The number of cases is triple that of severe influenza illnesses recorded annually, according to the World Health Organization.
Many hard-hit countries are easing lockdowns put in place to slow the spread of the virus, while others, such as China and Australia, implement another round of shutdowns in response to a resurgence in infections. Experts say alterations to work and social life could last until a vaccine is available.
The first case was reported in China in early January and it took 149 days to hit 6m cases. It has taken less than a third of that time, just 39 days to double to 12m cases, the tally shows.
There have been more than 546,000 deaths linked to the virus so far, within the same range as the number of yearly influenza deaths reported worldwide. The first death was reported on 10 January in Wuhan, China before infections and fatalities surged in Europe and then later in the United States.
The United States reported a daily global record of 56,818 new Covid-19 infections on 3 July when global cases reached the 11m mark. The United States recorded a total of 3m cases on Tuesday, and accounts for more than a quarter of both global cases and global fatalities putting President Donald Trump's pandemic strategy under scrutiny.
Brazilian President Jair Bolsonaro tested positive for coronavirus after downplaying the seriousness of the pandemic. The country has reported between 20,000 and 50,000 new cases daily since 1 July. Brazil has more than 1.7m cases and nearly 68,000 deaths.
The Reuters tally, which is based on government reports, shows the disease is spreading the fastest in Latin America. The Americas account for more than half the world's infections and almost half its deaths. Brazil and the United States account for around 45 percent of all new cases since the beginning of July.
India, the country with the third highest number of infections, is battling an outbreak of more than 20,000 new cases each day.
In countries with limited testing capacity, case numbers reflect only a proportion of total infections. Experts caution that official data likely underrepresents both cases and deaths.