3 Day Fast Weight Loss Calculator

Estimate Potential Calorie Deficit and Weight Loss Over 72 Hours

Your 3 Day Fast Weight Loss Estimate

Your Estimated Results

Key Assumptions

Fasting Duration: 3 days (72 hours)

Calorie Equivalence: 1 lb of fat ≈ 3500 kcal

Initial Weight Loss Composition: Assumes approximately 70% water loss and 30% fat loss for initial rapid drop.

Formula Explanation:
1. Calculate Basal Metabolic Rate (BMR) using the Mifflin-St Jeor equation.
2. Estimate Total Daily Energy Expenditure (TDEE) by multiplying BMR by the activity level factor.
3. Calculate Total Calorie Deficit: (TDEE * 3 days) – (Average Daily Calorie Intake * 3 days).
4. Calculate Estimated Fat Loss: Total Calorie Deficit * 0.30 / 3500.
5. Calculate Estimated Water Loss: (Starting Weight – Target Weight) – Estimated Fat Loss.
Note: Initial rapid weight loss is heavily influenced by water and glycogen depletion, not solely fat.

Chart showing estimated cumulative calorie deficit and weight loss over the 3 days.

3 Day Fast Weight Loss Breakdown

Metric	Value	Unit
Starting Weight	—	lbs
Target Weight	—	lbs
Weight Lost	—	lbs
Estimated Fat Loss	—	lbs
Estimated Water Loss	—	lbs
Total Calorie Deficit	—	kcal
Average Daily Deficit	—	kcal/day

What is 3 Day Fast Weight Loss?

The concept of a "3 day fast weight loss" refers to a short-term, intensive dietary intervention typically involving a period of 72 hours with minimal or no caloric intake, designed to induce rapid weight reduction. This method is often pursued for quick results, sometimes before an event or as an initial kick-start to a more comprehensive weight management program. It's crucial to understand that the majority of weight lost during such a short fast is not solely fat but also includes water and depleted glycogen stores. While it can lead to a noticeable drop on the scale, it is not a sustainable or healthy long-term strategy for fat loss. The 3 day fast weight loss is a temporary measure, and rapid re-gain is common once normal eating resumes. It's essential to approach this type of rapid weight reduction with caution and awareness of its limitations.

Who should use it: Individuals considering a 3 day fast weight loss should ideally be healthy and free from underlying medical conditions. It might be considered by those seeking a very short-term, dramatic scale shift, understanding that it's primarily water loss. However, it is generally NOT recommended for individuals with a history of eating disorders, pregnant or breastfeeding women, those with chronic illnesses like diabetes, or individuals who are underweight. Always consult with a healthcare professional before undertaking any extreme dietary changes like a 3 day fast weight loss.

Common Misconceptions: A primary misconception about the 3 day fast weight loss is that all the lost weight is fat. In reality, a significant portion is water weight due to reduced carbohydrate intake (glycogen stores bind water) and potential fluid loss. Another misconception is that this method leads to lasting fat loss; without subsequent lifestyle changes, the lost weight, especially water, is quickly regained. Finally, some believe it's a healthy way to "detox," a claim not supported by scientific evidence for short-term fasting.

3 Day Fast Weight Loss: Formula and Mathematical Explanation

Understanding the mathematics behind the 3 day fast weight loss involves estimating energy expenditure versus intake, alongside physiological responses like water retention. We can approximate the potential weight loss using several formulas.

Basal Metabolic Rate (BMR) Calculation

The Mifflin-St Jeor equation is widely considered more accurate for estimating BMR:

For men: BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5
For women: BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161

We first convert inputs:

Weight in kg = Weight in lbs / 2.20462
Height in cm = (Height in feet × 12 + Height in inches) × 2.54

Total Daily Energy Expenditure (TDEE)

TDEE is calculated by multiplying BMR by an activity factor:

TDEE = BMR × Activity Level (METs)

Note: For simplicity and a more conservative estimate in a fasting context, we use the MET value directly as a multiplier, assuming the MET value represents the average daily expenditure relative to BMR.

