"Eat less, move more" is technically correct and almost completely useless advice. How much you should eat depends on your body size, age, sex, and activity level in ways that vary enormously between individuals. Here's the science-based approach to actually figuring out your number.
Start With Your BMR
Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest just to stay alive — keeping your heart beating, lungs breathing, and cells functioning. For most adults, BMR ranges from 1,400 to 2,000 calories per day.
The Mifflin-St Jeor equation (the most validated formula) calculates BMR as: Men: (10 × kg) + (6.25 × cm) − (5 × age) + 5 and Women: (10 × kg) + (6.25 × cm) − (5 × age) − 161.
Then Multiply by Your Activity Level (TDEE)
Total Daily Energy Expenditure (TDEE) is what you actually burn in a day. Multiply your BMR by your activity factor:
- 1.2 — sedentary (desk job, little exercise)
- 1.375 — lightly active (1–3 workouts/week)
- 1.55 — moderately active (3–5 workouts/week)
- 1.725 — very active (hard exercise 6–7 days/week)
- 1.9 — extremely active (physical job + daily exercise)
💡 Most people overestimate their activity level. If you exercise 3 days a week but sit at a desk for 8 hours, you're probably a 1.375, not a 1.55. When in doubt, use the lower multiplier and adjust based on results.
Adjusting for Your Goal
Once you have your TDEE, adjusting is straightforward. As a rough rule of thumb, a calorie deficit of 500/day produces roughly 1 lb of fat loss per week — based on the traditional estimate that 3,500 calories equals one pound of fat. In practice, the body adapts over time, so real-world results vary, but it's a reliable starting point for planning. A 300–500 calorie surplus supports muscle gain alongside strength training. Deficits larger than 1,000 cal/day tend to backfire — muscle loss increases, metabolism adapts, and adherence drops.
Why Extreme Low-Calorie Diets Often Fail
1,200 calories is a commonly cited clinical threshold — approximately the minimum estimated for a small sedentary adult woman to avoid serious nutritional deficiency. It's not a universal biological floor, and it isn't a target. For most people, eating at 1,200 calories creates a deficit so large that the body responds by reducing metabolic rate, losing muscle, and triggering intense hunger. A more sustainable deficit is 15–20% below your TDEE.
Tracking Calories: The 80/20 Rule
You don't need to track every calorie forever. A few weeks of honest food logging reveals your true eating patterns, which most people dramatically underestimate. After that, most people can maintain their goals with rough awareness, occasional check-ins, and attention to protein intake.