Your daily calorie needs depend on four things: your size, age, sex, and how active you are. The most reliable way to figure out your number is a two-step process: first, estimate how many calories your body burns at rest, then multiply that by a factor that accounts for your activity level. For most adults, this lands somewhere between 1,600 and 3,000 calories per day, but the range is wide enough that guessing without doing the math can leave you hundreds of calories off.
Step 1: Estimate Your Resting Calorie Burn
Your body uses the majority of its daily calories just keeping you alive: breathing, circulating blood, maintaining body temperature, repairing cells. This baseline burn is called your resting metabolic rate, and it’s the foundation of any calorie calculation.
The Mifflin-St Jeor equation, developed in the 1990s, is considered the most accurate standard formula for most people. It uses your weight in kilograms, height in centimeters, and age in years:
- Men: (10 × weight) + (6.25 × height) – (5 × age) + 5
- Women: (10 × weight) + (6.25 × height) – (5 × age) – 161
To convert: divide your weight in pounds by 2.2 to get kilograms, and multiply your height in inches by 2.54 to get centimeters. 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 = 1,366 calories per day at rest.
That number represents what her body burns doing absolutely nothing. It doesn’t account for walking to the car, exercising, or even digesting food.
Step 2: Factor In Your Activity Level
To get your total daily calorie needs, multiply your resting number by an activity factor:
- Sedentary (desk job, little exercise): multiply by 1.2
- Lightly active (light exercise 1–3 days per week): multiply by 1.375
- Moderately active (moderate exercise 3–5 days per week): multiply by 1.55
- Very active (hard exercise 6–7 days per week): multiply by 1.725
- Extremely active (physical job plus intense training): multiply by 1.9
Using the example above, that 35-year-old woman with a resting burn of 1,366 who exercises moderately three to four times a week would multiply by 1.55, giving her roughly 2,117 calories per day. That’s her maintenance number, the intake at which her weight stays stable. General population estimates from the Merck Manual line up with this: sedentary adult women typically need 1,600 to 2,000 calories, while sedentary adult men need 2,200 to 2,600. Active adults of either sex need several hundred more.
If You Know Your Body Fat Percentage
Standard formulas treat all body weight the same, but muscle tissue burns significantly more calories than fat tissue. If you know your body fat percentage (from a DEXA scan, skinfold calipers, or a reliable smart scale), you can use a formula that accounts for this difference.
First, calculate your lean body mass: subtract your fat weight from your total weight. If you weigh 80 kg at 20% body fat, your lean mass is 64 kg (80 minus 16). Then plug that into the Katch-McArdle formula: 370 + (21.6 × lean mass in kg). For that 64 kg lean mass, the result is 1,752 calories at rest, which is often higher than what the standard equation produces for the same person. This approach is especially useful if you carry more muscle than average or if your body composition doesn’t match population norms.
Adjusting for Weight Loss or Gain
Once you know your maintenance calories, adjusting for a goal is straightforward in principle. Eating below your maintenance number creates a calorie deficit that leads to weight loss. Eating above it creates a surplus for weight gain.
A common starting point for weight loss is cutting about 500 calories per day from your maintenance number. The Mayo Clinic notes this typically produces about half a pound to one pound of loss per week. But the old rule that 3,500 calories always equals one pound of fat is an oversimplification. Your body adapts as you lose weight: your resting calorie burn drops, your movements become more efficient, and hormonal signals shift to conserve energy. The result is that weight loss slows over time even if your intake stays the same. Dynamic models of weight change account for this, which is why real-world results rarely match the neat arithmetic.
For weight gain, particularly building muscle, a surplus of 250 to 500 calories per day is a reasonable range when paired with resistance training. Going much higher tends to add more fat than muscle.
Why Your Number Changes Over Time
The calorie target you calculate today won’t stay accurate forever. Several factors shift it:
As you lose weight, your body gets smaller and burns fewer calories at rest. A person who drops 20 pounds may need 150 to 200 fewer daily calories than before, simply because there’s less tissue to maintain. If weight loss stalls despite consistent effort, this metabolic adaptation is the most likely explanation. Research from the University of Alabama at Birmingham found that metabolic adaptation diminishes after just a couple of weeks of weight stabilization. So if you’ve plateaued, holding your weight steady for two to three weeks and then resuming a deficit can help your metabolism reset.
Age also matters. The formulas subtract calories for each year because resting metabolism genuinely declines with age, largely due to gradual loss of muscle mass. Adults over 61 typically need 200 to 400 fewer calories than they did in their 30s at the same activity level.
Recalculating every 10 to 15 pounds of weight change, or every couple of months during active dieting, keeps your target relevant.
What You Eat Affects How Many Calories You Burn
Not all calories cost the same amount of energy to digest. Your body spends calories breaking down and processing food, a phenomenon called the thermic effect of food. Protein is the most “expensive” macronutrient to process: digesting it burns 15 to 30% of the protein calories you consume. Carbohydrates cost 5 to 10%, and fats cost just 0 to 3%.
This doesn’t mean you should eat nothing but protein, but it does explain why higher-protein diets tend to support weight management. If two people eat the same number of total calories but one gets 30% from protein and the other gets 15%, the higher-protein eater will have a slightly higher net calorie burn from digestion alone. Over weeks and months, that difference adds up.
Calorie Floors You Shouldn’t Go Below
Cutting calories too aggressively backfires. Very low intakes, generally below about 1,200 calories per day for women and 1,500 for men, make it difficult to get enough essential nutrients from food alone. They also accelerate muscle loss and can trigger a sharper metabolic slowdown that makes further progress harder. If running the math puts your target below these levels, a smaller deficit paired with more physical activity is a safer path to the same result.
Population-level data shows sedentary adult women maintaining health at 1,600 to 2,000 calories and sedentary adult men at 2,200 to 2,600. These ranges give you a sanity check. If your calculated target for weight loss falls dramatically below the sedentary range for your sex, the deficit is probably too aggressive.
Putting It All Together
Here’s the practical sequence. Weigh yourself and measure your height. Run the Mifflin-St Jeor equation (or the Katch-McArdle if you know your body fat percentage). Multiply by the activity factor that honestly reflects your week. That’s your maintenance number. Subtract 300 to 500 calories if you want to lose weight, or add 250 to 500 if you want to gain. Track your actual weight over two to three weeks to see if the number is working. If your weight isn’t moving in the expected direction, adjust by 100 to 200 calories and reassess. The formula gives you a starting point, but your body’s real response is the final word.