The number of calories you need to eat depends on your body size, age, sex, and how active you are. Most adults need somewhere between 1,600 and 3,000 calories per day, but that range is wide enough to be almost useless. To find your actual number, you need to estimate how much energy your body burns at rest and then adjust for your daily movement.
What Your Body Burns Without Moving
Your body uses a surprising amount of energy just keeping you alive. Breathing, circulating blood, regulating temperature, and maintaining cells account for 60% to 70% of the total calories you burn each day. This baseline is called your basal metabolic rate, or BMR. On top of that, your body spends roughly 10% of its energy digesting and processing food. The remaining 20% to 30% comes from all physical activity, whether that’s formal exercise or just walking to the kitchen.
This is why two people with the same exercise routine can need very different calorie intakes. The biggest piece of the puzzle isn’t how hard you work out. It’s how much energy your body demands at rest, which is largely determined by how much you weigh, how tall you are, and how much of your weight is muscle versus fat. Muscle tissue burns roughly 5 to 7 calories per pound per day at rest, while fat tissue burns far less. So a 180-pound person with more muscle mass will have a higher baseline than a 180-pound person with less.
The Best Formula to Estimate Your Needs
The Academy of Nutrition and Dietetics recommends the Mifflin-St. Jeor equation as the most accurate formula for estimating resting metabolic rate, including for people who are overweight or obese. It predicted resting metabolism within 10% of the actual measured value in 70% of people tested. Here’s how it works:
- Men: (9.99 × weight in kg) + (6.25 × height in cm) – (4.92 × age in years) + 5
- Women: (9.99 × weight in kg) + (6.25 × height in cm) – (4.92 × age in years) – 161
To convert your weight, divide pounds by 2.2. To convert your height, multiply inches by 2.54. A 35-year-old woman who weighs 155 pounds (70.5 kg) and stands 5’6″ (167.6 cm) would calculate: (9.99 × 70.5) + (6.25 × 167.6) – (4.92 × 35) – 161, which comes out to roughly 1,400 calories at rest.
That number only covers what your body burns lying in bed all day. To get your total daily calorie needs, you multiply it by an activity factor.
Adjusting for Your Activity Level
Researchers classify daily activity into three broad tiers, each with a multiplier you apply to your resting metabolic rate:
- Sedentary or lightly active (desk job, little intentional exercise): multiply by 1.4 to 1.69
- Moderately active (regular exercise or a job that keeps you on your feet): multiply by 1.7 to 1.99
- Vigorously active (intense daily training or heavy physical labor): multiply by 2.0 to 2.4
Using the example above, our 35-year-old woman with a resting rate of 1,400 calories who exercises a few times a week and walks regularly might use a multiplier around 1.7, putting her total daily needs near 2,380 calories. If she has a desk job and rarely moves outside of it, a multiplier closer to 1.5 gives her roughly 2,100. These are estimates, not exact figures. But they give you a realistic starting point.
Why Age Matters Less Than You Think
You’ve probably heard that metabolism slows dramatically as you get older. Recent research from Harvard Health tells a more nuanced story. Total energy expenditure and basal metabolic rate stay remarkably stable between ages 20 and 60, regardless of sex. The decline only begins around age 60, at a rate of about 0.7% per year. That means a 70-year-old’s metabolism is roughly 7% lower than it was at 60, which matters but isn’t the dramatic age-related crash many people assume is happening in their 30s and 40s.
What often changes in middle age isn’t metabolism itself but muscle mass and activity levels. Less movement and less muscle mean fewer calories burned, which gets misread as a “slower metabolism.” Staying active and maintaining muscle largely offsets this effect.
Using Your Number for Weight Loss or Gain
Once you have your estimated daily calorie needs, the math for changing your weight is straightforward in principle. To lose weight, eat fewer calories than you burn. To gain weight, eat more. The old rule of thumb said that cutting 500 calories per day would produce one pound of weight loss per week, based on the idea that a pound of fat equals 3,500 calories. The Mayo Clinic notes this rule doesn’t hold for everyone because your body adapts as you lose weight, gradually burning fewer calories and slowing your progress. You may need to readjust your intake over time.
A reasonable starting deficit for most people is 300 to 500 calories below maintenance. Larger deficits produce faster initial results but are harder to sustain and more likely to cost you muscle along with fat.
Calorie Floors You Shouldn’t Drop Below
Cutting calories too aggressively creates real risks. Harvard Health recommends that women eat no fewer than 1,200 calories per day and men no fewer than 1,500 per day unless supervised by a healthcare provider. Below those thresholds, it becomes very difficult to get adequate vitamins, minerals, protein, and fiber from food alone. Chronic undereating can also trigger your body to slow its metabolic rate further, making weight loss harder in the long run, not easier.
Why Trackers and Apps Can Mislead You
Fitness wearables are useful for tracking trends in your movement, but the calorie counts on your smartwatch are often far from accurate. Research from Harvard’s School of Engineering found that wrist-worn trackers have estimated error rates of 30% to 80% for calories burned. That means if your watch says you burned 400 calories during a workout, the real number could be anywhere from 80 to 560. Eating back all the calories your watch says you burned is one of the most common reasons people stall on a weight loss plan.
Online calorie calculators that use the Mifflin-St. Jeor formula are a better starting point than wearable data, but they’re still estimates. The most reliable approach is to calculate your number, eat at that level for two to three weeks, and then check what actually happened. If your weight stayed stable, you found your maintenance level. If it went up or down, adjust by 200 to 300 calories and reassess. Your real-world results will always be more accurate than any formula.
Putting It All Together
Start by running the Mifflin-St. Jeor equation with your current weight, height, and age. Multiply the result by the activity factor that honestly reflects your daily life, not your best week. That gives you an estimated maintenance number. From there, subtract 300 to 500 calories if you want to lose weight, or add 250 to 500 if you want to gain. Stay above 1,200 (women) or 1,500 (men) calories per day. Track your weight over two to three weeks, and let the scale tell you whether your estimate was close. Adjust in small increments rather than dramatic swings, and recalculate every 10 to 15 pounds of change, since your body’s needs shift as your size does.