National guidelines that advise people how to lose weight are seriously flawed and grossly overstate how quickly they will reach their targets, scientists say.

Researchers found people lost only half as much weight as expected in a year if they followed the advice given by the NHS and US health organisations.

The problem came to light when government scientists at the US National Institutes of Health in Maryland realised standard weight loss advice – including its own – failed to account for changes in metabolism as people lost weight.

The rule of thumb used by the NHS and other health services assumes that if a person cuts 500 calories from their daily diet, they will lose about 450g (1lb) each week, regardless of how long they adhered to the regime.

But as people lose weight, their metabolism slows until they reach a new stable bodyweight. Their weight loss is further complicated by how much body fat and muscle they have when they start dieting.

"Dietitians and physicians have used this rule of thumb for a long time, but it turns out to be completely wrong. It doesn't account for metabolic changes that happen when people change diet," said Kevin Hall, who led the research.

"If you change calories, your metabolism slows down and eventually you reach a plateau."

He told the annual meeting of the American Association for the Advancement of Science: "If you want to lose 10lb of weight eventually, you need to cut 100 calories a day from your diet. You will get halfway there in a year, but then plateau after three years. The old rule of thumb predicts twice as much weight loss after a year and gets worse after that."

The glitch has consequences that go far beyond the frustration of obese and overweight people who are trying to shed weight. Public policies drawn up to tackle the rising obesity epidemic have to be reassessed, Hall said, and in cases where this has been done, their effectiveness looked much less impressive than before.

Hall re-examined a proposal for a "fat tax" that added 20% to the cost of sweet and unhealthy foods. Using the flawed weight loss guidelines, obesity rates in the US could be slashed from 67% to 50% in five years. But when he did the calculation again, taking metabolism into account, the figures were less rosy. After five years of the tax, obesity might drop to 62%, but crucially fall no further.

Last year, researchers writing in the Lancet called on governments to bring in measures to reverse the rise of obesity, which could affect an extra 11 million people in the UK in the next 20 years. Almost 50% of British men could be obese by 2030, they wrote.

Based on the work, Hall and his colleagues have created an online tool that people from adolescence to late middle age can use to get more accurate advice on how to lose weight. The tool, which can be used by GPs and patients, then works out how many calories they need to cut, either through dieting or more exercise, and how they can then maintain the healthier weight.

Rather than demoralising people, Hall hopes more accurate advice would help overweight and obese people have realistic expectations.

"If you expected to lose twice as much weight after a year than you actually lose, I think that's horribly demotivating for people. I would rather have realistic figures out there for people to gauge their expectations and gauge their interventions," he said.

The tool can be used to design personalised weight loss programmes that tailor the pace of a person's weight loss.

Hall said they provided "a good reality check for how long weight loss takes, and what changes in eating and exercise are required to achieve and maintain a goal weight."

A Department of Health spokesman said: "We do not give specific recommendations on how much people should eat while dieting as this will be different for each of us. Anyone who wants specific advice should speak to a health professional. We keep all international evidence under review."