If Charter Communications is allowed to buy cable rivals Time Warner Cable (TWC) and Bright House Networks (BHN), just two Internet service providers could control about 70 percent of the nation's 25Mbps-and-up broadband subscriptions.

Comcast would remain the country's largest ISP with 22.87 million Internet subscribers, while Charter's merger will push it into second place with at least 20.56 million. (AT&T has 15.83 million.) Combined, Comcast and Charter would account for less than half of home Internet connections. But under the Federal Communications Commission definition of "broadband," which requires download speeds of at least 25Mbps, Comcast and Charter would reign supreme in the US.

The FCC changed its definition of broadband from 4Mbps downstream/1Mbps upstream to 25Mbps/3Mbps a year ago and uses the new definition to evaluate whether broadband is being deployed to all Americans at a reasonable and timely pace. Though many Americans get by with slower speeds, the FCC argues that higher speeds are necessary for multi-person families using the Internet to quickly download big files and use high-quality video and voice applications.

Market share measurements based on the 25Mbps standard could factor into the FCC's ongoing review of Charter's merger. Comcast's attempt to buy TWC was prevented by the FCC over fears Comcast would use its greater size to harm online video services. But Charter argues that it isn't a danger in this case, because Charter doesn't impose data caps and promised not to charge online video services like Netflix for network interconnection deals that improve video quality.

Charter said in November that it would serve 23 percent of the nation's 25Mbps-and-up broadband subscribers if it can buy TWC and BHN. Comcast has about 47.6 percent based on our calculations, pushing the two companies' total over 70 percent. The vast majority of Comcast subscribers have speeds of at least 25Mbps.

90 percent market-share claim is exaggerated

These figures are not exact, as we have to rely on some FCC data that is over a year old. But our estimate should be significantly more accurate than one that has been spread by merger opponents that just formed a group called the Stop Mega Cable Coalition. The group claimed that "Mega Cable [i.e. Charter after its mergers], along with Comcast, would control the vast majority—nearly 90 percent of broadband homes in America."

The group is trying to make the case that Comcast and Charter would constitute a duopoly that could try to snuff out competition from online video services that provide alternatives to cable TV.

Dish Network made a similar claim, telling the FCC last month that the mergers would "result in two broadband providers controlling about 90 percent of the nation’s high-speed broadband homes between them." Dish is a member of the Stop Mega Cable group, along with Public Knowledge; Cincinnati Bell; Common Cause; Consumers Union; Dish Network; FairPoint Communications; Future of Music Coalition; Greenlining Institute; ITTA Media Alliance; Open Technology Institute at New America; NTCA-The Rural Broadband Association; Sports Fans Coalition USTelecom; Writers Guild of America, East; Writers Guild of America, West; and Zoom Telephonics.

Stop Mega Cable didn't provide any public justification for its 90-percent claim, but we exchanged a few e-mails with a group spokesperson to figure out how they made their calculation. Stop Mega Cable is using the FCC's 25Mbps definition of broadband. But it turns out the group relied on an out-of-date estimate of the nationwide total of broadband subscribers, among other mistakes. The group used December 2013 data that said 29.37 million subscribers nationwide have 25Mbps speeds.

Stop Mega Cable claimed this is "the most recent data," but in fact the FCC has newer data going through December 2014, which counted 41.65 million US Internet subscribers with speeds of at least 25Mbps. (The data doesn't seem to account for upload speeds.) Using the outdated 2013 numbers, Stop Mega Cable claimed that Comcast has 63.25 percent market share in the 25Mbps-and-up category, far higher than Comcast's actual share.

In one e-mail to Ars, the group's spokesperson claimed that Comcast and Charter will actually have 93 percent market share, even though the group's public statement asserted that it's actually "nearly 90 percent."

One thing that is clear from FCC data is cable companies in general dominate the high-speed broadband market:

Charter getting bigger, but how big?

Stop Mega Cable also credited Charter with about 30 percent market share, despite Charter's statement that it will have 23 percent after the mergers.

Stop Mega Cable's only justification for that was another Charter filing that said the expanded company would serve "less than 30 percent of [25Mbps] broadband customers."

"We can only rely on the numbers Charter chooses to make public," Stop Mega Cable told Ars. "That said, it would behoove Charter to cite their actual market share if the figure was truly significantly less than 30 percent. It feels more likely to us that the 30 percent figure cited reflects roughly their true market share."

It's really no surprise that Charter would stress the "less than 30 percent" figure because that's a threshold the FCC used in the past to determine whether a cable company's market share is too high. (The 30-percent cap was thrown out in court, but companies still point to it in FCC filings if their market share is lower than 30.)

Charter's "less than 30 percent" quote came in May 2015, and the company followed up with its more precise estimate of 23 percent in December.

A Charter spokesperson told Ars that it calculated the 23 percent estimate by comparing the merging companies' December 2014 subscriber numbers to the December 2014 FCC data, so that means the expanded Charter would have about 9.58 million subscribers with 25Mbps speeds.

Charter currently has 5.44 million Internet subscribers, and it says all of them have speeds of at least 60Mbps. TWC has 13.02 million subscribers, but many don't have 25Mbps service. BHN has at least 2.1 million subscribers, with download speeds ranging from 15 to 150Mbps.

Comcast's share of 25Mbps connections has dropped

As for Comcast's share, the company said in March 2015 that "over 90 percent" of its customers buy at least 25Mbps service. While Comcast has 22.87 million Internet subscribers today, it had 21.96 million at the end of 2014. 90 percent of that is 19.77 million, giving the company at least a 47.5-percent market share in the 25Mbps-and-up category in December 2014.

The market-share figures would probably look different now if we had completely up-to-date data. TWC and other cable companies have been upgrading speeds, and AT&T has been turning on new fiber-to-the-home deployments, both of which could alter the total number of 25Mbps subscribers. Comcast's share of the 25Mbps market changed quite a bit between 2013 and 2014, dropping from about 56 percent to our calculation of 47.6 percent.

With those caveats in mind, our best estimate is that Comcast and Charter would have a bit more than 70 percent of the 25Mbps-and-up market—if Charter's merger is approved—and less than half of the total home Internet service market.

UPDATE: After this story published, consumer advocacy group Free Press pointed us to its own analysis, which got a similar result. Free Press says the poster-merger Charter and Comcast "together would form a national broadband duopoly controlling nearly two-thirds of current customers and the telecommunications wires connected to nearly 8 of every 10 homes." Free Press's analysis uses estimated market shares as of June 30, 2015:

Disclosure: Bright House is owned by the Advance/Newhouse Partnership, which is part of Advance Publications. Advance Publications owns Condé Nast, which owns Ars Technica. Advance/Newhouse would own 13 percent of Charter after the proposed transactions.