How many calories should I eat to lose weight? A dietitian's guide

To lose weight steadily, eat about 20% fewer calories than your body uses in a day. For many adults that's roughly 400–600 kcal a day below their maintenance calories. Don't go below 1,200 kcal a day for women or 1,500 kcal for men without medical supervision, and don't aim to lose weight at all if your BMI is under 18.5.

The rest of this guide shows how to work out your own number in two steps, with worked examples. If you'd rather skip the maths, the macro calculator does it for you from your weight, height, age, activity and goal.

Step 1: work out your maintenance calories

Your maintenance calories are what you'd eat to stay the same weight. They're made of two parts: the energy your body uses at rest, and how active you are.

The energy you use at rest is best estimated with the Mifflin-St Jeor equation (Mifflin et al., 1990). A systematic review found it predicted resting energy use within 10% of the measured figure for more people than any other common equation (Frankenfield et al., 2005).

• Women: 10 × weight in kg + 6.25 × height in cm − 5 × age − 161
• Men: 10 × weight in kg + 6.25 × height in cm − 5 × age + 5

Then multiply by your activity level:

Treat the result as a starting point, not an exact figure. Equations estimate the average person of your size, and your real needs may be around 10% higher or lower. Your weight over the next few weeks tells you which.

Step 2: take off about 20%

Multiply your maintenance calories by 0.8. That's your daily target for losing weight.

You'll often see "eat 500 calories less" instead. The problem with a fixed number is that it means very different things for different people. For a smaller woman with maintenance calories around 1,800, a 500 kcal cut is a deficit of nearly 28%. For a large man burning around 3,600, it's under 14%. A percentage scales with you.

A 20% cut sits comfortably within clinical guidance. The 2013 American guideline on managing obesity recommends a deficit of 500–750 kcal a day, or a daily intake of 1,200–1,500 kcal for women and 1,500–1,800 kcal for men, adjusted for body weight (Jensen et al., 2014).

How fast will you lose weight?

You'll often read that a 500 kcal daily deficit loses 0.5 kg a week. That rule overestimates over time. As you lose weight, your body needs less energy, so the same intake creates a smaller deficit and weight loss gradually slows (Hall, 2008; Hall et al., 2011).

Two practical points:

• The first week usually drops faster than the weeks after, mostly from water. Don't expect that pace to continue.
• Day-to-day weight jumps around with water, salt and digestion. Judge progress on the trend over two to four weeks, not on one morning.

If your weight hasn't moved after four weeks, first check how you're weighing and logging food (see below). Then lower your target a little, without going under the limits that follow.

The safety limits

Don't go below 1,200 kcal a day (women) or 1,500 kcal (men) without medical supervision. These are the bottom of the ranges in the guideline above. Below them, it gets hard to eat enough protein, vitamins and minerals. The same guideline says very-low-calorie diets, under 800 kcal a day, should only be used with medical supervision.

Don't aim to lose weight if your BMI is under 18.5. That's the World Health Organization's cut-off for underweight (WHO). If you want to change your weight or shape, speak to your GP or a dietitian first.

Speak to your GP or a dietitian before cutting calories if any of these apply:

• you're pregnant or breastfeeding;
• you have, or have had, an eating disorder;
• you're under 18;
• you take medicine for diabetes, especially insulin or tablets that can cause low blood sugar;
• you have a long-term health condition that affects what you eat.

Worked examples

These figures were worked out with BiteSync's own code, using the method above.

Two of these show the safety limits at work:

• The 60-year-old woman: 20% off her maintenance would be 1,101 kcal, which is under the 1,200 kcal floor, so her target is raised to 1,200.
• The 25-year-old woman: her BMI is 18.0, under 18.5, so there's no weight-loss target. Her figure is for maintaining her weight.

Calories aren't the whole story

Protein matters while you're losing weight. When you eat less, some of the weight lost comes from muscle, and eating more protein helps you keep it. Around 1.2–1.6 g per kg of body weight a day is a sensible range while dieting. How much protein you need.

How you weigh food changes the numbers. Rice triples in weight when cooked and chicken loses about 30%, so mixing raw and cooked figures can put a day's calories out by hundreds. Raw or cooked weight: how to weigh food.

Protein also helps a little with fullness (Leidy et al., 2015), which can make a lower calorie target easier to live with. Why protein keeps you fuller.

How BiteSync sets your calories

BiteSync is our app.

BiteSync uses the same method as this guide:

• Maintenance: the Mifflin-St Jeor equation × your activity level.
• Your goal: 20% below maintenance to lose weight, maintenance to stay the same, or 10% above to build muscle.
• Safety limits built in: never below 1,200 kcal (women) or 1,500 kcal (men), and no weight-loss target if your BMI is under 18.5.

It then plans your meals to hit the number. As you place meals on the week, each day's calories and protein update, so you can see a day is off before you shop. On Pro, the AI fills the whole week from your recipes, aiming for every day to land within 150 kcal or 7% of your target (whichever is tighter) and at least 95% of your protein.

Work out your calories and protein. It's free, needs no sign-up, and nothing you enter is sent anywhere.

Quick answers

How many calories should a woman eat to lose weight? It depends on her size, age and activity. In the examples above, two women's weight-loss targets were 1,200 and 1,590 kcal a day. Don't go below 1,200 kcal without medical supervision.

How many calories should a man eat to lose weight? Again, it depends. In the examples above, the targets were 1,679 kcal for a 30-year-old with a desk job and 2,331 kcal for an active 40-year-old. Don't go below 1,500 kcal without medical supervision.

Is 1,200 calories a day enough? It's the lowest intake most guidance suggests for women without supervision, not a target. Most women need more, and eating too little makes it harder to get enough protein, vitamins and minerals, and harder to stick with.

Should I eat back the calories I burn exercising? Not if you already chose an activity level that includes your exercise. That counts it once. Calorie figures from fitness trackers are estimates, so adding them on top can cancel out your deficit.

Why am I not losing weight on a calorie deficit? The most common reasons are water changes hiding fat loss over a week or two, and food being weighed or logged inconsistently. Look at the trend over four weeks, check how you weigh food, and recalculate your target as your weight changes.