Sedentary (Office job, little exercise)
Lightly Active (1-3 days/week exercise)
Moderately Active (3-5 days/week exercise)
Very Active (6-7 days/week hard exercise)
Extremely Active (Athletic training/Physical job)
Fat Loss (Deficit)
Maintenance
Muscle Growth (Lean Bulk)
Your Science-Based Results
Protein
–
Carbs
–
Fats
–
function calculateBWS() {
var gender = document.getElementById("bwsGender").value;
var age = parseFloat(document.getElementById("bwsAge").value);
var weight = parseFloat(document.getElementById("bwsWeight").value);
var height = parseFloat(document.getElementById("bwsHeight").value);
var activity = parseFloat(document.getElementById("bwsActivity").value);
var goal = document.getElementById("bwsGoal").value;
if (!age || !weight || !height) {
alert("Please enter all required fields.");
return;
}
// Mifflin-St Jeor Equation
var bmr;
if (gender === "male") {
bmr = (10 * weight) + (6.25 * height) – (5 * age) + 5;
} else {
bmr = (10 * weight) + (6.25 * height) – (5 * age) – 161;
}
var tdee = bmr * activity;
var targetCalories;
if (goal === "fatloss") {
targetCalories = tdee – 500;
} else if (goal === "muscle") {
targetCalories = tdee + 250;
} else {
targetCalories = tdee;
}
// Macros based on Built With Science Principles
// Protein: ~2.2g per kg (approx 1g/lb)
var proteinGrams = weight * 2.2;
var proteinCals = proteinGrams * 4;
// Fats: ~25% of total calories
var fatCals = targetCalories * 0.25;
var fatGrams = fatCals / 9;
// Carbs: Remainder
var carbCals = targetCalories – proteinCals – fatCals;
var carbGrams = carbCals / 4;
document.getElementById("bwsSummary").innerHTML = "To reach your goal, your daily target is " + Math.round(targetCalories) + " calories.Based on your Maintenance (TDEE) of " + Math.round(tdee) + " calories.";
document.getElementById("resProtein").innerText = Math.round(proteinGrams) + "g";
document.getElementById("resCarbs").innerText = Math.round(carbGrams) + "g";
document.getElementById("resFats").innerText = Math.round(fatGrams) + "g";
document.getElementById("bwsResult").style.display = "block";
}
Understanding the Science of Body Transformation
Achieving a physique like those seen in Jeremy Ethier's Built With Science programs requires more than just hard work; it requires precision. This calculator uses the Mifflin-St Jeor equation, widely considered by research to be the most accurate for estimating Basal Metabolic Rate (BMR) in healthy individuals.
How This Calculator Works
The "Built With Science" methodology focuses on three primary levers:
Energy Balance: Whether you are in a caloric surplus (muscle growth) or a caloric deficit (fat loss).
Protein Optimization: We calculate protein at 2.2g per kg of bodyweight to ensure muscle protein synthesis is maximized, even during a cut.
Strategic Macro Split: Fats are kept at a healthy baseline (25%) to support hormonal health, while the remaining calories are allocated to carbohydrates to fuel high-intensity training sessions.
Realistic Examples
Consider a 30-year-old male weighing 85kg at 180cm who is moderately active:
Nutrition is dynamic. As you lose weight, your BMR will slightly decrease (adaptive thermogenesis). It is recommended to re-calculate your numbers every 4-6 weeks to ensure you haven't hit a plateau. Furthermore, "Active" calories from exercise are already accounted for in the TDEE multiplier—be careful not to "double count" calories burned in the gym.