Admittedly, before this virus came along, I never even knew what an R value was. This article explains the fundamentals,

and has other good information wrt how far droplets can travel depending upon atmospheric conditions:

"R0" is the basic reproduction number and is pronounced "R naught", not "R-zero" btw!

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"What is the R number, and why is it relevant to coronavirus?

The reproduction number – often called the R value or R number – is a measure of a disease’s ability to spread. It tells us how many people a single infected person will pass on the disease to.

The R number for COVID-19 that’s being quoted in the media and government briefings is what’s known as the ‘effective’ reproduction number. This value can go up and down.

We can reduce R by making it harder for the disease to spread, by implementing measures such as social distancing, closing restaurants and non-essential shops, and encouraging people to stay at home.

Every disease also has what’s called a ‘basic’ reproduction number, R0, which is the fixed value of R if no measures are put in place. For example, measles is highly contagious, with a R0 as high as 18, while COVID-19 has a R0 of around three.

So if COVID-19 was allowed to spread through the population, an infected person would, on average, give the disease to three other people.

But if all these people are practising physical distancing, then the virus can’t spread so easily and the effective R value goes down.

The crucial thing is to keep R below 1. If we can do this, then the number of new cases dwindles and the outbreak will eventually come to a halt.

Conversely, if R rises above 1, then we run the risk of rapidly escalating case numbers that would require stronger measures to keep the virus under control.

Because of this, R is used by governments to assess how we are doing in our efforts to stop the spread of COVID-19, and to adjust our actions, if needed."