The newly-designed Body Weight Planner gets nitty-gritty with personal details to calculate how many calories you should eat for weight loss

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Losing weight comes down to a very specific, well-established formula: You have to consume 3,500 less (or burn 3,500 more) calories per week to shed one pound. This number dates back 50 years to when a doctor named Max Washnofsky calculated that someone would need to reduce their calories by 500 every day in order to lose weight. The only problem? This number isn't actually correct for everyone. (But it is helpful! Find out more in Should You Count Calories to Lose Weight?)

Luckily, the National Institutes of Health has created a far more specialized and accurate calculator, called the Body Weight Planner (BWP). The calculator wasn't created by an M.D., but instead by a NIH mathematician Kevin Hall, Ph.D. Hall analyzed the best weight-loss studies out there and then constructed an algorithm that incorporated all the factors these studies proved impacted weight loss the most.

What makes this weight loss calculator so much better than the rest? It asks you to answer the typical questions like age, current weight, goal weight, and timeframe you're wanting to work within, but you're also asked your physical activity level on a scale of 0 to 2.5 and the exact percentage you're willing to change your physical activity by to reach your goal. And since most of us don't know these numbers off the top of our head, Hall has created a subset of genius questions us answer them. To determine the percentage you're willing to change, the calculator asks "I plan to add light/medium/intense walking/running/cycling for 5/50/120 minutes, 1/5/10 times per day/week" (there's an option for every five minutes between 0 and 120, and every frequency between one to 10). This level of specificity gets into the nitty-gritty of what a realistic amount of exercise-and therefore potential calorie burn-is for you specifically.

For example, if you're 135 pounds and exercise lightly, the BWP estimates that you can eat 2,270 calories a day to maintain your current weight. But you'd only have to cut 400 calories a day-100 less than the standard suggestion-to lose five pounds in a month (by jogging for 30 minutes twice a week). (Learn about Your Brain On: A Calorie Count.)

"The biggest flaw with the 500-calorie rule is that it assumes weight loss will continue in a linear fashion over time," Hall told Runner's World. "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."

People require a different calorie deficit to lose a single pound, depending on their current weight-which also means that if you're looking to shed a high number of pounds, the calorie deficit is going to be different for the last 10 pounds than it was for the first 10.

While the 100-calories-a-day difference may not seem like much, that's roughly one glass of wine a night. And when it's framed that way, we think you'll agree-this calculator can not only help you set more realistic weight loss goals, but also help you enjoy getting healthy a whole lot more.

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