It's the moment in a relationship many people dread - when a partner asks how many people you've slept with.

Now, a new calculator could make the conversation even more awkward, as it reveals how many people a person has indirectly been exposed to.

It also reveals the risk of catching a sexually transmitted infection (STI) as a result.

For example, a person who has had nine lovers - the UK average - and suspects their spouse has slept with the same number of people - has been indirectly exposed to 597,870 individuals.

This is the equivalent of the entire population of Belfast in the UK, or Milwaukee in the US, the Sexual Exposure and STI Risk Calculator found.

This person has a 10 per cent risk of catching gonorrhoea, a 9 per cent risk of syphilis and a 6 per cent risk of chlamydia, it revealed.

The tool, which draws on the theory of six degrees of separation, was developed by Dr Felix, an online doctor service based in the UK.

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Using official statistics for STI rates in each country, it was designed to emphasise the importance of safe sex and having regular tests for sexually transmitted diseases.

The tool asks people to enter in which country and city they live in, followed by how many sexual partners they've had.

It also asks for how many lovers they suspect their partner has had too.

Using a mathematical formula based on the theory of six degrees of separation, the calculator works out how many people we've indirectly had sex with.

Writing on its website, Dr Felix said: 'Most people are probably familiar with this term from John Guare’s play from the 90’s, but “six degrees of separation” is actually a theory originally introduced by Frigyes Karinthy in 1929.

'"Six degrees of separation” is the simple idea that everyone is six or fewer steps away from any other person in the world.

'So a simple chain of connection connects any two people in a maximum of six steps.'

The tool was created using data for STIs from a range of countries, from reliable sources such as World Health Organisation (WHO), Centers for Disease Control and Prevention (CDC) and other Government websites.

A new calculator reveals how many people a person has 'indirectly' slept with - and their risk of catching a sexually transmitted infection (STI) as a result. For example, a person who has had nine lovers - the UK average - and suspects their spouse has slept with the same number of people - has been indirectly exposed to 597,870 individuals. This is the equivalent of the entire population of Belfast, the calculator claims

A person who has had sex with 10 people and suspects their partner has too - has been indirectly exposed to 1,111,110 individuals. Apparently, this is the equivalent of 92 per cent of people in Glasgow

This information was analysed in relation to each country's population to calculate the probability of becoming infection with any of the eight common STIs.

These were chlamydia, gonorrhoea, syphilis, trichomoniasis, herpes virus, Human Papilloma Virus (HPV), HIV and Hepatitis B.

In countries without accurate statistics, mathematical modelling was used to fill in the gaps.

Writing on its website, Dr Felix emphasised the calculator is not a diagnostic tool, it simply highlights a person's potential sexual exposure and risk of STIs based on the theory of six degrees of separation.

It said: 'This is not just your sexual partner(s) but their partners, and their sexual partners etc so don’t be alarmed if you see some alarmingly high percentages if you are a sexually active individual.'