IT TURNS out that condom sizes are all wrong.

Despite the well-known fact that condoms are the only birth-control method that protects against most sexually transmitted diseases, condom usage in Australia is on the decline.

A recent study of single, heterosexual men in Australia found only one in three (35 per cent) used a condom at their last sexual encounter with a casual partner.

And a report from the Centre for Social Research in Health found that less than half of the gay and bisexual men surveyed (47 per cent) always used condoms with casual partners.

Men often complain of discomfort, diminished sensation and poor fit when it comes to wearing them — and apparently it is because standard condom sizes are too big for the average penis.

Until recently, standard condoms had to be at least 6.69 inches (17cm) long, but studies find the average erect penis is roughly 2.5cm shorter, the New York Times reported.

“The idea was it had to be long enough to fit most men, and excess length could just be rolled,” Debby Herbenick, a sexual health expert at Indiana University told the publication.

She and her colleagues published a study of 1661 men living throughout the United States that found that 83 per cent had penile lengths shorter than standard condoms. The average length was 5.57 inches (14.15cm).

According to a more global study by British researchers published in the BJU International journal of urology, the average penis is 13.12cm in length when erect.

Now, a custom-fit condom company in the US, Global Protection Corp., has rallied the Food and Drug Administration (FDA) and industry standards associations for changes, allowing them to open the door to the condom equivalent of bespoke suits, the New York Times reports.

The Boston-based company has begun selling custom-fit condoms in 60 sizes, in combinations of 10 lengths and nine circumferences. It has succeeded where many other ideas for improving condoms have failed, impeded by the costs of testing required to satisfy the FDA, which considers condoms to be medical devices.

“If they bought a small condom before and it was still too big, it’s horrible for men to have that experience,” Global Protection Corp. president, Davin Wedel told the New York Times. Within hours of going on sale, he added, customers had ordered condoms in all 60 sizes.

In a study called Errors and Predictors of Confidence in Condom Use Amongst Young Australians Attending a Music Festival, which surveyed people between 18 and 29 who attended a northern New South Wales music festival, 48 per cent had seen a condom slip off during intercourse in the past year and 51 per cent had had a condom slip off when withdrawing the penis.

The custom condoms, marketed under the brand name myONE Perfect Fit, come in lengths of 4.9 (12.45cm) to 9.4 inches (23.88cm) and circumferences of 3.5 (8.90cm) to 5 inches (12.7cm). Standard condoms are typically 6.7 (17cm) to 8.3 inches (21.1cm) long and 3.9 (9.9cm) to 4.5 inches (11.43cm) in circumference.

However, to protect a man’s modesty, the measurement guide for the custom condoms use randomly ordered letters and numbers, rather than inches or centimetres. One man might be B17, for instance.

The idea for custom condoms was originally born in Europe in 2011 and sold by a company called TheyFit. Global Protection purchased the company but it took years of pressing the FDA and two standards organisations, ASTM International and ISO, for the devices to reach the United States.

However, while Mr Wedel says the bespoke condoms are flying off the shelves, not everyone is convinced that they will be the solution to declining condom usage rates.

Dr Herbenick said condom education, along with tips like adding lubricant, are more important than access to 60 sizes.

She and her colleagues published a study that found custom-fit condoms less likely to break but, for some men, more likely to slip.

Some men might “prefer a condom that they think fits their penis,” she told the New York Times. “But for the most part, men and their partners are fine with existing condoms.”