennials said they regularly choose the lowest option, and nearly 1 in 5 gives no tip — more than any other age group.
In addition, reports that Instacart and other delivery companies, including Doordash, were using customer tips to subsidize the fees the company paid to drivers have added to the confusion around tipping, Rossman added. (Both companies have since stopped that practice.)
"When in doubt, it's nice to give something," Rossman said. CreditCards.com surveyed more than 2,500 adults in August.