How many calories should I eat to lose weight? A simple way to find your number

Every weight-loss plan starts with the same question: how much should I actually eat? Online calculators give you a number in seconds, but they rarely tell you where it comes from, or what to do when it stops working. Here's the simple maths, a worked example and the honest limits.

The quick answer

• Step 1: work out roughly how many calories your body burns in a day. This is your maintenance number.
• Step 2: eat about 300 to 500 kcal a day less than that. For most adults this means slow, steady loss you can live with.
• Step 3: weigh yourself regularly for 2 to 3 weeks, look at the trend, and adjust by 100 to 200 kcal if nothing is moving.

The number from any formula is a starting guess, not a rule. Your scale tells you the real answer.

Step 1: find your maintenance calories

Your body burns energy just to keep you alive (breathing, heartbeat, staying warm). This is your resting metabolic rate, or RMR. The most widely used way to estimate it is the Mifflin-St Jeor equation, published in 1990. A systematic review of the common formulas found it was the most reliable of them, landing within 10% of the measured value for more people than the others.

Here it is, with weight in kg, height in cm and age in years:

• Men: (10 × weight) + (6.25 × height) − (5 × age) + 5
• Women: (10 × weight) + (6.25 × height) − (5 × age) − 161

Then multiply your RMR by an activity factor to get your maintenance calories. These multipliers are the ones most calculators use:

Be honest here. Most people pick a higher activity level than they really have, and that alone can erase a whole deficit. If you're not sure, choose the lower one.

A worked example

Let's take two people and run the numbers.

Notice how different the two targets are. That's why a fixed "1,200 calories for everyone" plan doesn't make sense: it's a big cut for some people and far too little for others.

Step 2: choose your deficit

A deficit of about 500 kcal a day is the classic starting point. Over a week that's 3,500 kcal, which is roughly 0.45 kg (1 lb) of body fat, so early weight loss of around 0.5 kg a week is a fair expectation.

Smaller deficits of 300 kcal work too. They're slower, but easier to stick with, and you keep more energy for training. Bigger deficits feel fast at first, but hunger, low energy and lost muscle usually catch up with you. Very low-calorie diets are normally done only with medical supervision, so don't go there on your own.

Whatever deficit you pick, keep your protein high. It protects your muscle while you eat less and keeps you full. Our guide on how much protein you need a day gives you the number.

Why the weight loss slows down

The "3,500 kcal = 1 lb" rule is fine for the first few weeks, but it falls apart over months. As you get lighter, your body burns less energy, so the same calories make a smaller and smaller deficit.

Researchers at the US National Institutes of Health built a model of this. Their rule of thumb: for an average overweight adult, every 100 kJ (about 24 kcal) a day you cut leads to an eventual loss of about 1 kg, with half of it coming off in about a year and nearly all of it within about three years. So a slowdown after a few months is normal, not a sign that you're doing something wrong.

The practical fix is simple: every time you lose about 5 kg, run the numbers again with your new weight. The NIH also has a free online Body Weight Planner if you want to see the curve for yourself.

Step 3: let your results correct the number

Formulas are averages. Your real maintenance can easily be 10% higher or lower than the estimate. So treat your target as an experiment:

• Weigh yourself in the morning, a few times a week, and look at the weekly average, not single days.
• If the average hasn't moved in 2 to 3 weeks and you've tracked carefully, drop 100 to 200 kcal a day.
• If you're losing more than about 1% of your body weight a week and feel drained, add 100 to 200 kcal back.

Most of the time, a stalled scale is a tracking problem, not a metabolism problem: oil, drinks, snacks and weekend meals out add up fast. Our posts on why you're not losing weight and counting calories when you eat out cover the usual suspects.

The short version

• Estimate your maintenance with Mifflin-St Jeor × your honest activity level.
• Eat 300 to 500 kcal a day below it, and keep protein high.
• Expect about 0.5 kg a week at first, then a slower pace. That's normal.
• Use your 2 to 3 week weight trend to adjust, and redo the maths every 5 kg lost.

Knowing your number is the easy part; hitting it every day is the hard part. AIMS sets your calorie target for you and adjusts it as your weight changes, so all you do is text, snap or say your meal.