By LIZ HULL

Last updated at 07:40 08 November 2007

They regularly proclaim that finding the right size bra is as good for a woman's figure as a splashing out on a boob job.

Now style gurus Trinny Woodall and Susannah Constantine appear to have gone one step further, by encouraging women to plunge their breasts into water to help them work out the exact size of their bosoms.

The bizarre technique, they say, is a far more accurate because it determines the weight, rather than the cup size of a woman's breasts.

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And it could help scientists design the ultimate, perfect-fitting bra for all women in the future.

With just a bowl of luke warm water, a baking tray and kitchen scales, housewives across the nation could soon all be using simple physics to work out the precise size of their bosoms.

The technique works by immersing each breast in a bowl of water and weighing the amount of water displaced.

One litre of water weighs one kilogram, so the volume of water can be easily converted to work out the weight of each individual bosom.

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While Trinny spent a day as a big busted woman courtesy of a painted on 32D silicone enlargement, Susannah tried out the measuring technique to highlight the problems women face finding the right bra size.

According to recent surveys, as many as 75 per cent of women either do not know their true bra size or are regularly wearing ill-fiiting underwear.

She visited three high street stores who all came up with different sizes for her 34D chest for a new series of ITV's Undress the Nation.

One shop advised her she was a 36D, while another assistant claimed she needed to wear a 36C bra and a third 32F.

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But after dunking her assets into a bowl of luke warm water, design engineer Dr John Tyrer, was able to work out that Susannah's breasts weighed two pounds each - or the equivalent of four bags of sugar.

He is currently designing a prototype for the perfect bra based on weight, rather than cup size.

Dr Tyrer said: "Women have this wonderful methodology about their breasts. They say 'well, they're different, they're this shape, they're that size.'

"The question is how do you know? Because the measurement process you have is an irrelevance.

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"If you're an engineer or a designer the first thing you want to know is how heavy - how heavy are each of your breasts.

"Knowing the displaced amount of water tells us what the breast weight is.

Then, using a conversion factor we can then convert the weight of displaced water to the weight of the breast.

"We used water displacement because it is a good practical science experiment you could do at home, if you so wished.

"Our techniques have allowed us to pioneer new designs of bras which, unlike existing bra designs, now actually work."

Susannah said that finding bras that fit perfectly could revolutionise the lives of women and stop those unhappy with their body shape from going under the knife for cosmetic surgery.

"It's astonishing that so many women have no concept of how big their boobs are," she said.

"Hardly anyone has the right size bra on. Bras are either way too big, or way too small."

Trinny added: “The most awful thing is that every single bra manufacturer makes a different size bra again and again and again. So, you might be a B cup in one store and a D cup in the other, it is crazy."

Trinny and Susannah's Undress the Nation is on ITV1 at 8pm tonight.