Calorie Deficit and Weight Loss Estimation

The core of 3 day fast weight loss calculation is the calorie deficit.

Total Calorie Deficit over 3 days = (TDEE × 3) – (Average Daily Calorie Intake × 3)

We know that approximately 3500 calories equal one pound of fat. However, rapid weight loss in the initial days includes significant water loss. A common estimate is that initial rapid loss might consist of up to 70% water and 30% fat.

Estimated Fat Loss = (Total Calorie Deficit × 0.30) / 3500

Estimated Water Loss = Total Weight Lost – Estimated Fat Loss

Total Weight Lost = Starting Weight – Target Weight

Variables Table

Variables Used in 3 Day Fast Weight Loss Calculation

Variable	Meaning	Unit	Typical Range
Starting Weight	Initial body weight before the fast.	lbs	50 – 500+
Target Weight	Desired body weight after the 3-day fast.	lbs	Starting Weight – 1 to 10+
Daily Calorie Intake	Average calories consumed per day during the fast.	kcal	0 – 1500 (typically very low or zero)
Daily Activity Level (METs)	Energy expenditure multiplier based on physical activity.	Unitless	1.2 (sedentary) – 2.0+ (moderate activity)
Height	Individual's height.	Feet/Inches	3'0″ – 7'0″
Age	Individual's age.	Years	18 – 80+
Gender	Biological sex.	Male/Female	N/A
BMR	Calories burned at rest.	kcal/day	~1200 – 2500+
TDEE	Total calories burned per day including activity.	kcal/day	~1440 – 5000+
Calorie Deficit	Difference between calories burned and consumed.	kcal	Significant positive value

Practical Examples

Let's illustrate the 3 day fast weight loss calculator with two distinct scenarios.

Example 1: Moderately Active Woman

Sarah is a 30-year-old female, 5'8″ tall, weighing 160 lbs. She plans a 3-day fast, consuming only about 800 kcal per day, and considers herself moderately active (METs = 1.7). She aims to reach 157 lbs after 3 days.

• Starting Weight: 160 lbs
• Target Weight: 157 lbs
• Daily Calorie Intake: 800 kcal
• Height: 5'8″ (68 inches)
• Age: 30
• Gender: Female
• Activity Level: 1.7

Calculation Steps:

1. Convert to metric: 160 lbs ≈ 72.57 kg; 68 inches ≈ 172.72 cm.
2. Calculate BMR (Female): (10 × 72.57) + (6.25 × 172.72) – (5 × 30) – 161 ≈ 725.7 + 1079.5 – 150 – 161 ≈ 1594 kcal/day.
3. Calculate TDEE: 1594 × 1.7 ≈ 2709.8 kcal/day.
4. Total Calorie Deficit (3 days): (2709.8 × 3) – (800 × 3) ≈ 8129.4 – 2400 ≈ 5729.4 kcal.
5. Estimated Fat Loss: (5729.4 × 0.30) / 3500 ≈ 1718.8 / 3500 ≈ 0.49 lbs.
6. Total Weight Lost: 160 lbs – 157 lbs = 3 lbs.
7. Estimated Water Loss: 3 lbs – 0.49 lbs ≈ 2.51 lbs.

Interpretation: Sarah's 3 day fast weight loss is estimated at 3 lbs. Of this, roughly 0.5 lbs is likely fat, and 2.5 lbs is water/glycogen. The substantial calorie deficit drives the scale change, primarily through fluid reduction. This aligns with typical expectations for such a short period.

Example 2: Sedentary Male

John is a 45-year-old male, 6'0″ tall, weighing 220 lbs. He decides to do a strict 3-day water fast (0 kcal intake) and has a very sedentary lifestyle (METs = 1.3). His goal is to reach 215 lbs.

