You might not expect it, but the majority of the American workforce is a pretty happy lot. This was revealed in the first CNBC/Survey Monkey Workplace Happiness Index, announced at the @Work Talent + HR Summit on Tuesday.

The quarterly index measures how Americans feel about their jobs across five key categories — pay, opportunities for advancement, recognition, autonomy and meaning. CNBC and SurveyMonkey poll 8,664 professionals nationwide across a wide swath of industries March 13–18. Data has been weighted for age, race, sex, education, employment status and geography using Census Bureau's Current Population Survey to reflect the demographic composition of United States employed population.

The CNBC/SurveyMonkey Workplace Happiness Index is an optimistic 71 out of 100, while the survey found 85 percent of respondents are either somewhat or very satisfied with their jobs.

"Meaningfulness" was the biggest contributor to overall happiness, with 17 out of 20 possible points contributed to the overall index score of 71. Recognition, or how much colleagues value employee contributions, added 15 points, and autonomy, or work/life balance, added another 15. Opportunities for advancement was the biggest "drag" on the overall index.

Despite the overall optimism, only 9 percent of workers gave top ratings across all five categories of the Workplace Happiness Index. Twenty-seven percent of workers surveyed say they are not well paid, and 30 percent have seriously considered quitting their job in the last three months.

"Our study clearly reveals that workplace happiness is richly nuanced. While a big majority of U.S. workers are at least somewhat satisfied with their jobs, there are a lot of negatives when it comes to how people relate to their work," said Jon Cohen, SurveyMonkey's chief research officer.

The survey found that providing workers with opportunities to advance their careers may be the best way to keep them from leaving. Just 13 percent of workers who say their companies provide "excellent" opportunities for advancement were thinking of quitting their jobs. But there are no guarantees, and the survey did not reveal a single solution to keeping employees happy.

"If companies want to hire and retain great employees, they need to open up feedback loops to get at the 'why' —learning what makes people happy and productive," Cohen said.

Survey Methodology: This SurveyMonkey online poll was conducted March 13-18, 2019, among a national sample of 8,664 workers in the United States. Respondents for this survey were selected from the more than 2 million people who take surveys on the SurveyMonkey platform each day. The modeled error estimate for this survey is plus or minus 1.5 percentage points. Data have been weighted initially for age, race, sex, education, and geography using the Census Bureau's American Community Survey to reflect the demographic composition of the United States age 18 and over, then weighted for age, race, sex, education, employment status, and geography using Census Bureau's Current Population Survey to reflect the demographic composition of United States employed population. Full results available here.