In short: this calculator turns your age, size, activity level and goal into a daily calorie target and a protein, carbohydrate and fat split you can actually eat to. It uses the Mifflin-St Jeor equation for resting metabolism with standard activity multipliers, and the working is set out in full below so you can check it rather than take it on trust.
Macro and calorie calculator
Work out roughly how many calories and how much protein, carbohydrate and fat to aim for each day.
These are estimates from a standard equation, not medical or dietary advice. If you take insulin or other diabetes medication, are pregnant or breastfeeding, or manage a kidney condition, speak to your doctor or dietitian before changing how you eat.
How these numbers are worked out
Nothing here is a black box, so here is the arithmetic in the order the calculator does it.
Resting metabolism (BMR). The calculator uses the Mifflin-St Jeor equation, which estimates the calories you would burn at complete rest: 10 times your weight in kilograms, plus 6.25 times your height in centimetres, minus 5 times your age, then plus 5 if you are male or minus 161 if you are female. It has been the standard estimate in clinical practice for decades precisely because it is simple and reasonably accurate across most adults.
Daily burn (TDEE). That resting figure is then multiplied by an activity factor, from 1.2 for a desk job with little exercise up to 1.9 for a physical job plus hard training. This is the single biggest source of error in any calculator, including this one, because people routinely overestimate how active they are. If your weight does not move in the direction you expect over three weeks, the activity factor is usually the culprit, not the equation.
Goal. Losing fat uses 20 per cent below your maintenance estimate, maintaining uses it unchanged, and building muscle uses roughly 12 per cent above it. A 20 per cent deficit is a deliberate choice: it is large enough to see progress in weeks while still being sustainable for most people, where more aggressive deficits tend to cost muscle and end in a rebound.
Protein. The calculator sets protein at 1.8 grams per kilogram of body weight when you are losing fat, and 1.6 g/kg otherwise. Both sit inside the 1.2 to 1.7 g/kg working range that the evidence supports for most adults, with the higher end used during a deficit because protein is what protects lean mass when calories are short. Protein is deliberately set first, before fat and carbohydrate, because it is the target with the strongest evidence behind it.
Fat and carbohydrate. Fat takes a fixed share of your calories, which you choose: 40 per cent on the lower-carb setting, 30 per cent balanced, 22 per cent higher-carb. Carbohydrate is whatever calories remain, divided by four. If you pick a low enough calorie target with a high fat share, carbohydrate can approach zero, and the calculator will never show you a negative number.
Every figure is rounded for readability, so the three macronutrient totals may sit a few calories either side of the headline number. That is rounding, not an error.
What to do with the numbers
Treat the calorie figure as a starting hypothesis and the protein figure as the part worth defending. If you want to know why protein is set where it is, and what happens between the official minimum and the fitness aisle, that is covered in how much protein you actually need. If you are wondering whether you need to track macros this precisely at all, the honest answer, with the cases where it is and is not worth it, is in whether you should calculate your macros.
Two practical points. Eat the protein across the day rather than in one sitting, and put a real portion of it in your first meal, whichever meal that is, because that is where it does the most work: why the first meal of the day matters. And weigh yourself weekly, not daily, then adjust the activity factor up or down before you change anything else.
What a calculator cannot do
This is an estimate from a population equation, not a measurement of you, and it is not medical or dietary advice. Two people of identical age, height and weight can have metabolisms that differ by several hundred calories a day. If you take insulin or other diabetes medication, are pregnant or breastfeeding, are underweight, have a history of an eating disorder, or manage a kidney or liver condition, do not act on these numbers without speaking to your doctor or a registered dietitian first. For most healthy adults, the figure is a sensible place to start and a decent thing to adjust from.