How Many Calories Should I Eat a Day?

Your daily calorie number is not a fixed rule. It is an estimate of how much energy your body uses, built from your size, age, sex and how much you move. Once you understand where that estimate comes from, it becomes much easier to pick a sensible target and adjust it when real life does not match the math.

Where your calories go

Every day your body spends energy in three broad ways:

• Basal metabolic rate (BMR). The energy needed just to stay alive at rest: breathing, circulation, keeping warm, repairing cells. For most people this is the biggest share, often around 60 to 70 percent of the total.
• Digesting food. Breaking down and storing what you eat costs roughly 10 percent of your intake. Protein costs a little more to process than fat or carbohydrate.
• Movement. Planned exercise plus everything else: walking to the car, standing, fidgeting, housework. This is the part that varies most from person to person and from week to week.

Add them together and you get your total daily energy expenditure, or TDEE. That is your maintenance level: eat about this much and your weight should stay roughly steady.

How the estimate is built, step by step

Nobody can measure your metabolism from a form, so calculators use a prediction equation. The most widely used one today is Mifflin-St Jeor, which estimates BMR from weight in kilograms, height in centimeters and age in years, with a small adjustment for sex. The equation then gets multiplied by an activity factor, from 1.2 for a mostly seated life up to 1.9 for very hard daily training or physical work.

The process looks like this:

1. Convert your weight and height to metric if needed.
2. Calculate BMR: 10 × weight + 6.25 × height − 5 × age, then add 5 for men or subtract 161 for women.
3. Multiply BMR by the activity factor that matches a typical week.
4. Adjust the result up or down depending on your goal.

A worked example

Take a 35-year-old woman who weighs 68 kg (about 150 lb), is 165 cm (about 5 ft 5 in) tall and does light exercise two or three times a week.

1. BMR = 10 × 68 + 6.25 × 165 − 5 × 35 − 161 = 680 + 1,031.25 − 175 − 161 = 1,375 kcal.
2. Light activity uses a factor of 1.375, so maintenance = 1,375 × 1.375 ≈ 1,891 kcal per day.
3. A mild deficit of 250 kcal gives about 1,641 kcal; a 500 kcal deficit gives about 1,391 kcal.

Entering 150 lb and 5 ft 5 in instead of the metric values gives almost identical results (1,376 and 1,892), because the conversions are not perfectly round.

Now look at how much the activity choice matters for the same person:

The difference between "sedentary" and "extra active" is nearly 1,000 kcal a day. That is why choosing your activity level honestly matters more than any decimal in the formula. If she picked sedentary, her maintenance would be 1,650 kcal and a 500 kcal cut would land at 1,150 kcal, below the 1,200 kcal floor that is generally considered the minimum for women without medical supervision.

Using the Calorie Calculator

The Calorie Calculator does all of the steps above in one go. Choose US or metric units, enter your age, sex, weight and height, pick an activity level and press Calculate. You get your BMR, your maintenance calories and five daily targets: two for losing weight, one for maintaining and two for gaining. Any target that falls below the usual minimum (1,200 kcal for women, 1,500 kcal for men) is marked so you can see it at a glance.

Treat the number as a starting point, not a verdict. The real test is what your body does over the next few weeks.

How to fine-tune your target

Equations describe an average person with your measurements. You might burn 10 percent more or less than that. A simple way to find your own number:

1. Eat close to your estimated target for two to three weeks.
2. Weigh yourself a few mornings a week under the same conditions and look at the weekly average, not single days.
3. If the average is not moving the way you expected, change your intake by about 100 to 200 kcal and watch for another two weeks.

Day-to-day scale changes are mostly water, salt and food in the gut; the multi-week trend is what counts.

Why your number changes over time

Your target is not a one-time calculation. Two things shift it:

• Weight change. If the woman in the example loses 5 kg and reaches 63 kg, her maintenance drops from about 1,891 to about 1,822 kcal. The same 1,391 kcal intake that was a 500 kcal deficit is now only about 430 below maintenance, so progress slows. Recalculating every 4 to 5 kg keeps the plan realistic.
• Age. The equation subtracts 5 kcal of BMR for every year. At 55 instead of 35, with the same weight and height, her maintenance would be about 1,753 kcal.

Your activity can change too. A new job, an injury, a training plan or simply more walking in summer can move your real needs by a few hundred calories.

Common mistakes

• Choosing your best week as your activity level. Pick an ordinary week. Most people with desk jobs fit light or moderate even if they train hard sometimes.
• Counting exercise twice. The activity factor already includes your workouts. Adding fitness-tracker calories on top usually overestimates what you can eat.
• Cutting too hard. A huge deficit is difficult to sustain, makes it harder to get enough protein and nutrients, and often ends in rebound eating.
• Expecting a perfectly straight line. The old "3,500 kcal equals one pound" rule is a rough guide. Real weight loss tends to slow as you get lighter and your body adapts.

These calculations give estimates, not medical advice. If you are pregnant, breastfeeding, managing a health condition or planning a big change, talk to a doctor or registered dietitian first.

FAQ

Should I eat back the calories I burn exercising?

Usually not if you picked an activity level that includes your workouts. The activity factor already accounts for typical exercise. If you chose sedentary and then train, adding back part of the exercise calories, such as half, is a more cautious approach than adding all of them.

Why is my calorie target lower than my friend's?

Calorie needs scale with body size, muscle mass, age and activity. A taller, heavier or more active person burns more at rest and during movement. Two people eating the same amount can see very different results.

How accurate is the Mifflin-St Jeor equation?

For most healthy adults it lands within about 10 percent of measured resting energy use, which makes it one of the better options available. It can be less accurate for very muscular people, older adults and people with obesity, so real-world tracking is the best check.

Is it better to cut 250 or 500 calories a day?

A 250 kcal deficit is gentler and easier to keep up, with slower results. A 500 kcal deficit roughly doubles the pace. Many people start with the smaller cut, see how it feels and adjust from there.