What You'll Learn
- ✓ What calories actually are (spoiler: they're just energy)
- ✓ BMR vs TDEE—and why you need to know both
- ✓ The Mifflin-St Jeor formula decoded
- ✓ How to set calories for weight loss, maintenance, or muscle gain
- ✓ Common calorie myths that need to die
What Are Calories, Really?
A calorie is just a unit of energy. That's it. When you eat 100 calories, you're consuming 100 units of energy that your body can use for everything from breathing to running marathons.
Technically, a "calorie" (lowercase) is the energy needed to heat 1 gram of water by 1°C. But when we talk about food calories, we're actually talking about kilocalories (kcal)—which is 1,000 calories. That protein bar labeled "200 calories"? It's really 200,000 calories. But nobody says that because it sounds terrifying.
Quick Science Lesson
Your body burns calories doing three main things:
- Basal functions (60-75%): Breathing, pumping blood, thinking
- Physical activity (15-30%): Exercise, walking, fidgeting
- Digesting food (10%): Yes, eating burns calories!
So when a calorie calculator tells you that you need 2,000 calories per day, it's estimating how much energy your body uses for all these processes. Simple in theory. Complex in practice.
BMR vs TDEE: The Two Numbers You Need to Know
Calorie calculators give you two critical numbers. Most people confuse them. Don't be most people.
BMR (Basal Metabolic Rate)
This is the number of calories you'd burn if you stayed in bed all day doing absolutely nothing. It's your body's baseline energy requirement just to stay alive.
Example: A 30-year-old woman who's 5'6" and 150 lbs has a BMR of about 1,450 calories.
TDEE (Total Daily Energy Expenditure)
This is your BMR plus all the calories you burn through daily activity and exercise. This is the number you actually care about.
Example: That same woman with a moderately active lifestyle has a TDEE of about 2,250 calories.
🎯 Remember: You eat to your TDEE, not your BMR.
Eating at your BMR every day means you're chronically undereating. Your body needs fuel for movement, not just staying alive.
The Mifflin-St Jeor Formula (Most Accurate)
Most modern calorie calculators (including ours) use the Mifflin-St Jeor equation. It was developed in 1990 and is considered the gold standard for estimating BMR.
The Formula
For Men:
BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age) + 5
For Women:
BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age) - 161
Notice the only difference? Men get +5, women get -161. That's because men naturally have more muscle mass, which burns more calories at rest.
Real Example
Let's calculate BMR for a 35-year-old woman, 165 cm tall, weighing 70 kg:
BMR = (10 × 70) + (6.25 × 165) - (5 × 35) - 161
BMR = 700 + 1,031.25 - 175 - 161
BMR = 1,395 calories/day
Or just use our free calorie calculator and skip the math. We won't judge.
Activity Level Multipliers Explained
Once you have your BMR, you multiply it by an activity factor to get your TDEE. This is where people screw up. Be honest with yourself.
Sedentary (×1.2)
Desk job, little to no exercise, mostly sitting or lying down.
Example TDEE: 1,395 × 1.2 = 1,674 calories
Lightly Active (×1.375)
Light exercise 1-3 days/week, or a job with moderate walking.
Example TDEE: 1,395 × 1.375 = 1,918 calories
Moderately Active (×1.55)
Moderate exercise 3-5 days/week, or physically demanding job.
Example TDEE: 1,395 × 1.55 = 2,162 calories
Very Active (×1.725)
Hard exercise 6-7 days/week, or training for competitive sports.
Example TDEE: 1,395 × 1.725 = 2,406 calories
Extremely Active (×1.9)
Very hard exercise twice per day, physical job, or competitive athlete.
Example TDEE: 1,395 × 1.9 = 2,651 calories
⚠️ Reality Check
Most people overestimate their activity level. That 30-minute gym session 3x/week? That's "Lightly Active," not "Very Active." Going for walks doesn't count unless you're walking several miles daily.
Setting Calories for Your Goals
Now that you know your TDEE, here's how to adjust calories based on your goal:
🔥 Weight Loss
Target: TDEE minus 300-500 calories
A 500-calorie daily deficit equals about 1 pound of fat loss per week (3,500 calories = 1 pound). Don't go more aggressive unless you're supervised by a professional.
Example: TDEE of 2,162 calories → Eat 1,662-1,862 calories/day
⚖️ Maintenance
Target: Eat at your TDEE
Simple. Eat the same number of calories you burn. Your weight stays stable.
Example: TDEE of 2,162 calories → Eat 2,162 calories/day
💪 Muscle Gain
Target: TDEE plus 200-300 calories
You need a calorie surplus to build muscle. But don't go crazy—a small surplus minimizes fat gain. And you need to be lifting weights, obviously.
Example: TDEE of 2,162 calories → Eat 2,362-2,462 calories/day
Common Calorie Myths (Debunked)
Myth 1: "All calories are equal"
FALSE. 100 calories of broccoli affects your body differently than 100 calories of candy. Protein has a higher thermic effect (burns more calories during digestion). Fiber slows absorption. Nutrients matter.
Myth 2: "1,200 calories is safe for everyone"
NOPE. 1,200 is often cited as a "minimum" for women, but it's way too low for most people. If your TDEE is 2,500 and you drop to 1,200, your metabolism will slow down and you'll feel like death. Read our guide on safe calorie deficits.
Myth 3: "Eating late makes you gain weight"
FALSE. Total daily calories matter, not timing. Your body doesn't care if you eat at 6 PM or 10 PM. That said, late eating can disrupt sleep for some people.
Myth 4: "Cardio burns the most calories"
MISLEADING. Cardio burns calories during the workout. Strength training burns fewer calories during exercise but increases your BMR long-term by building muscle. Muscle burns more calories at rest.
Myth 5: "Calorie calculators are exact"
FALSE. Calculators give estimates based on averages. Your actual needs can vary by 10-20% based on genetics, metabolism, gut health, and more. Use calculators as a starting point, then adjust based on real-world results. Check out the 5 most common calculator mistakes.
Calculate Your Daily Calorie Needs
Ready to find your numbers? Our free calculator uses the Mifflin-St Jeor formula to give you accurate BMR and TDEE estimates in seconds.
Try Calorie Calculator →