• Starting Weight: 220 lbs
• Target Weight: 215 lbs
• Daily Calorie Intake: 0 kcal
• Height: 6'0″ (72 inches)
• Age: 45
• Gender: Male
• Activity Level: 1.3

Calculation Steps:

1. Convert to metric: 220 lbs ≈ 99.79 kg; 72 inches ≈ 182.88 cm.
2. Calculate BMR (Male): (10 × 99.79) + (6.25 × 182.88) – (5 × 45) + 5 ≈ 997.9 + 1143 – 225 + 5 ≈ 1921 kcal/day.
3. Calculate TDEE: 1921 × 1.3 ≈ 2497.3 kcal/day.
4. Total Calorie Deficit (3 days): (2497.3 × 3) – (0 × 3) ≈ 7491.9 – 0 ≈ 7491.9 kcal.
5. Estimated Fat Loss: (7491.9 × 0.30) / 3500 ≈ 2247.6 / 3500 ≈ 0.64 lbs.
6. Total Weight Lost: 220 lbs – 215 lbs = 5 lbs.
7. Estimated Water Loss: 5 lbs – 0.64 lbs ≈ 4.36 lbs.

Interpretation: John's estimated 3 day fast weight loss is 5 lbs. With zero calorie intake, the deficit is maximized. Approximately 0.64 lbs is fat loss, while the majority (4.36 lbs) is water and glycogen. This demonstrates how the body reacts to extreme fasting, shedding water rapidly before significant fat mobilization occurs.

How to Use This 3 Day Fast Weight Loss Calculator

1. Enter Starting Weight: Input your current weight in pounds (lbs).
2. Enter Target Weight: Specify the weight you hope to achieve after the 3-day period in pounds (lbs). This should be a realistic reduction, acknowledging that rapid loss includes water.
3. Estimate Daily Calorie Intake: If you plan to consume any calories, enter the average amount per day (kcal). For a water fast, this would be 0.
4. Set Daily Activity Level: Choose a number representing your average daily physical activity using the METs scale. A sedentary lifestyle is around 1.2, light activity 1.5, moderate 1.7-2.0, and high activity 2.1+.
5. Input Height: Enter your height in feet and inches separately.
6. Input Age: Provide your current age in years.
7. Select Gender: Choose 'Male' or 'Female' for BMR calculation accuracy.
8. Click 'Calculate': The calculator will instantly provide your estimated results.

Reading Your Results:

• Main Result (Weight Lost): This is the total pounds you are estimated to lose.
• Total Calorie Deficit: The total number of calories your body is estimated to burn beyond what you consume over the 3 days.
• Estimated Fat Loss: The portion of weight loss attributed to actual body fat.
• Estimated Water Loss: The portion of weight loss attributed to water and glycogen depletion.
• BMR & TDEE: Your estimated resting and total daily energy expenditures, providing context for the deficit.
• Chart & Table: Visualize the deficit progression and see a breakdown of all key metrics.

Decision-Making Guidance: Use these estimates to understand the physiological impact of a short fast. Remember that rapid rehydration and food intake will likely reverse much of the water weight. This tool is best used for informational purposes to gauge potential immediate scale changes, not as a plan for sustainable fat loss. For long-term goals, focus on balanced nutrition and consistent exercise.

Key Factors That Affect 3 Day Fast Weight Loss Results

Several variables significantly influence the outcome of a 3 day fast weight loss, extending beyond simple calorie math. Understanding these is key to interpreting your results realistically.

