Losing weight is twice as difficult as official guidelines claim, according to US research that will offer some reassurance to dieters struggling to shed the pounds.
A widely-accepted rule of thumb states that cutting 500 calories per day from your diet will translate to a weight loss of 1lb (half a kilogram) per week.
But the advice, which appears on the NHS website and a variety of diet plans, is wrong because losing weight becomes harder the thinner you get, experts said.
This is because cutting the number of calories in your diet slows your metabolism down, meaning that your weight loss will tail off and eventually reach a plateau.
To help redress the balance, scientists from the US National Institute of Diabetes and Digestive and Kidney Disease have programmed an online calculator which designs a personalised weight loss plan based on information about your diet, physical activity and body shape.
Although the tool is primarily designed for doctors, it can also be used by members of the public to calculate how much weight they would lose by making certain lifestyle changes.
A general guide, they said, is that eating 100 fewer calories per day per day will make you lose 10 lbs over the course of three years, including 5lb in the first year.
According to the old and allegedly incorrect rule of thumb, it would have taken just six months to lose 5lb on the same diet.
Speaking at the annual meeting of the American Academy for the Advancement of Science (AAAS) in Vancouver Dr Kevin Hall, one of the designers of the online calculator, said: "The contrast is that the old rule of thumb predicts twice as much weight loss after a year and it gets worse after that.
"People have used this rule of thumb to predict how much weight they should lose for decades now, and it turns out to be completely wrong.
"If you cut the calories in somebody's diet their metabolism starts to slow down, and it slows down more the more weight is lost, so eventually you'll reach a plateau."
The online calculator takes into account factors like people's age, sex, height, weight and physical activity along with their diet to predict how much weight they will lose under certain dietary or exercise regimes.
It then simulates what changes of diet or exercise a person would need to meet their desired weight in a set time frame.
Dr Hall said the tool would help people "match up their expectations about weight loss with the changes in diet and physical activity that would be required to both reach those goals and maintain them."
He said: "If you've expected to lose twice as much weight after a year as you actually lose, I think that's horribly demotivating to people."
Speaking at the same seminar Dr Boyd Swinburn, of Deakin University in Australia, said the growing global obesity epidemic could be blamed largely on an increase in food consumption rather than a decrease in exercise.
He said there were a range of interventions which could help tackle the problem, but the most cost-effective of those were "the very same policies that governments do not want", such as restrictions on the advertising of junk food to children, taxes on unhealthy foods and "traffic light" labelling of products to signal how healthy they are.