In order for the UK's coronavirus lockdown to be relaxed, five tests need to be met.

The NHS must be able to cope, there needs to be enough testing and PPE to meet further demand, a relaxing in restrictions must not lead to a second peak, there needs to be a consistent fall in daily death rates, and there needs to be a consistent fall in the rate of infection.

To ensure these tests are being met, the government has established a new Covid Alert System that is determined by the R number and the number of coronavirus cases.

This system will determine how strict the lockdown measures need to be.

But what is the R number, how is it calculated, and what does it mean?

Here's what you need to know.

What is the R number?

The R number is how many people each infected person will pass the disease to, on average.

R stands for reproduction, as the R number is how scientists measure how quickly a disease is spreading.

If the R number is two, it means each infected person will infect another two, who will each infect another two, and so on.

What is the R number for coronavirus?

At the start of the pandemic the R number for coronavirus in the UK was three - meaning that each person who caught COVID-19 passed it on to an average of three others.

The lockdown and social distancing measures have brought down the R number to somewhere between 0.5 and 0.9.

If the R number remains below one, then the infection will gradually decrease and eventually the pandemic will end.

If it rises above one, there could be a second peak of COVID-19 in the UK, and the lockdown may need to get stricter.

How is the R number calculated?

Six groups of scientists look at data including hospital admissions, deaths, intensive care occupancy and total confirmed cases. They each make separate recommendations to SAGE, who meet twice a week to consider the evidence.

SAGE will be closely monitoring the R number as the lockdown measures are relaxed.

What happens if the R number rises?

The government may re-impose stricter coronavirus lockdown measures if the R number rises above one.

It could lead to an increase in cases, and a second peak.

How do you reduce an R number?

At the moment, social distancing is preventing the spread of coronavirus.

Tracking and tracing the virus - with extensive testing or location tracking apps - will also help limit the spread.

A vaccine would also reduce the R number, as large portions of the population would be protected from the disease.