Men are more than twice as likely as women to underestimate their weight, finds a project with 300,000 participants

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Millions of men may be putting their health at risk by not realising they're overweight, Guardian research suggests.

Men are more than twice as likely as women to underestimate their weight - and therefore may not be aware that their lifestyle is putting them at the risk of a range of diseases – including heart disease, diabetes and some cancers – associated with carrying extra pounds.

Almost one in four men asked to say whether they were of a healthy weight, overweight or obese by the Guardian guessed they were at least one category thinner than they really were – indicating many are taking risks with their health without even knowing it.

More than 300,000 people fed their height and weight into a Guardian interactive project since its launch last month, and each was asked in advance which category they would fall into – underweight, healthy, overweight, obese, or morbidly obese.

Almost one in four men (23%) who participated guessed their weight range was at least one category less than it was in reality – meaning men who guessed they were in the healthy weight range were in fact overweight (or heavier), for example.

The findings, which come from a very large sample (but not a scientifically selected one), suggest millions of men may be unaware they are overweight, and thus of the consequent health risks they are taking: in the UK, 68% of men and 58% of women are overweight or obese.

By contrast, women were if anything possibly too self-conscious about their weight. Only 8% under-estimated which category they fell into, while 27% thought they weighed more than they did – versus just 12% of men.

This could suggest women are more influenced by images of thin women shown across the media, are more worried and aware about their health, or could be tied to a myriad of other factors.

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The categories are based on Body Mass Index (BMI), an approximate (and simplified) measure of someone's body shape based solely on their height and weight. Generally, a score of under 18.5 is regarded as underweight, 18.5 to 25 is 'ideal', and more than 25 signifies someone who may be overweight.

The findings also revealed stark differences in self-knowledge between people of different weights.

Perhaps unsurprisingly, people whose BMI fell into the healthy range were the most likely to guess their weight correctly, with 70% of respondents in this group correctly guessing their category. Those who did get it wrong generally thought they were heavier than they actually were: about five times as many wrongly thought they were overweight as thought they were underweight.

As respondents' BMI increases, accuracy of guesses drops. Overweight respondents were only very slightly less likely than those in their healthy range to correctly guess their weight, with 67% getting it right. However, those who got it wrong generally thought they were thinner than they really are: 27% guessed their were healthy (or under) weight, versus just 7% who wrongly thought they were obese.

In the obese and morbidly obese categories, the majority of people did not correctly guess their weight, at 44% and 46% correctly. In both cases, more people underestimated their BMI group than got it right.

But – as the chart below reveals, these weren't the people who were least likely to get their BMI right.

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It is, in reality, those people with a BMI of less than 18.5 – underweight people, who are least likely to correct guess their grouping. Of the respondents in the Guardian research, only 40% of underweight people correctly placed themselves in that group. The majority guessed they'd be normal weight, or even higher.

The findings – though only indicative, rather than formal research – show the importance of breaking down the data: even though overall most of us know our bodies, the people most likely to get it wrong are in the groups most as risk of health consequences as a result.

There's also a thorny dilemma for public health advocates on obesity: is there a way to make men worry more about their waistlines, while at the same time making women worry at least slightly less?

You can take the Guardian's BMI test yourself, or read more on the issues in the obesity crisis series.