How the numbers are worked out
Nothing here is a secret formula. It's the same arithmetic a dietitian would do, with the assumptions shown so you can disagree with them.
Resting energy. The Mifflin-St Jeor equation, which in validation studies comes closest to measured values for most adults: 10 × weight (kg) + 6.25 × height (cm) − 5 × age, plus 5 for men or minus 161 for women.
Daily maintenance. Resting energy multiplied by an activity factor: 1.2 sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active. These are the standard multipliers, and most people pick one too high — see the FAQ.
Your goal. Losing fat takes 15% off maintenance, roughly 0.5 kg (1 lb) a week for most people. Building muscle adds 10%, enough to support growth without most of the surplus going to fat. Maintain is maintenance.
Protein. Set per kilogram of bodyweight, because that's how the research reports it: 1.6 g/kg to maintain, 1.8 g/kg to build, and 2.0 g/kg when cutting, where a higher intake helps hold on to muscle while you lose weight.
Fat and carbs. Fat at 25% of calories when cutting and 30% otherwise, which keeps it above the floor hormones need. Carbohydrate takes whatever calories are left — it's the flexible one, and the one that fuels training.
Use the numbers for two or three weeks, watch the trend on the scale and how sessions feel, then adjust by 100–200 calories a day. The calculator gets you to the right neighbourhood; your own data gets you to the house.
FAQs
How does this macro calculator work?
It estimates your resting energy use with the Mifflin-St Jeor equation, multiplies by an activity factor to get your daily maintenance calories, then adjusts for your goal: about 15% below maintenance to lose fat, 10% above to build muscle. Protein is set per kilogram of bodyweight, fat at around 25–30% of calories, and carbohydrate fills the rest.
How much protein do I need to build muscle?
Research supports at least 1.6 grams per kilogram of bodyweight a day for adults doing resistance training, with little extra benefit above about 2.2 g/kg. This calculator uses 1.8 g/kg for building muscle and 2.0 g/kg when losing fat, where a higher intake helps protect muscle.
Are macro calculators accurate?
They are a starting point, not a measurement. Mifflin-St Jeor is accurate to within about 10% for most people, but individual metabolism varies. Use the numbers for two to three weeks, watch your bodyweight trend and how training feels, then adjust calories by 100–200 a day in whichever direction you need.
Should I count calories or macros?
Calories decide whether you gain or lose weight. Macros decide what that weight is made of. If you only track one thing, make it protein: hitting your protein target and staying roughly on calories gets most people most of the result. Precise carb and fat splits matter much less.
What activity level should I pick?
Most people overestimate. A desk job with three gym sessions a week is 'lightly active', not 'moderately active'. If you are unsure, pick the lower option — it is easier to add food when you are losing weight too fast than to work out why you are not losing at all.