Nutrition Basics: How to Calculate BMI, TDEE, and Your Macros for Any Goal

Leandro Cordova

Jul 30

7 min read

Updated: Sep 28

Written by Dr. Leandro Cordova, Ph.D. — Co-Founder & Formulation Scientist at Proven Science Nutrition Science First. Results Always.

"Nutrition basics — how to calculate BMI, TDEE, and macros for your goals — Proven Science Nutrition" — Guide to understanding basic nutrition concepts like BMI, TDEE, and macronutrients to help you achieve your health and fitness goals.

Nutrition is the foundation of physical performance and health — but most people train hard while guessing at the numbers that actually drive results. How many calories do you burn in a day? How many should you eat to lose fat or build muscle? How should those calories be split between protein, carbs, and fat?

This guide answers all of it, with the real formulas and worked examples. By the end, you'll be able to calculate your own BMI, estimate your daily calorie burn (TDEE), set a calorie target for your goal, and design a macronutrient split that fits it. No apps required — just the math the pros actually use.

What Is BMI, and How Do You Calculate It?

Body Mass Index (BMI) is a quick screening number that relates your weight to your height. It's simple to calculate:

Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = [weight (lbs) ÷ height (in)²] × 703

The standard categories:

BMI	Category
Under 18.5	Underweight
18.5 – 24.9	Normal weight
25.0 – 29.9	Overweight
30.0 and above	Obese

Here's the important caveat: BMI doesn't distinguish between muscle and fat. It only knows your total weight and height. That makes it a reasonable population-level screening tool but a poor individual one — a lean, muscular athlete can easily land in the "overweight" or even "obese" range despite low body fat, and someone at a "normal" BMI can carry excess fat with little muscle.[2] Use BMI as a rough starting reference, not a verdict on your health or physique. Waist measurement, body-fat percentage, and how you look and perform are all more informative.

What Is TDEE, and How Do You Calculate It?

Your Total Daily Energy Expenditure (TDEE) is the total number of calories you burn in a day. This is the single most useful number in nutrition, because everything — fat loss, muscle gain, maintenance — is set relative to it.

TDEE is built in two steps. First you estimate your Basal Metabolic Rate (BMR) — the calories your body burns at complete rest just to stay alive — then you multiply it by an activity factor.

The most accurate widely-used formula for BMR is the Mifflin-St Jeor equation, validated as more accurate than the older Harris-Benedict equation for modern populations:[1]

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

Then multiply BMR by your activity factor:

Activity Level	Multiplier
Sedentary (little/no exercise)	1.2
Lightly active (1–3 days/week)	1.375
Moderately active (3–5 days/week)	1.55
Very active (6–7 days/week)	1.725
Extra active (physical job + training)	1.9

TDEE = BMR × Activity Factor

What Your TDEE Is Actually Made Of

It helps to understand why TDEE works the way it does. Your daily burn is the sum of three components:

"Components of TDEE chart: BMR ~65%, physical activity ~25%, thermic effect of food ~10%" — Illustrative typical proportions in adults. Individual values vary.

Basal Metabolic Rate (~60–70%) — the energy to run your organs, brain, and cells at rest. It's the biggest piece, which is why BMR anchors the whole calculation.
Physical activity (~15–30%) — both structured exercise and everyday movement (walking, fidgeting, chores), which vary hugely between people.
Thermic effect of food (~10%) — the energy used to digest and process what you eat. Protein has the highest thermic effect of the three macronutrients.[4]

This is also why "I have a slow metabolism" is rarely the whole story: the activity component is the one you have the most control over day to day.

Setting Your Calorie Target by Goal

Once you know your TDEE, your goal sets your calorie target. Body weight is governed by energy balance — calories in versus calories out — so you adjust intake relative to your burn:[3]

Maintain weight: eat at your TDEE
Lose fat: eat 10–20% below TDEE
Gain muscle: eat 10–20% above TDEE

Example: For a 2,500-calorie TDEE, a 20% deficit puts your target around 2,000 calories for fat loss; a 15% surplus puts it around 2,875 calories for muscle gain.

A moderate deficit or surplus is deliberate. Aggressive deficits burn muscle along with fat and are hard to sustain; aggressive surpluses add more fat than muscle. Slower and steadier wins on both sides.

Macronutrient Splits: Dividing Up Your Calories

Total calories decide whether you gain or lose weight; macronutrients decide the quality of that change — how much of it is muscle versus fat, and how good you feel getting there.