• Initial Hydration Levels: Dehydration can skew weight readings. Furthermore, the body's initial response to a fast includes shedding excess water, contributing significantly to the early scale drop. Higher initial water retention can lead to a larger apparent weight loss in the first 72 hours.
• Glycogen Stores: Carbohydrates are stored in the muscles and liver as glycogen, which binds to water (roughly 3-4 grams of water per gram of glycogen). When you fast or drastically reduce carbs, these stores are depleted, leading to a rapid loss of both glycogen and associated water weight. The fuller your glycogen stores are at the start, the more water weight you stand to lose quickly.
• Metabolic Adaptation: While a 3-day fast is short, the body may begin to slightly slow its metabolism as an energy-saving response. This calculator uses a standard TDEE estimate, but individual metabolic rates can vary and adapt, potentially reducing the overall calorie deficit slightly over time.
• Muscle Mass: Individuals with higher muscle mass generally have a higher BMR and TDEE, meaning they burn more calories at rest and during activity. This can lead to a larger calorie deficit and potentially more fat loss compared to someone with less muscle mass, assuming similar dietary and activity levels.
• Hormonal Responses: Fasting triggers hormonal shifts, including changes in insulin, glucagon, and stress hormones like cortisol. These can affect appetite, energy levels, and how the body utilizes stored energy. Cortisol, in particular, can sometimes influence water retention.
• Accuracy of Input Data: The calculator relies on accurate inputs for weight, height, age, activity level, and calorie intake. Inaccurate estimations, especially for activity levels or calorie consumption, will lead to less precise results for the 3 day fast weight loss. Even slight overestimation of TDEE or underestimation of intake can inflate the projected deficit.
• Digestive Tract Contents: The weight of undigested food and waste in the digestive system also contributes to the initial "weight loss." This is temporary and not indicative of fat or water loss from the body's tissues.

Frequently Asked Questions (FAQ)

Q1: Is a 3 day fast weight loss safe?
A: For most healthy adults, a 3-day fast is generally considered safe when properly hydrated. However, it's not suitable for everyone. Individuals with certain medical conditions (like diabetes, kidney disease), pregnant or breastfeeding women, those with a history of eating disorders, or people on specific medications should consult a healthcare provider before attempting it.

Q2: Will I lose only fat during a 3 day fast?
A: No, it's highly unlikely. The initial rapid weight loss observed during a 3 day fast primarily consists of water weight (due to glycogen depletion) and some initial fat mobilization. Actual fat loss is usually a smaller percentage compared to water loss in such a short timeframe.

Q3: How much weight can I realistically expect to lose?
A: This varies greatly. Factors like starting weight, body composition, hydration, and activity levels play a role. Typically, individuals might see a loss of 2-6 lbs over 3 days, with a significant portion being water. Our calculator provides an estimate based on your inputs.

Q4: What should I do after the 3 day fast?
A: It's crucial to reintroduce food gradually. Start with small, easily digestible meals like broths, steamed vegetables, and small portions of lean protein. Avoid heavy, processed, or sugary foods immediately, as they can cause digestive distress and rapid weight re-gain. Transitioning to a balanced, sustainable diet is key for long-term results.

Q5: Can I drink water during a 3 day fast?
A: Yes, staying hydrated is essential. Drinking plenty of water is recommended during fasting to prevent dehydration and support bodily functions. You may also consume black coffee or plain tea without sugar or milk.

Q6: Is this calculator accurate for long-term weight loss?
A: No, this 3 day fast weight loss calculator is designed specifically for short-term, rapid weight changes associated with fasting. It does not accurately predict sustainable, long-term fat loss, which requires consistent caloric deficits over weeks and months through balanced diet and exercise.

Q7: What are the risks of prolonged fasting beyond 3 days?
A: Longer fasts (beyond 3-5 days) carry increased risks, including nutrient deficiencies, electrolyte imbalances, muscle loss, dizziness, fatigue, and potential impacts on organ function. Medical supervision is strongly advised for fasts exceeding a few days.

Q8: How does activity level impact 3 day fast weight loss calculations?
A: Higher activity levels increase your Total Daily Energy Expenditure (TDEE), meaning you burn more calories. This results in a larger calorie deficit for the same dietary intake (or lack thereof), potentially leading to more significant weight loss, primarily driven by both water and fat loss over the 3 days.