What Are Calories? How Many You Burn and Need Each Day

A calorie is a unit of energy. Food gives your body calories, and your body uses them to keep you alive and to move. How many you need each day depends on your size, age, sex and how active you are - the NHS puts the average at about 2,000 calories a day for women and 2,500 for men, but your own number can be quite different.

What is a calorie?

Strictly, the "calories" on food labels are kilocalories (kcal) - the energy needed to raise the temperature of one kilogram of water by one degree Celsius. In everyday use, "calorie" and "kcal" mean the same thing. Some countries also list energy in kilojoules (kJ); one kcal is about 4.2 kJ.

According to the NIDDK, the nutrients in food provide different amounts of energy:

This is why foods high in fat are usually high in calories for their size, and why drinks with alcohol can add up quickly.

Where do the calories you burn go?

Your total daily energy expenditure (often shortened to TDEE) is made up of three parts:

• Basal metabolic rate (BMR): the energy your body uses at complete rest to breathe, pump blood, keep warm and repair cells. For most people this is the largest part of the day's total, even for people who exercise.
• Digesting food: breaking down and storing what you eat uses a small amount of energy, known as the thermic effect of food.
• Movement: everything from workouts to walking, climbing stairs, standing and fidgeting. This is the part that changes most from person to person and day to day.

BMR depends mostly on your body size and composition - larger bodies and more muscle use more energy - and it tends to fall gradually with age. Movement is the part you have the most control over.

How to estimate your BMR

No formula is exact for an individual, but the Mifflin-St Jeor equation is one of the most widely used. It was published in 1990 from measurements of healthy adults:

• Men: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) + 5
• Women: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) - 161

Example: a 30-year-old woman who is 165 cm tall and weighs 65 kg has an estimated BMR of (10 x 65) + (6.25 x 165) - (5 x 30) - 161 = 650 + 1,031 - 150 - 161 = about 1,370 calories a day.

To estimate your total daily needs, add the energy you use moving around. Free planners from health bodies, such as the NIDDK Body Weight Planner, do this for you from your age, sex, height, weight and activity level. Treat any result as a starting estimate - the NIDDK itself notes that its values will be too high for people who are very sedentary or have an unusually low metabolism.

How do wearables estimate calories burned?

No watch can measure calories directly. Instead, wearables estimate them. In general terms:

• Resting energy is estimated from personal details such as age, sex, height and weight - much like the BMR formula above.
• Active energy is estimated from how you move (motion sensors, and GPS for outdoor workouts) and from your heart rate, combined with your personal details.

On iPhone and Apple Watch these appear in Apple Health as Resting Energy and Active Energy. Keeping your height, weight and age up to date in the Health app helps the estimates.

These numbers are useful for comparing one day with another, but they are estimates. A 2017 study of several wrist-worn devices found that heart rate readings were generally reasonable, while energy expenditure estimates were much less accurate. That is worth remembering if you plan to "eat back" the calories a device says you burned.

How do calories relate to weight change?

Over time, eating more energy than you use leads to weight gain, and eating less leads to weight loss. In practice the body is not a simple calculator: water shifts, changes in muscle, and the way your body adjusts to eating less all mean weight rarely changes in a straight line.

If you want to lose weight, the CDC notes that people who lose it at a gradual, steady pace - about 1 to 2 pounds (roughly 0.5 to 1 kg) a week - are more likely to keep it off. A few habits help most people:

• Make small, lasting changes to what you eat rather than cutting out whole food groups.
• Fill up on vegetables, fruit, whole grains and protein, which tend to be more filling for their calories.
• Cut back on sugary drinks and alcohol, which add calories without making you feel full.
• Move more through the day, not just in workouts. See our guide to how many steps a day for an easy target.
• Protect your sleep - short sleep tends to make hunger and food choices harder to manage.

Avoid crash diets and very low calorie plans unless a doctor is supervising them. Eating far too little can leave you tired, short of nutrients and losing muscle as well as fat, and the weight often comes back.

When to talk to a professional

• You have a health condition such as diabetes or heart disease, take regular medicine, or are pregnant or breastfeeding and want to change how you eat.
• You are losing weight without trying.
• You feel dizzy, faint or unusually tired after cutting back on food.
• Thoughts about food, eating or your weight are causing you distress. A doctor can help, and you do not need to be at a particular weight to ask.

A registered dietitian can help you work out an eating plan that fits your health and goals.

Seeing your energy in FitRest

FitRest does not log food. It reads the energy your Apple Watch or other devices save to Apple Health and shows it as your Energy score on the Dashboard: the calories you have burned so far today compared with a daily target. You can see how workouts add to the day on the workouts page, and Personal Metrics shows your BMR alongside your BMI and weight trend.

Sources

• NHS - Understanding calories
• NIDDK - Calorie (definition)
• NIDDK - Body Weight Planner
• CDC - Steps for Losing Weight
• Mifflin MD et al. - A new predictive equation for resting energy expenditure in healthy individuals (American Journal of Clinical Nutrition, 1990)
• Shcherbina A et al. - Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort (Journal of Personalized Medicine, 2017)