First, the energy each macro provides:

Protein: 4 calories per gram
Carbohydrate: 4 calories per gram
Fat: 9 calories per gram

Evidence-based starting ratios by goal:

Goal	Protein	Carbs	Fat
Fat loss	30–40%	30–40%	20–30%
Muscle gain	25–35%	40–50%	20–30%
Maintenance	25–30%	40–50%	20–30%

Notice protein stays high across every goal. That's deliberate: adequate protein preserves muscle in a deficit and builds it in a surplus, and research consistently supports a higher protein intake — roughly 1.6–2.2 g per kg of body weight — for anyone training to change their body composition.[5][6]

How to Turn Percentages Into Grams

Take your calorie target, split it by percentage, then divide by the calories per gram.

Example — 2,400 calories, muscle gain (30% protein / 45% carbs / 25% fat):

Protein: 2,400 × 0.30 = 720 cal ÷ 4 = 180 g
Carbs: 2,400 × 0.45 = 1,080 cal ÷ 4 = 270 g
Fat: 2,400 × 0.25 = 600 cal ÷ 9 = 67 g

Worked Case Studies

Case 1 — 30-year-old man, fat loss. 180 lbs, 70 in, moderately active.

BMR ≈ 1,782 cal → TDEE (×1.55) ≈ 2,762 cal
20% deficit → ~2,210 cal
Macros (35/35/30): 193 g protein, 193 g carbs, 74 g fat

Case 2 — 25-year-old woman, muscle gain. 140 lbs, 65 in, very active.

BMR ≈ 1,380 cal → TDEE (×1.725) ≈ 2,379 cal
15% surplus → ~2,736 cal
Macros (30/45/25): 205 g protein, 308 g carbs, 76 g fat

Putting It All Together

Here's the whole system in five steps:

Calculate BMR with Mifflin-St Jeor.
Multiply by your activity factor to get TDEE.
Adjust for your goal (−10–20% for fat loss, +10–20% for muscle gain).
Split into macros using the ratios for your goal.
Track, then adjust. These formulas are excellent estimates — your real-world results over 2–3 weeks are the true feedback. If the scale and mirror aren't moving as intended, nudge calories by ~100–200 and reassess.

That last step matters most. The math gives you a smart starting point; your body gives you the answer.

Want This Done For You?

If you'd rather not run the numbers yourself, we've packaged all of this — the formulas, the macro guidance, and practical meal strategies — into our free 30-page Nutritional & Supplement Guide. Grab it here: psnutrition.net/free-guide.

And if you want a plan built specifically around you, my wife and PSN co-founder Michelle Oliveira coaches women and athletes through exactly this at Fit by Mi — a NASM-certified nutrition coach and two-time Ms. Indiana Bikini champion who'll set your targets and build the training and nutrition to match.

The Bottom Line

Nutrition stops being guesswork the moment you know your numbers. Calculate your BMI for a rough reference, your TDEE for your true daily burn, set your calories by your goal, and split your macros with protein kept high. Then track your results and adjust. That simple, evidence-based framework is the foundation everything else — supplements included — is built on.

Science first. Results always.

Frequently Asked Questions

Is BMI an accurate measure of health? BMI is a useful population-level screening tool but a limited individual one, because it can't tell muscle from fat. A muscular athlete may register as "overweight" despite low body fat. Use it as a rough reference alongside better measures like waist circumference, body-fat percentage, and performance.

Which BMR formula is most accurate? The Mifflin-St Jeor equation is currently the most accurate widely-used predictive formula for resting energy expenditure in healthy people, and it's more reliable than the older Harris-Benedict equation. Remember it's still an estimate — track your results and adjust.

How big should a calorie deficit be for fat loss? A moderate deficit of about 10–20% below your TDEE is the sweet spot. It's large enough to drive steady fat loss but small enough to preserve muscle and stay sustainable. Very aggressive deficits tend to cost you muscle and backfire.

How much protein should I eat? For anyone training to change their body composition, roughly 1.6–2.2 grams of protein per kilogram of body weight per day is well supported by research. Protein preserves muscle in a deficit and builds it in a surplus, which is why it stays high across every goal.

Do I really need to count macros? Not forever, but tracking for a while is one of the best ways to learn what your intake actually looks like. Most people dramatically misjudge their calories and protein. Once you've calibrated your eye, you can often maintain results with looser tracking.

Why isn't my TDEE calculation matching my results? Because it's an estimate — activity, NEAT (everyday movement), and individual metabolism all vary. Treat the calculated number as a starting point, track your weight and measurements for 2–3 weeks, and adjust calories by 100–200 based on what actually happens.

Written by Dr. Leandro Cordova, Ph.D., Co-Founder and Formulation Scientist at Proven Science Nutrition. With a Ph.D. and 15+ years in product development, Leo leads the development of every PSN formula — built on peer-reviewed science, with no proprietary blends.

Questions about our formulas or the research behind them? Reach us at contact@psnutrition.com.

*This content is for educational purposes only and does not replace professional medical or dietary advice. Individual needs vary. Consult a qualified healthcare professional before making significant changes to your diet, especially if you have a medical condition.