Macro Calculator

How this macro calculator works

Calories

Your resting burn comes from the Mifflin-St Jeor equation, built in 1990 from measurements of 498 adults aged 19 to 78. Enter a body fat percentage and the calculator switches to Katch-McArdle, which works from your lean mass and ignores sex and age.

That number gets multiplied by an activity factor, from 1.2 for a desk job with no training up to 1.9 for twice a day training. The result is your maintenance.

Lose fat puts you 20% under maintenance. Lose fat slowly is 10% under. Build muscle is 10% over. The calculator won’t go under 1,200 calories for women or 1,500 for men, which is the line Harvard Health draws for dieting without a health professional involved.

Protein

Maintaining or gaining, you get 0.73 g per pound of bodyweight (1.6 g per kg). That’s where muscle gains leveled off in a 2018 analysis of 49 studies and 1,863 people on lifting programs.

On a cut it goes up to 1 g per pound (2.2 g per kg), the top of the range from that same analysis. A deficit pulls muscle along with the fat, and more protein is one of the ways to hold onto it. How you spread it across the day matters far less than the total.

If your weight is well above where you want it, enter a goal weight and the protein number is set from that. Fat tissue doesn’t need protein to maintain it. That’s a coaching shortcut, not a study finding. Protein is also capped at 40% of your calories so carbs and fat still fit, and that cap is a calculator choice.

Fat and carbs

Fat is set at 25% of calories, inside the 20 to 35% range US dietary guidance gives for adults. Carbs get every calorie that’s left. Fiber is 14 g per 1,000 calories, also from the US guidelines. If most of your carbs come from sugar, the 30 day less sugar challenge is the place to start.

How close is it?

Close enough to start with. A 2005 review of 4 resting burn equations found Mifflin-St Jeor the most reliable, and it still misses by more than 10% for some people. The activity factor is the roughest part of the math, because “3 to 5 days a week” covers a lot of different weeks.

Eat the number for 2 weeks and weigh yourself most mornings. If your weight isn’t moving the way your goal says it should, shift calories by 100 to 200 a day and run another 2 weeks.