How Many Calories Should You Eat?

There is no single number of calories everyone should eat. A reasonable estimate for an adult comes in three steps: estimate resting energy use (BMR) from sex, age, height and weight, multiply by an activity factor to get total daily energy expenditure (TDEE), then set a target above, at or below TDEE depending on whether you want to gain, hold or lose weight. Eating at TDEE is your estimated maintenance level. The result is an estimate for a healthy adult, so expect to adjust it using your own weight trend.

Step by step: a 45-year-old woman

Assumptions: 5 ft 7 in, 160 lb, entered in imperial units. The calculator converts to 170.18 cm and 72.57 kg. The Mifflin-St Jeor equation for women is 10 x 72.57 + 6.25 x 170.18 - 5 x 45 - 161, which gives a BMR of about 1,403 kcal. Multiplying by each activity factor gives:

The second column of numbers is a different person for comparison: a 25-year-old man, 180 cm, 75 kg, with a BMR of 1,755 kcal. Rounded to the nearest kcal, the two differ by 420 to 670 kcal a day at the same activity level. The BMR gap of 352 kcal splits into the sex constant in the equation (+5 for men, -161 for women, worth 166 kcal), the 20-year age difference (100 kcal) and body size (about 86 kcal). This is why advice like 2,000 calories a day for everyone means little for any one person.

Turning TDEE into a goal

Take the woman's light-exercise TDEE of 1,930 kcal. Common planning adjustments are a deficit of 300 to 500 kcal for weight loss and a surplus of 250 to 500 kcal for weight gain. The weekly change uses the approximation of 7,700 kcal per kg (3,500 per lb), which is a rule of thumb and not an exact law.

Notice that for a smaller person a 500 kcal deficit is a bigger slice of intake: 26% of this woman's TDEE, against 19% for a 95 kg man in our calorie deficit guide. A fixed number of calories is not equally gentle for everyone. If a plan leaves you with a very low target, that is the moment to get individual advice from a doctor or registered dietitian rather than pushing on.

Picking the activity level honestly

The activity factor is the largest source of avoidable error. The calculator's own example shows how big the range is: with a BMR of 1,600 kcal, sedentary (1.2) gives 1,920 kcal and very active (1.9) gives 3,040 kcal, a difference of 1,120 kcal. For the woman above, choosing moderate (1.55) when light (1.375) fits better would overstate her maintenance level by 245 kcal a day. Choose the level that matches your typical week, including your job and daily movement, not the week you hope to have.

Calibrate with your own data

1. Eat roughly your estimated maintenance for two weeks while weighing yourself at a consistent time.
2. Compare the average weight of week one with week two. If it is stable, your estimate is close.
3. If it moves, convert it back: a drift of 0.25 kg a week is roughly 275 kcal a day by the 7,700 kcal per kg rule (0.25 x 7,700 / 7). Adjust your target by about that amount and repeat.
4. Recalculate after any large change in weight or activity.

Common mistakes

• Copying a calorie target from someone else with a different size, age or activity pattern.
• Counting exercise twice by choosing a high activity factor and then also eating back workout calories.
• Using a number to the exact calorie. The equation is an estimate that can be around 10 percent off for an individual.
• Ignoring who the equation does not cover: children, pregnant or breastfeeding people, and people with conditions such as diabetes or thyroid disease need individual guidance.