How many calories to lose weight

The 500-calorie rule

A deficit of 500 calories a day works out to about 1 pound a week, because a pound of fat stores roughly 3,500 calories. Seven days at 500 is 3,500. It's a good starting estimate.

It doesn't hold forever. As you get lighter, you burn fewer calories, so the same intake produces a smaller deficit. A mathematical model from NIH researchers published in The Lancet estimated that the body's response to a change in intake has a half-time of about 1 year. That's why weight loss slows on a plan that hasn't changed, and it's not a sign you've done anything wrong.

Why crash diets backfire

Eating far too little can leave you tired and hungry and can lead to muscle loss, and many people swing back into rebound eating, guilt and more restriction. It's a hamster wheel. Your metabolism also adapts. A follow-up of 14 Biggest Loser contestants found that 6 years later, their resting metabolism was still about 500 calories a day lower than expected for their body size. That's an extreme case, a very small group and a very large weight loss, so don't read it as your future. Still, it's a good reason not to treat harder as better.

Why keeping it off is harder

A calorie target gets the weight off, and keeping it off is its own job. A review in Perspectives on Psychological Science argues that simply trying to eat less, without counting calories, has limited influence, because your body responds by lowering metabolic rate and raising appetite. That's a review making an argument, not a trial, and the guideline trials above show calorie targets do work when people follow them. But the warning is fair.

Another review, in Medical Clinics of North America, describes the usual pattern as early weight loss, then a plateau, then gradual regain, and says long-term management takes ongoing attention to behavior and environment. In plain terms, you need a setup that keeps working once your motivation fades. So when you ask what you can eat, the useful answer isn't only a number. It's a number you can follow with the meals, habits and schedule you actually have.

If you know your body fat

The Katch-McArdle formula uses lean body mass (LBM), your weight without the fat:

• LBM = (1 minus body fat % as a decimal) x your weight in kg
• BMR = 370 + (21.6 x LBM)

Skinfold calipers are the cheapest way to estimate body fat, though the result depends a lot on who's using them. If your number is a guess, your BMR is a guess too.

If you don't

Most people don't know their body fat, and that's fine. The Mifflin-St Jeor equation only needs your weight, height and age. A systematic review found it more likely than other common equations to land within 10% of a measured resting rate, though it can still be off for individuals.

• Women: (10 x weight in kg) + (6.25 x height in cm), minus (5 x age), minus 161
• Men: (10 x weight in kg) + (6.25 x height in cm), minus (5 x age), plus 5

Multiply by your activity

Standard activity multipliers

A worked example

Take a 200-pound man (about 90 kg) with 20% body fat who trains 4 days a week.

1. LBM = (1 minus 0.20) x 90 = 72 kg
2. BMR = 370 + (21.6 x 72) = about 1,925 calories
3. Moderate activity: 1,925 x 1.55 = about 2,984 calories
4. Light activity, if the rest of his day is at a desk: 1,925 x 1.375 = about 2,647 calories

That's a 337-calorie gap depending on one judgment call, which tells you how rough this is. Start from the lower number, because you can always add food. Then let the scale correct you over the next few weeks.

If you'd rather not do the math, the Body Weight Planner from the National Institutes of Health does it for you, and it accounts for your burn falling as you lose.

Start small, if you like

A cut of 100 to 200 calories is a fine way to start if a bigger one feels like too much, as long as you know what you're getting. It's only 0.2 to 0.4 pounds a week, smaller than normal day-to-day swings on the scale, so it can take a month to see anything. Hold it for a couple of weeks, and if you're not losing at the rate you want, cut another 100.

Calorie floors

Don't go under the guideline ranges above without a clinician involved. Eating under about 800 calories a day counts as a very low-calorie diet, which NIH describes as a diet supervised by a health care professional. Your BMR is also a useful check. It isn't a medical cutoff, but if your target is below it, your deficit is probably bigger than you need.

Going too low can bring extreme hunger, poor nutrition, muscle loss, mood changes and missed periods. Muscle matters most in the long run, because low muscle mass is tied to worse metabolic health as we age. If you're pregnant or breastfeeding, have diabetes, take medication for your weight or blood sugar, or have a history of disordered eating, set your target with your doctor or a registered dietitian.

Filling in carbs & fat

Set protein first, then split what's left. Keep fat at 20% or more of calories, which is the floor of the National Academies' range, and give the rest to carbohydrate. Here's how that looks for our 200-pound man at 2,150 calories, which is the lower maintenance number minus about 500.

A sample split at 2,150 calories

Our guide to macronutrient ratios for weight loss has more on where those numbers come from.