Calorie deficit calculator: how long to lose weight

How many calories should I eat to lose weight?

Your TDEE minus a deficit. Each pound of weight loss is counted as about 3,500 calories (7,700 per kilogram), so:

The calculator estimates your TDEE from the Mifflin-St Jeor equation and your activity level, then subtracts the deficit for the rate you choose. US guidance from the CDC and NIH is 1 to 2 pounds a week. For lighter people that can be more than 1 percent of body weight a week, the point beyond which studies of keeping muscle during a diet suggest slowing down, and the calculator warns you when you pass it.

It also will not go below about 1,200 calories a day for women or 1,500 for men, a common clinical rule of thumb. Below those levels it is hard to get enough protein, fiber, vitamins and minerals without medical supervision, so if the rate you pick would take you lower, the calculator uses the fastest rate that does not. It is not meant for anyone under 18, who is still growing, or during pregnancy or breastfeeding.

How long will it take to lose the weight?

The calculator gives two answers, because the honest one depends on what you do next.

• Adjusting intake as you go assumes you recalculate and eat a little less as you get lighter, so the rate stays steady.
• Never adjusting intake holds your calories where they are now. As your weight falls, so does your TDEE, the deficit shrinks, and loss slows. Sometimes it levels off before the goal, and the calculator shows where.

For example, a 35-year-old woman, 5 ft 5 in, going from 200 to 180 lb at a pound a week with light activity: about 1,705 calories a day, and about 20 weeks if she adjusts as she goes, or about 23 if she keeps eating 1,705.

Both are the earliest likely dates, not promises. The 3,500-calorie rule is a fair approximation for the first couple of months, apart from the early water loss, and a poor one after that. Research by Kevin Hall, then at the NIH, showed it overstates long-term weight loss. For a detailed model, the NIH's free Body Weight Planner simulates the whole process.

Why weight loss slows down

• Water first. The first week can show a drop of 2 to 4 pounds, more on a low-carb diet, mostly water released from stored carbohydrate. After that, the pace settles.
• A smaller body burns less. Every 10 pounds lost lowers your TDEE by roughly 50 to 100 calories a day, which eats into the deficit.
• You move less without noticing. In a long deficit, everyday movement tends to drop.
• Logging drifts. Under-reporting is the best-documented reason a deficit stops working: in a 1992 study in the New England Journal of Medicine, people who said they could not lose weight under-reported their intake by about 47 percent.

When the scale has not moved for three weeks or more, recalculate at your new weight, check your logging, and then decide whether to cut a further 100 to 200 calories.

Making a deficit easier to keep

Keep protein high, about 0.7 to 1 g per pound (1.6 to 2.2 g/kg). If you have a lot to lose, base it on your goal weight rather than your current one. Protein protects muscle and is the most filling of the macronutrients; the macro calculator sets the grams for you. Low-calorie, high-volume foods help too: in Penn State studies, a low-calorie soup before a meal cut calories at that meal by about 20 percent. A bowl of vegetable soup before dinner is an easy place to start. Why the size of the deficit matters, and why it drifts, is covered in the calorie deficit article.