Super Bowl LI overnight ratings are big -- but not as big as last year's

The game, which saw the Patriots make the largest comeback in Super Bowl history, brought in an overnight rating of 48.8, according to Nielsen. That's down slightly from last year, which saw a 49.0 overnight rating, and the year before, which had a 49.7.

Still, it is Fox's best metered market rating ever, according to the network.

To come up with overnight ratings, Nielsen takes the percentage of households watching in 56 US markets and comes up with an average. A 48.8 means 48.8% of households in those 56 markets tuned into the game Sunday night.

Last year's Super Bowl ultimately went on to bring in 111.9 million viewers, making it the third-most-watched broadcast in U.S. TV history.

The Patriots' last appearance in a Super Bowl, in 2015, brought in the biggest audience in history with an average of 114.4 million viewers.

The actual number of people who tuned in to a Super Bowl is always harder to figure out, and most likely larger than the ratings portray, because Nielsen doesn't count the groups of people watching at parties or in bars and restaurants.

It had appeared for most of the game as if ratings could take a hit due to a less-than-exciting contest, as the Atlanta Falcons dominated the Patriots for the first three quarters.

But then quarterback Tom Brady led the Patriots to a shocking comeback, forcing the game into overtime -- a Super Bowl first -- and likely keeping many viewers glued to their televisions in the process.

The numbers are subject to change as the day goes on and final viewership numbers are produced. Fox will announce those figures Monday afternoon.