Built With Science Calculator
Your evidence-based tool for calculating daily calories and macronutrients.
Enter your age in years.
Enter your current body weight.
Enter your height.
Your Target Calories & Macros
This is your estimated daily calorie target to reach your goal.
This is your TDEE (Total Daily Energy Expenditure).
What is a Built With Science Calculator?
A builtwithscience calculator is a tool designed to apply scientific principles to fitness and nutrition. Unlike generic calorie counters, this calculator is based on the evidence-based methodologies popularized by Jeremy Ethier and the Built With Science platform. It helps you determine your specific daily calorie and macronutrient (protein, carbs, and fat) needs based on your unique body metrics, activity level, and primary fitness goal.
This calculator is for anyone serious about achieving their fitness goals—whether it’s losing fat, building muscle, or maintaining their current physique. It removes the guesswork by providing a science-based starting point for your diet. A common misunderstanding is that all calories are equal; however, the source of those calories (macros) is crucial for body composition, which this calculator helps you optimize.
The Built With Science Calculator Formula and Explanation
The calculator works in a sequence of calculations to determine your needs. First, it calculates your Basal Metabolic Rate (BMR), then your Total Daily Energy Expenditure (TDEE), and finally adjusts for your goals to find your target macros.
- Basal Metabolic Rate (BMR): This is the energy your body burns at complete rest. We use the Mifflin-St Jeor equation, which is widely considered the most accurate formula.
- Men: BMR = 10 * weight (kg) + 6.25 * height (cm) – 5 * age + 5
- Women: BMR = 10 * weight (kg) + 6.25 * height (cm) – 5 * age – 161
- Total Daily Energy Expenditure (TDEE): This is your BMR multiplied by an activity factor. It represents your total “maintenance” calories. For a deep dive, see our fat loss fundamentals guide.
- Goal Adjustment: Your TDEE is adjusted based on your goal: a calorie deficit for fat loss (e.g., -500 calories) or a surplus for muscle gain (e.g., +300 calories).
- Macronutrient Split:
- Protein: Set at a science-based level for muscle repair and growth, typically around 2.0g per kg of bodyweight.
- Fat: Set to about 25% of total goal calories for hormonal health.
- Carbohydrates: The remaining calories are allocated to carbs to fuel your workouts.
Variables Table
| Variable | Meaning | Unit (Auto-Inferred) | Typical Range |
|---|---|---|---|
| Age | Your current age | Years | 18 – 80 |
| Weight | Your body weight | kg or lbs | 40 – 150 kg |
| Height | Your body height | cm or in | 140 – 220 cm |
| Activity Factor | Your daily physical activity level | Multiplier | 1.2 – 1.9 |
| Goal Adjustment | Calorie adjustment for goal | Calories | -500 to +300 |
Practical Examples
Example 1: Fat Loss Goal
Consider a 35-year-old male who weighs 90kg (198 lbs), is 183cm (6 ft) tall, and is lightly active.
- Inputs: Male, 35 years, 90kg, 183cm, Lightly Active, Lose Fat goal.
- TDEE (Maintenance): Approx. 2484 kcal.
- Results (Fat Loss Target): Approx. 1984 kcal/day.
- Protein: ~180g
- Fat: ~55g
- Carbs: ~194g
Example 2: Muscle Gain Goal
A 25-year-old female who weighs 60kg (132 lbs), is 165cm (5′ 5″) tall, and is moderately active wants to build muscle. Understanding the progressive overload guide is key here.
- Inputs: Female, 25 years, 60kg, 165cm, Moderately Active, Build Muscle goal.
- TDEE (Maintenance): Approx. 2004 kcal.
- Results (Muscle Gain Target): Approx. 2304 kcal/day.
- Protein: ~120g
- Fat: ~64g
- Carbs: ~311g
How to Use This Built With Science Calculator
Using the calculator is a straightforward process designed to give you actionable numbers quickly.
- Select Units: Start by choosing your preferred measurement system: Metric (kg, cm) or Imperial (lbs, in). The calculator will convert everything for you.
- Enter Personal Details: Input your gender, age, weight, and height. Be as accurate as possible.
- Choose Activity Level: Be honest about your daily activity. This is one of the most significant variables. If you’re unsure, it’s often better to underestimate slightly.
- Set Your Goal: Select whether you want to lose fat, maintain your weight, or build muscle. This applies the correct calorie surplus or deficit.
- Interpret Results: The calculator instantly displays your target daily calories (primary result), your maintenance calories, and your macronutrient breakdown in grams. Use these numbers as the starting point for your nutrition plan. Check out our guide on nutrition for fitness for more info.
Key Factors That Affect Your Results
Several factors influence your calorie and macro needs. Understanding them helps you use this builtwithscience calculator more effectively.
- Body Composition: An individual with more muscle mass will have a higher BMR than someone of the same weight with more body fat.
- Age: Metabolic rate naturally slows as we age, reducing calorie needs.
- Genetics: Some individuals have naturally faster or slower metabolisms.
- Workout Intensity: The “Hard Exercise” of a professional athlete is different from that of a casual gym-goer. Your actual energy expenditure can vary.
- Non-Exercise Activity Thermogenesis (NEAT): This includes all the calories you burn from activities that aren’t formal exercise, like walking, fidgeting, or doing chores. It can have a huge impact.
- Diet History: If you’ve been in a calorie deficit for a long time, your metabolism may have adapted downwards, which this calculator can’t account for initially. See our information on best workout splits to optimize your training.
Frequently Asked Questions (FAQ)
It’s very accurate as a starting point. It uses the Mifflin-St Jeor equation, which is scientifically validated. However, any calculator provides an estimate. The best approach is to use these numbers for 2-3 weeks, monitor your weight, and adjust your intake up or down as needed.
The formulas require metric units (kg and cm) for accuracy. This calculator automatically converts from imperial (lbs and inches) behind the scenes, so you just need to select the correct unit system and enter your numbers.
Evidence-based guidelines suggest a higher protein intake (around 1.6-2.2g per kg of bodyweight) is optimal for building and preserving muscle mass, especially during a fat loss phase. It also promotes satiety.
A plateau is normal. It usually means your TDEE has decreased as your body weight has changed. Simply re-run your numbers in the calculator with your new weight to get updated targets, or slightly decrease your calories (by ~100) to restart progress.
While flexible dieting (IIFYM) works, it’s crucial to prioritize nutrient-dense, whole foods for overall health, micronutrients, and fiber. A diet based on 80% whole foods and 20% “fun” foods is a sustainable approach.
Different calculators may use different formulas (e.g., Harris-Benedict vs. Mifflin-St Jeor) or different activity multipliers. Our calculator uses the most current, evidence-based standards to align with the Built With Science philosophy.
A good rule of thumb is to recalculate your numbers after every 5-10 lbs (or 2-5 kg) of weight change, or whenever you hit a consistent two-week plateau.
Be honest and conservative. If you work a desk job and go to the gym 3 times a week for an hour, you are “Lightly Active,” not “Moderately Active.” Most people overestimate their activity level.