Mr Z

AI Prompt to Calculate Macros: By an 18-Year Coach

Francesco Zen, founder of ZTraining · Published 9 July 2026 · Updated 9 July 2026

The prompt

Copy-paste prompt, built from 18 years of coachingAct as my coach for calculating my macros, not a generic macro calculator. Before giving me anything, ask me for: my height, current weight, a rough body-fat estimate (or an honest description if I do not know it), my goal (fat loss, muscle gain, or recomposition), my activity level, and how many days a week I train. Once you have that: 1. Calculate my lean body mass: weight x (1 - body fat % / 100). Show me this number and use it, not my total weight, as the basis for protein. 2. Set my daily protein target from lean mass, using roughly 0.7g per lb of lean mass as a baseline, scaled up for more structured or advanced training and down if I am mostly inactive or over 50. 3. Set a daily fat target of roughly 40-50g a day, adjusted slightly for my sex and bodyweight. Do not set it lower than that without a specific reason tied to my goal. 4. Give me the remaining calories as carbohydrate, based on my goal (a deficit for fat loss, maintenance for recomposition, a modest surplus for muscle gain). Do not treat carbs as the macro to cut first. 5. Only count complete proteins toward my protein target: meat, fish, eggs, dairy, whey, EAA. Bread, rice, oats and legumes are carbs, not protein, even though they contain some. 6. Show me the maths at each step, not just the final numbers, so I understand where they came from. 7. Tell me these numbers are a starting point, not a permanent fact, and should be recalculated as my weight and body fat change. Ask me the setup questions now, before giving me any numbers.

Why this prompt works

Most online macro calculators use total bodyweight and a generic activity multiplier, which quietly overstates protein for anyone carrying meaningful body fat. Lean body mass is the correct basis because muscle tissue is what actually uses dietary protein, fat mass mostly does not, so two people at the same scale weight but different body-fat percentages need genuinely different protein targets, not the same one.

Carbohydrate is calculated last and treated as the flexible macro that fills the remaining calories, never the first one cut. That ordering, protein and fat set from real targets first, carbs filling the rest, is what separates an actual macro calculation from a diet app's default 40/30/30 split applied to everyone regardless of goal or body composition.

A real-world example

Take two people at the same 80kg bodyweight: one at 15% body fat, one at 30%. Their lean mass differs by roughly 12kg, which is a meaningful difference in the protein target this prompt would set for each of them, even though a generic calculator using total bodyweight would hand them the identical number.

That gap is exactly why lean-mass-based calculation matters more than which macro app someone happens to open.

Where a single prompt hits its ceiling

These numbers are a snapshot. The moment your weight or body fat changes, and both change continuously through a fat loss or muscle gain phase, the calculation is technically out of date, but a one-off prompt has no way to know that unless you notice and recalculate yourself.

It also cannot verify your body-fat estimate is accurate. Macro calculations are only as good as the inputs, and a self-estimated body-fat percentage that is off by five or ten points changes the lean mass figure enough to shift every number downstream of it.

Common questions

How do I calculate my macros properly?

Start from lean body mass, not total bodyweight, then set protein and fat first and let carbohydrate fill the remaining calories based on your goal. Most calculators skip the lean-mass step entirely and use total weight instead.

Why do macro calculators give different answers?

Because they use different formulas and different assumptions about activity level, and most default to total bodyweight rather than lean mass. Two calculators can give meaningfully different protein targets for the exact same person.

Do I need to recalculate my macros as I lose weight?

Yes. Your lean mass and body fat change as you progress, so a macro target set at the start of a fat loss phase is not the correct target by the end of it, and should be recalculated periodically.

Francesco Zen is the founder of ZTraining and ZLIFE, with 18 years of coaching experience working with executives across the UK, US, and UAE. He built Mr Z, an AI coach trained on his methodology to bring the same approach to a wider audience.