How is your calorie target found?

First, your BMR (basal metabolic rate) is calculated from your sex, age, height, and weight using the widely-used Mifflin-St Jeor equation. Then, a multiplier is applied to this number based on your selected activity level (this number is now your TDEE, or total daily energy expenditure - also just called maintenance calories). Finally, another multiplier is applied to that number to account for your selected goal. Those multipliers are:

1.0 (Maintain)
0.9 (Lose Weight Slowly)
0.8 (Lose Weight Quickly)
1.1 (Lean Bulk)
1.2 (Gain Weight Quickly)

How are your macronutrient targets found?

Protein

Protein is set to 1g per lb of body weight for Maintain and Lean Bulk goals. For Lose Weight Slowly and Lose Weight Quickly goals, it's set to 1.1g and 1.2g per lb of body weight, respectively. When eating in a calorie deficit, I believe it's important to get these levels of protein for muscle preservation and satiety.
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For the Gain Weight Quickly goal, protein is set to only 0.8 g per lb of body weight because when eating in this much of a surplus a) your body doesn't need as much protein in order to preserve muscle, b) eating more protein than this may be difficult, and c) because of a and b you might as well cut your grocery bill slightly.

Fats

Fat is calculated at 25% of daily calories for all goal settings. A widely accepted guideline is to get minimum 20% of daily calories from fat.
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Getting enough fat is critical for hormone production (in both men and women), joint and skin health, brain function, and mood. All of these are especially critical when training.

Carbohydrates

Carbohydrates are simply set to fill remaining calories after protein and fat are calculated. That isn't to say they aren't important, though. Your body needs them to fuel workouts, build muscle, and recover properly. Don't be afraid of them.

You can adjust your carbs and fats slightly depending on your own needs, and it's fine if some days you end up eating more carbs and less fat or vice versa. I recommend following the protein guideline more strictly, however, and making it a priority to hit your target each day.

Note: if the calculator gives you less than 50 g of fat per day, DM me for a more tailored approach.


-Sam