It's a common refrain whenever the cheap, effective Canadian health care system is brought up: "what about all those Canadians who come to the US for health care?"
Here is the simple answer. All numbers are from 2014. Links go to the sources.
In 2014, the Census Bureau reported that a sharp drop in uninsured Americans (caused by the first year of implementation of ObamaCare) meant there were 33 million uninsured Americans. The Census Bureau also reports that the population of the US in 2014 was about 319 million, so the uninsured population was about 10%.
The Canadian health care system underserves about 1/7th of a percent. The American system underserves slightly over 10% -- after the first year of Obamacare. The number the year before would have been about 13%.
In a single number, the American health care system underserves 70 times as much of the population as the Canadian system.
This number is conservative, as it includes Canadians who get non-emergency health care in other countries because of convenience or scheduling, when they might have gotten it at home, and reserves American under service to those completely without insurance, not including anyone who can't afford a co-pay or deductible.
(Cross posted to Group News Blog)