Built With Science Calculator

Your science-based tool for calculating daily energy expenditure and macronutrient targets.

What is the Built With Science Calculator?

The Built With Science Calculator is a comprehensive tool designed to estimate your Total Daily Energy Expenditure (TDEE) and provide personalized macronutrient targets based on your fitness goals. Unlike generic calculators, this tool is grounded in the principles of nutritional science, similar to the approaches advocated by fitness experts like Jeremy Ethier of Built With Science. It helps you understand the energy your body needs to function and how to structure your diet to effectively lose fat, gain muscle, or maintain your current weight.

The core of this calculator is determining your TDEE, which is the total number of calories you burn in a 24-hour period. It accounts for your resting metabolism (BMR) and your physical activity. Once your TDEE is known, you can set a precise calorie target: a deficit for fat loss, a surplus for muscle gain, or simply eat at maintenance. This Built With Science Calculator then breaks down those calories into macronutrients—protein, carbohydrates, and fats—which are the building blocks of any effective nutrition plan.

Built With Science Calculator Formula and Explanation

The calculator uses scientifically validated formulas to provide accurate estimations. The process involves two main steps: calculating your Basal Metabolic Rate (BMR) and then adjusting it for your activity level to find your TDEE.

1. Basal Metabolic Rate (BMR) Calculation

BMR is the energy your body uses at rest just to stay alive (e.g., breathing, circulation). We use the Mifflin-St Jeor equation, which is widely considered one of the most accurate methods.

Men: BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) + 5

Women: BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) - 161

2. Total Daily Energy Expenditure (TDEE) Calculation

Your BMR is then multiplied by an activity factor to estimate your daily maintenance calories (TDEE).

TDEE = BMR × Activity Multiplier

Activity Level Multipliers

Activity Level	Description	Multiplier
Sedentary	Office job, little to no formal exercise	1.2
Lightly Active	Light exercise or sports 1-3 days/week	1.375
Moderately Active	Moderate exercise or sports 3-5 days/week	1.55
Very Active	Hard exercise or sports 6-7 days/week	1.725
Extra Active	Very hard exercise/physical job or training twice a day	1.9

3. Goal-Based Calorie and Macro Calculation

• Fat Loss: A 20% calorie deficit is applied to your TDEE. This is a sustainable rate for losing fat while preserving muscle.
• Maintenance: Your calorie goal is equal to your TDEE.
• Lean Muscle Gain: A modest surplus of 300 calories is added to your TDEE to fuel muscle growth while minimizing fat gain. You can learn more about this in our article on the muscle gain diet.
• Protein: Set at 2.2g per kg of body weight, a science-backed amount for maximizing muscle repair and growth.
• Fat: Set at 25% of your total calorie intake, crucial for hormone function.
• Carbohydrates: The remaining calories are allocated to carbs, which fuel your workouts.

Practical Examples

Example 1: Male Seeking Muscle Gain

A 25-year-old male, weighing 80kg at 185cm tall, who is moderately active and wants to build muscle.

• BMR: ~1830 kcal
• TDEE (Maintenance): 1830 kcal * 1.55 = ~2837 kcal
• Goal Calories (Lean Gain): 2837 + 300 = 3137 kcal
• Macros: Protein ~176g, Fats ~87g, Carbs ~412g

Example 2: Female Seeking Fat Loss

A 35-year-old female, weighing 70kg at 165cm tall, who is lightly active and wants to lose fat.

• BMR: ~1421 kcal
• TDEE (Maintenance): 1421 kcal * 1.375 = ~1954 kcal
• Goal Calories (Fat Loss): 1954 * 0.80 = 1563 kcal
• Macros: Protein ~154g, Fats ~43g, Carbs ~140g

For more specific tips, see our guide on fat loss tips.

How to Use This Built With Science Calculator

1. Select Your Units: Choose between Metric (kg, cm) or Imperial (lbs, in).
2. Enter Your Details: Input your gender, age, weight, and height.
3. Choose Activity Level: Select the option that best describes your weekly exercise routine. This is a crucial step for accuracy.
4. Set Your Goal: Decide whether you want to lose fat, maintain, or gain muscle. The calculator will adjust your calories accordingly.
5. Review Your Results: The calculator will instantly show your goal calories, maintenance (TDEE), and BMR. It also provides a full macronutrient breakdown in grams.
6. Use the Chart: The pie chart visualizes the percentage of your calories coming from protein, carbs, and fat, helping you understand your diet’s composition. Many people find a visual like this helpful, especially when learning about a bodybuilding diet.

Key Factors That Affect Your Calorie & Macro Needs

• Training Intensity: Harder, more frequent training requires more energy, increasing your TDEE. The chosen activity multiplier accounts for this.
• Lean Body Mass: Individuals with more muscle mass have a higher BMR than those with less, even at the same total weight. This calculator uses formulas that are reliable even without knowing your body fat percentage. If you know it, a calculator using the Katch-McArdle formula could be even more precise.
• Dietary Adherence: Your results depend on consistently hitting your calorie and macro targets. Using a food tracking app can be very helpful.
• Non-Exercise Activity Thermogenesis (NEAT): This is the energy expended for everything we do that is not sleeping, eating, or sports-like exercise. It includes walking, fidgeting, and even typing. A more active lifestyle outside the gym means a higher TDEE.
• Thermic Effect of Food (TEF): Your body burns calories to digest, absorb, and metabolize food. Protein has the highest TEF, meaning your body uses more energy to process it compared to carbs and fats.
• Age and Gender: Metabolism naturally slows with age, and men generally have a higher BMR than women due to differences in body composition. Our TDEE formula guide explains this in more detail.

Frequently Asked Questions (FAQ)

Q: How accurate is this Built With Science Calculator?
A: This calculator uses the Mifflin-St Jeor equation, which is highly regarded for its accuracy in predicting BMR. However, any calculator provides an estimate. The best approach is to use this as a starting point, monitor your weight for 2-3 weeks, and adjust your calorie intake up or down if your weight isn’t changing as desired.

Q: Should I recalculate my numbers after I lose weight?
A: Yes. As your body weight changes, so does your energy expenditure. It’s a good practice to recalculate your TDEE and macros after every 5-10 lbs (or 2-5 kg) of weight change to ensure your targets remain accurate.

Q: Why is protein so high?
A: A high protein intake (around 1.8-2.2g per kg of body weight) is crucial for preserving muscle mass during a fat loss phase and for building new muscle during a gaining phase. It also promotes satiety, helping you feel fuller for longer.

Q: Can I build muscle and lose fat at the same time?
A: This process, known as body recomposition, is possible, especially for beginners or individuals returning to training after a break. It typically requires eating at or slightly below maintenance calories with a high protein intake and a solid strength training program.

Q: What if I don’t track macros, only calories?
A: Tracking only calories can still lead to weight loss or gain. However, tracking macros ensures your body gets the right fuel for your goals. If you only track one thing besides calories, prioritize hitting your daily protein target.

Q: Does it matter what units I use?
A: No, the calculator automatically converts between Metric and Imperial systems. Just ensure you select the correct unit system and enter your weight and height in the corresponding units. The final calculations will be correct regardless.

Q: How does this compare to the official Jeremy Ethier program?
A: This calculator is based on the same scientific principles that are foundational to the Built With Science programs. It provides the caloric and macronutrient starting point. The full programs offer complete, adaptive workout and nutrition plans that evolve with your progress.

Q: What should I do on rest days?
A: For simplicity and consistency, it’s recommended to eat the same number of calories and macros every day, whether you train or not. Your body uses energy on rest days to recover and build muscle, so consistent nutrition is key.