How many calories should I eat per day?

Use the Mifflin-St Jeor equation: BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age) - 161 for women (+5 for men). Multiply by activity factor (1.2 sedentary to 1.9 very active) for TDEE. Example: A 30-year-old male, 180 lbs, 5'10", moderately active needs ~2,650 cal/day.