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BMI, TDEE and Macros Explained: What the Numbers Mean and Their Limits

Health · Updated October 2, 2026 · 4 min read

Fitness calculators turn a few measurements into numbers that look precise: a BMI of 25.9, a maintenance intake of 2,732 calories, 148 grams of protein. Each comes from a formula fitted to large groups of people, so each is a starting estimate for an individual. This guide shows how the numbers are produced, using one example person, and where each one stops being reliable.

This is general information, not medical advice. Talk to a doctor or registered dietitian before major changes, particularly if you are pregnant, under 18, or managing a health condition.

BMI: a screening number

Body mass index is weight in kilograms divided by height in metres squared. Our example is a 35-year-old man, 178 cm tall and weighing 82 kg: 82 ÷ 1.78² = 25.9.

  • WHO adult categories: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and above obese.
  • BMI cannot tell muscle from fat: a muscular athlete can be “overweight” with low body fat.
  • Several Asian countries use lower cut-offs because health risks rise at lower BMI in those populations — China treats 24 as overweight.
  • Children and teenagers are assessed against age- and sex-specific growth charts, not adult categories.

Waist-to-height ratio is a useful companion: keeping your waist under half your height is the guidance used by the UK’s NICE. The body fat calculator estimates body-fat percentage from tape measurements.

BMR: energy at rest

Basal metabolic rate is the energy your body uses at complete rest. The Mifflin–St Jeor equation is the most widely used estimate:

Men:    10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
Women:  10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161

Example: 10 × 82 + 6.25 × 178 − 5 × 35 + 5 = 1,762.5 kcal

For most people it lands within about 10% of a laboratory measurement, but individuals can sit well outside that range, especially at very high or low body fat.

TDEE: what you burn in a day

Total daily energy expenditure multiplies BMR by an activity factor: 1.2 for sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active and 1.9 for hard daily training or physical work. Our example man is moderately active: 1,762.5 × 1.55 ≈ 2,732 kcal a day to maintain his weight.

The activity factor is the weakest link. People commonly overestimate it, and fitness trackers often overestimate the calories burned in workouts. Everyday movement — walking, standing, fidgeting — varies more between people than gym time does. When unsure, choose the lower level.

Calorie targets for a goal

  • Weight loss: a daily deficit of about 500 kcal (about 2,230 kcal for our example) is a common starting point, targeting roughly 0.5 kg or 1 lb a week at first.
  • Weight gain: a surplus of 250–500 kcal supports gradual gain with less fat.
  • Very low targets — under about 1,200 kcal for women or 1,500 for men — make it hard to meet nutrient needs and should be medically supervised.
  • Expect the first week to show a larger change, mostly water. Judge progress on two- to three-week trends.

Macros: dividing the calories

Protein and carbohydrate provide about 4 kcal per gram and fat about 9. A common approach is to set protein from body weight — 1.6–2.2 g per kg is widely used for active people and during weight loss — then split the remaining calories between carbohydrate and fat according to preference.

For our example at 2,232 kcal: 1.8 g × 82 kg ≈ 148 g of protein (590 kcal). Giving 30% of calories to fat is about 74 g (670 kcal), which leaves about 243 g of carbohydrate. Total calories drive weight change; the macro split mostly affects how full you feel, training performance and how much muscle you keep.

Using the numbers well

  1. Start with the calculators’ estimates rather than a number from a friend or a forum.
  2. Track intake and weekly average body weight for two to three weeks.
  3. Adjust calories by 100–200 a day based on the real trend, not the formula.
  4. Recalculate after every 5 kg or so of change.
  5. Get individual advice for pregnancy, eating disorders, diabetes, kidney disease and other medical conditions.

Scale weight versus body composition

Body weight combines fat, muscle, bone, water and the contents of your digestive system, and it can swing by a kilogram or more within a day. Two people with the same weight and BMI can have very different health profiles. When strength training while eating at a small deficit, it is common for weight to fall slowly while waist size falls faster — a sign of losing fat while keeping or gaining muscle.

For that reason, track more than one number: weekly average weight, waist circumference, and how your clothes fit or how your training is going.

Beyond calories and macros

  • Fibre: common recommendations are about 25 g a day for women and 38 g for men, from vegetables, fruit, legumes and whole grains. It also helps with fullness when eating less.
  • Protein spread: spreading protein across three or four meals is a practical way to reach a daily target.
  • Hydration, sleep and stress all affect appetite, training and the number on the scale.
  • Food quality matters for long-term health even when two diets have identical calories and macros.

When to get professional advice

  • Unexplained weight loss or gain, or a sudden change in appetite.
  • A BMI under 18.5 or a history of disordered eating — calorie targets can do harm here.
  • Diabetes, kidney disease, heart conditions or medications that affect appetite or weight.
  • Pregnancy and breastfeeding, when energy and nutrient needs change substantially.
  • Children and teenagers, whose needs depend on growth and should not be set by adult formulas.

Tools used in this guide