You've often heard the ubiquitous diet mantra "calories in, calories out." It's pretty much true; human beings are, after all, subject to the laws of thermodynamics, just like everything else in the universe is. But, like so many others, you're still wondering - "how many calories do I need to lose weight?" If you don't burn the energy you ingest as food, it stays on you in the form of fat - where else would it go? What's less commonly understood is just how many - or, rather, how few - calories you actually need on a day-to-day basis. Unfortunately, it's a bit tricky to find out. I'll explain the basics; after that, you should be well on your way to succeeding with a low-calorie diet!

Basal Metabolic Rate

The first thing you need to understand is your basal metabolic rate - this is the amount of calories you burn every hour, day in and day out, to sustain yourself at your current weight.

Men naturally have a somewhat higher BMR than women do; this is one reason why the FDA suggests 2500 calories a day for the average man, but only 2000 for the average woman. However, that's not the entire story either; BMR goes down with age, as almost everyone past the age of 30 knows. However, past the age of 20, it's only supposed to drop 2% every decade - so, let's be honest, your changing metabolism is not an excuse for gaining 20 pounds in one year!

You can try calculating BMR for yourself, but you need to know your lean body mass. Multiplying your lean body mass by 22 and then adding 500 will give you an estimate of your BMR. Take your BMR and multiply it by between 1.2 and 1.9 - depending on how active you are - and you have a good idea of how many calories you need in a day to maintain your current weight. You can also find BMR calculators online.

How many calories do I need to lose weight?

So now that you know your base metabolic rate, you're probably wondering "how many calories do I need to lose weight?" Under the calories in, calories out model, you literally only need to consume less than you need. Your BMR is the amount of calories you need every day to sustain you at your current weight, so anything less than that will result in weight loss. Easy, right?

So I can just cut that in half, right?

Cutting your calories drastically makes losing weight harder and slower, not faster. Your body is programmed to hold on to extra stored energy if you're in starvation mode, making fat loss more difficult. Men should not eat fewer than 1500 calories per day and women should eat no less than 1200.

Hopefully, you now know the answer to "how many calories do I need to lose weight?" Now that you know how much to eat, try to make those calories count!