Calorie Calculator

How your daily calorie needs are estimated

The energy you use in a day has three parts:

• Basal metabolism — keeping your body running at rest. For most people this is the largest part.
• Digesting food — breaking down and storing what you eat.
• Movement — everything from walking to the kitchen to a workout. This part varies the most between people and from day to day.

The calculator estimates your basal metabolic rate with the Mifflin-St Jeor equation, then multiplies it by an activity factor that stands in for the other two. The result is your maintenance intake: the calories that would, on average, keep your weight where it is. The BMR calculator compares Mifflin-St Jeor with two other equations if you want to see the range.

Choosing an activity level. Think about a typical week, not your best one. Your job counts as well as exercise: someone on their feet all day at work may move more than an office worker who runs twice a week. If you are between two levels, start with the lower one and adjust after a few weeks of seeing what your weight actually does.

The calculator asks for sex because the equation was fitted separately to data from men and from women. It was not developed for people whose body composition differs from both groups, such as during hormone therapy.

Formulas used

W

weight in kilograms

H

height in centimeters

A

age in years

The activity factors are 1.2 (sedentary), 1.375 (lightly active), 1.55 (moderately active), 1.725 (active) and 1.9 (very active). For a goal, the calculator adds or subtracts a fixed daily amount:

The energy per unit uses the long-standing rules of thumb of 3,500 kcal per pound and 7,700 kcal per kilogram. (They are rounded separately, so they don’t convert exactly: 3,500 ÷ 0.45359237 ≈ 7,716.)

Why the weekly rate is an overestimate. The 3,500-calorie rule treats every pound lost as if nothing else changes. In reality a smaller body burns less, so the same intake produces a shrinking deficit and weight loss slows. Modelling by Kevin Hall and colleagues (The Lancet, 2011) found that body weight responds to a lasting change in intake slowly, with a half-time of about a year. The NIH Body Weight Planner, built on that research, projects a curve rather than a straight line.

Worked example

A 35-year-old man, 5 ft 10 in and 180 lb, who exercises lightly a couple of times a week:

1. Height 70 in × 2.54 = 177.8 cm; weight 180 lb × 0.45359237 = 81.65 kg.
2. BMR = 10 × 81.65 + 6.25 × 177.8 − 5 × 35 + 5 = 1,757.7, rounded to 1,758 kcal.
3. Maintenance = 1,758 × 1.375 = 2,417 kcal/day (lightly active).
4. To lose 1 lb a week: 1 × 3,500 ÷ 7 = 500 kcal/day less, so 1,917 kcal/day. The mild pace (0.5 lb) gives 2,167 kcal/day.
5. The faster pace (2 lb a week) would need 2,417 − 1,000 = 1,417 kcal/day. That is under 1,500, the low end of the guideline range for men, so the calculator flags it.
6. To gain 0.5 lb a week: 2,417 + 250 = 2,667 kcal/day.

Setting a realistic target

• Slow and steady lasts. The CDC notes that people who lose about 1 to 2 pounds a week are more likely to keep the weight off than those who lose it faster.
• Mind the floor. The 2013 AHA/ACC/TOS guideline for adults with overweight or obesity lists usual weight-loss intakes of 1,200–1,500 kcal/day for women and 1,500–1,800 kcal/day for men. Targets under the lower end are flagged; going below them is normally done with medical supervision.
• Treat the number as a starting point. Weigh yourself under similar conditions for two to four weeks. If the trend doesn’t match your goal, move your intake by 100–200 kcal and check again. The first week often shows extra loss from water.
• Don’t count exercise twice. The activity factor already includes your usual exercise. Adding a fitness tracker’s “calories burned” on top counts it again.
• Gaining weight. Muscle is built slowly, so a large surplus tends to add more fat than muscle. Pair a mild surplus with strength training if muscle is the goal.

Who this calculator is not for

The estimates are for healthy adults. They don’t apply to children and teenagers, who need energy to grow; to pregnancy or breastfeeding, which raise needs; or to people with medical conditions or medications that change metabolism or appetite. Very muscular people and endurance athletes usually need more than the equation predicts. If you have a history of disordered eating, calorie targets can do more harm than good — a doctor or registered dietitian can help set goals that suit you.