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With something as challenging as trying to lose weight, it’s nice to rely on things like math and logic to figure out how to do it. For example, if you’ve ever calculated how much weight you’ll lose by cutting out a certain number of calories a day, you know the most famous equation in dieting: one pound of fat = 3,500 calories. Too bad that equation is wrong.


It may be right in a strict mathematical sense, like if you burn a pound of fat in a lab, but cutting out 500 calories a day for a week won’t result in a pound of fat loss. We’ve covered this here before: trying to balance calorie intake and burn never quite works out.

Now, there’s a new equation in town, or more specifically a calculator called the Body Weight Planner, based on research done by the National Institutes of Health. Amby Burfoot explains at Runner’s World why we should ditch the old rule and embrace the new one:



“The biggest flaw with the 500-calorie-rule is that it assumes weight loss will continue in a linear fashion over time,” says [mathematician Kevin] Hall. “That’s not the way the body responds. The body is a very dynamic system, and a change in one part of the system always produces changes in other parts.” What’s realistic? According to Hall, in the first year of a new weight-loss program, most overweight people will lose about half the weight that the 3,500-calories rule predicts. In other words, over 12 months, the new rule is 7,000 calories = one pound. (The math changes slightly over shorter and longer periods of time, with few managing to lose weight beyond 12 months.)


Of course, results will vary from person to person.

And the new number may sound discouraging at first, but long-term dieters can feel validated: the science has finally caught up to what your body was already telling you. It’s not that you’re not working hard enough—it’s just that we were given the wrong equation.

This story was originally published on 7/22/15 and was updated on 10/10/19 to provide more thorough and current information.