Most adults need somewhere between 1,600 and 3,000 calories per day to maintain their weight, but the exact number depends on your age, sex, body size, and how much you move. The most reliable way to estimate yours is to calculate your resting metabolic rate and then multiply it by an activity factor that reflects your daily life. Here’s how that works in practice and what can shift the number over time.
The Three Places Your Calories Go
Your body burns calories in three distinct ways, and understanding the split helps explain why two people of the same size can have very different maintenance numbers.
Your resting metabolic rate accounts for roughly 60 to 70 percent of everything you burn in a day. This is the energy your body uses just to keep you alive: pumping blood, breathing, maintaining body temperature, running your brain. It’s the single biggest piece of the equation, which is why it’s the starting point for any calorie estimate.
Digesting food takes about 10 percent of your daily energy. This cost varies by what you eat. Protein is the most expensive macronutrient to process, raising your metabolic rate by 15 to 30 percent of the calories in that food. Carbohydrates cost 5 to 10 percent, and fats cost almost nothing, just 0 to 3 percent. This doesn’t mean you should eat only protein, but it does explain why higher-protein diets slightly increase the calories you burn each day.
Physical activity makes up the remaining 20 to 30 percent. This includes deliberate exercise like running or lifting weights, but also every other movement you make: walking to the kitchen, fidgeting, standing at your desk, carrying groceries. For most people, these small everyday movements actually burn more calories than formal exercise does.
How to Estimate Your Number
The most accurate equation available without lab equipment is the Mifflin-St Jeor formula. The Academy of Nutrition and Dietetics considers it the best option for estimating resting metabolic rate, predicting within 10 percent of the true measured value in about 70 percent of people. The older Harris-Benedict equation, which you’ll still find on many websites, only hits that same accuracy window in 39 to 64 percent of people and tends to overestimate.
The Mifflin-St Jeor equations work like this:
- Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
The result is your resting metabolic rate in calories per day. To convert your weight from pounds to kilograms, divide by 2.2. For inches to centimeters, multiply by 2.54.
As a quick example: a 35-year-old woman who weighs 150 pounds (68 kg) and stands 5’6″ (168 cm) would get a resting metabolic rate of about 1,387 calories per day. That’s the energy her body needs before any movement at all.
Adjusting for How Active You Are
Once you have your resting metabolic rate, multiply it by an activity factor to get your total daily calorie needs. The standard multipliers used in clinical nutrition are:
- Sedentary (desk job, little exercise): 1.0 to 1.4
- Lightly active (some walking, light exercise 1 to 3 days per week): 1.4 to 1.6
- Moderately active (exercise 3 to 5 days per week, active job): 1.6 to 1.9
- Very active (hard daily exercise, physical labor): 1.9 to 2.5
Using the example above, that 35-year-old woman with a resting metabolic rate of 1,387 would need about 1,940 calories per day if she’s lightly active (1,387 × 1.4), or roughly 2,220 if she’s moderately active (1,387 × 1.6). Most people overestimate their activity level, so if you’re unsure, start with the lower end of the range that seems right.
Why Your Number Isn’t Fixed
Your maintenance calories shift over time, sometimes in ways you wouldn’t expect. Several factors push the number up or down.
Age matters less than people think, at least until your 60s. A large study published in Science found that metabolic rate stays remarkably stable between the ages of 20 and 60 when you account for changes in body size. After 60, metabolism declines by about 0.7 percent per year. By age 90, total energy expenditure drops roughly 26 percent below middle-aged levels, even after accounting for smaller body size. The common belief that your metabolism tanks in your 30s or 40s isn’t supported by the data. What usually changes is activity level and muscle mass.
Body composition plays a role too, though the effect of muscle is often exaggerated. A pound of muscle burns roughly 5 to 7 calories per day at rest. That’s meaningfully more than fat tissue, but adding 10 pounds of muscle (a significant amount that takes months or years of training) would only raise your resting burn by 50 to 70 calories. The real metabolic benefit of strength training comes from the exercise itself and the recovery afterward, not just the resting rate of the tissue.
Individual variation is surprisingly wide. Resting metabolism can vary up to threefold among people of the same body mass, age, sex, and activity level. Genetics, hormonal differences, and organ size all contribute. This is why two coworkers with similar builds can eat very different amounts and stay the same weight.
What Happens After Weight Loss
If you’ve recently lost weight, your maintenance calories will be lower than someone who has always been at your current size. Part of this is straightforward: a smaller body needs less energy. But there’s also a phenomenon called metabolic adaptation, where your body temporarily burns fewer calories than expected for your new size. This “energy conservation mode” has been one of the most debated topics in obesity research.
The good news: recent evidence suggests this effect is smaller and shorter-lived than previously feared. A study in the American Journal of Clinical Nutrition found that in overweight women, metabolic adaptation was detectable right after weight loss but had essentially disappeared by one and two years of follow-up, as long as weight was stable. The body does adjust, but it recalibrates over time rather than permanently fighting your new weight.
Why Calculators Are a Starting Point, Not an Answer
Every online calculator, including the equation above, gives you an estimate. Even the best formula is only accurate within 10 percent for most people, and some individuals fall outside that range. When you add the activity multiplier, which requires you to honestly categorize your lifestyle, the margin for error grows.
Tracking what you eat introduces its own inaccuracy. People consistently underreport their calorie intake, and fitness trackers are even worse on the output side, with estimated error rates of 30 to 80 percent for calories burned during exercise. Relying on a watch to tell you exactly how many calories you earned back from a run is not a reliable strategy.
The most practical approach is to use a calculator as your starting point, eat that amount consistently for two to three weeks, and watch what happens on the scale. If your weight is stable, you’ve found your maintenance level. If it’s creeping up or down, adjust by 100 to 200 calories and repeat. Your body gives you better data than any equation can.
A Faster Shorthand Method
If you want a rough estimate without running the full equation, multiply your current body weight in pounds by 14 to 16. The lower end works for people who are mostly sedentary, and the higher end for those who are regularly active. A 180-pound moderately active person would land around 2,520 to 2,880 calories per day. This method is less precise than Mifflin-St Jeor, but it gets you in the right neighborhood quickly and gives you a starting point to fine-tune from there.