Most adults need somewhere between 1,600 and 3,000 calories per day to maintain their weight, with the exact number depending on your size, age, sex, and how much you move. That’s a wide range, which is why a personalized estimate matters more than any single rule of thumb. The good news: you can get a solid estimate with a simple two-step calculation using your basic stats.
The Formula That Gets You Closest
The Mifflin-St Jeor equation is the formula most dietitians recommend for estimating your resting metabolic rate, meaning the calories your body burns just to keep you alive (breathing, circulating blood, maintaining body temperature). It uses your weight, height, age, and sex:
- Males: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5
- Females: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161
To convert pounds to kilograms, divide by 2.2. To convert inches to centimeters, multiply by 2.54. So a 35-year-old woman who weighs 150 pounds (68 kg) and stands 5’6″ (167.6 cm) would calculate: (10 × 68) + (6.25 × 167.6) – (5 × 35) – 161 = roughly 1,392 calories per day at rest.
That number only covers what your body burns while doing nothing. To find your actual maintenance calories, you need to account for movement.
How Activity Level Changes Everything
Your resting metabolic rate gets multiplied by an activity factor that reflects your typical daily movement. These multipliers, known as Physical Activity Levels, break down into three broad categories:
- Sedentary or lightly active (factor of 1.4 to 1.69): Desk job, minimal exercise, mostly sitting throughout the day
- Moderately active (factor of 1.7 to 1.99): Regular exercise several times a week or a physically demanding job
- Vigorously active (factor of 2.0 to 2.4): Intense daily training or heavy manual labor
Taking the example above, that 35-year-old woman with a resting rate of 1,392 calories would need about 1,950 to 2,350 calories if she’s sedentary, or 2,370 to 2,770 if she’s moderately active. The difference between sitting at a desk all day and hitting the gym regularly can easily be 500 or more calories per day. Activity levels above 2.4 are difficult to sustain over long periods, even for elite athletes.
Most people overestimate how active they are. If you exercise three or four times a week but sit for most of your workday, you’re likely in the upper end of the sedentary range or the lower end of moderately active, not solidly in the middle of “active.”
Why Age Matters Less Than You Think
The common belief that metabolism crashes in your 30s or 40s turns out to be largely wrong. A landmark study published in Science, drawing on data from thousands of people, found that both total daily energy expenditure and resting metabolic rate stay remarkably stable from age 20 to about 60, regardless of sex. The real decline doesn’t begin until around age 63, and even then it’s gradual: about 0.7% per year. By age 90, total energy expenditure drops roughly 26% below middle-aged levels.
What actually changes in your 30s and 40s is often your activity level and muscle mass, not your metabolism itself. People move less, lose some muscle, and gain fat. The formula captures the age effect with a small subtraction for each year, but the bigger driver of any calorie difference over the decades is what happens to your body composition and movement habits.
Muscle, Fat, and Your Metabolic Rate
A pound of muscle burns roughly 4.5 to 7 calories per day at rest, while fat tissue burns significantly less. That difference sounds small on a per-pound basis, but it adds up across your whole body. Someone carrying 20 extra pounds of muscle compared to someone of the same weight with 20 extra pounds of fat could burn 90 to 140 more calories daily before doing anything.
If you know your body fat percentage, there’s an alternative formula called the Katch-McArdle equation that factors in lean mass directly: 370 + (21.6 × fat-free mass in kg). This can be more accurate for people who are unusually muscular or carry more body fat than average, since the standard equations assume a typical body composition for your height and weight.
What Your Body Does With Food
Digesting food itself burns calories, a process called the thermic effect of food. Not all nutrients cost the same energy to process. Protein increases your metabolic rate by 15 to 30% of the calories consumed, carbohydrates by 5 to 10%, and fats by just 0 to 3%. This means a 300-calorie chicken breast effectively “costs” you 45 to 90 calories just to digest, while 300 calories of butter costs almost nothing.
This doesn’t mean you should eat only protein, but it does explain why higher-protein diets tend to make weight maintenance slightly easier. The thermic effect is already baked into most online calorie calculators, so you don’t need to subtract it separately. It’s worth understanding, though, because it’s one reason two people eating the same number of calories but different foods can see different results on the scale.
Why Your Estimate Might Be Off
Even with the right formula, there’s a gap between calculated estimates and real life. One major reason: people consistently underestimate how much they eat. Research from the Women’s Health Initiative found that participants underreported their calorie intake by about 25% on average, with some groups underreporting by as much as 32%. That’s not dishonesty. It’s the cumulative effect of forgotten snacks, underestimated portions, and cooking oils that don’t get counted.
If you’re using your maintenance number to guide your eating and your weight is creeping up, the formula probably isn’t wrong. Your tracking likely is. Weighing food on a kitchen scale for even a few days can be eye-opening. A “tablespoon” of peanut butter, eyeballed, often turns out to be closer to two.
Metabolic Adaptation After Weight Loss
If you’ve recently lost a significant amount of weight, your maintenance calories may be temporarily lower than the formula predicts. Your body can reduce its energy expenditure beyond what the loss of body mass alone would explain. This is sometimes called metabolic adaptation, and a systematic review of 33 studies found evidence of it in the majority of cases during active weight loss.
The encouraging finding, though, is that this effect appears to shrink or disappear once your weight stabilizes. When researchers measured metabolic adaptation specifically during periods of weight maintenance rather than active dieting, the values were reduced or not statistically significant. So if you’ve lost weight and kept it off for several months, the standard formulas should work reasonably well for you again. The amount of weight you lost doesn’t seem to determine how much adaptation occurs.
Finding Your Real Number
Formulas give you a starting point, not a final answer. The most reliable way to find your true maintenance calories is to track your food intake carefully for two to three weeks while monitoring your weight. Weigh yourself at the same time each day, ideally in the morning before eating, and look at the weekly average rather than any single day. Daily weight can swing 5 to 6 pounds based on water retention, sodium intake, and how much food is still moving through your digestive system.
If your average weight stays flat over two to three weeks, the calories you’re eating are your maintenance level. If you’re gaining, you’re above maintenance. If you’re losing, you’re below it. Adjust by 100 to 200 calories and reassess. This approach takes patience, but it accounts for all the individual variables that no equation can capture: your genetics, your gut bacteria, your NEAT (the unconscious fidgeting and movement that varies enormously between people), and the specific foods you eat.
For a quick starting estimate without any math, multiply your body weight in pounds by 14 to 16. Use 14 if you’re mostly sedentary, 16 if you’re fairly active. A 170-pound moderately active person lands around 2,550 to 2,720 calories. It’s a rougher estimate than the Mifflin-St Jeor equation, but it gets you in the right neighborhood fast.