Get your daily calories and a protein, carb and fat split based on your body, activity level and goal. Pick a macro ratio.
Calories use the Mifflin-St Jeor equation for resting metabolism, multiplied by your activity level, then adjusted for your goal. Macros split those calories at 4 kcal per gram of protein and carbs and 9 kcal per gram of fat. These are starting estimates, not medical advice.
Macros (macronutrients) are protein, carbohydrate and fat, the three nutrients that supply calories. Tracking them, not just total calories, helps with body composition because protein, carbs and fat do different jobs in the body.
The tool estimates your resting metabolic rate with the Mifflin-St Jeor equation from your sex, age, weight and height, multiplies it by an activity factor for your daily activity, then adds or subtracts calories for your goal.
Balanced (30/40/30) suits most people. Lower-carb ratios suit some people for fat loss, higher-carb suits endurance training, and higher-protein supports muscle retention while dieting. They are starting points to adjust based on results.