Calorie Deficit Calculator

Calculate the calorie deficit needed to reach your weight loss goal. See how long it will take and find a safe, sustainable deficit range.

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Gender

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Age (years)

Height (cm)

Current Weight (kg)

Goal Weight (kg)

Activity Level

Target Rate of Loss

About Calorie Deficit Calculator

Understanding Calorie Deficit

A calorie deficit is the foundation of nearly every weight loss approach. It simply means consuming fewer calories than your body expends in a given day. Your body requires energy for everything it does — from breathing and circulating blood to walking and exercising. When you provide less fuel through food than your body demands, it makes up the difference by tapping into stored energy reserves, primarily body fat.

The concept is governed by the first law of thermodynamics: energy cannot be created or destroyed, only transferred. In practical terms, if your body needs 2,500 calories per day and you eat 2,000, the remaining 500 calories must come from somewhere — and your body will draw from fat stores, glycogen, and to a lesser extent, muscle tissue.

It's important to understand that not all deficits are created equal. A moderate deficit of 500 calories per day is sustainable for most people and leads to steady weight loss of roughly one pound per week. Larger deficits can accelerate weight loss but often come with trade-offs: increased hunger, fatigue, potential muscle loss, and a greater risk of metabolic adaptation. The goal is to find a deficit that is large enough to produce meaningful results without compromising your health or quality of life.

The Science of Weight Loss

Weight loss ultimately comes down to energy balance, but the biology behind it is more nuanced than a simple math equation. When you create a calorie deficit, your body goes through several metabolic processes to compensate for the reduced energy intake.

One pound of body fat stores approximately 3,500 calories of energy. This figure, while a useful approximation, is not perfectly linear over time. In the early stages of a deficit, you may lose weight faster than expected because your body also sheds water and glycogen. As your body adapts, the rate of loss tends to slow down.

Weekly Rate	Daily Deficit	Monthly Loss	Suitability
0.5 lb/week	250 cal	~2 lbs	Those close to goal weight
1 lb/week	500 cal	~4 lbs	Most people (recommended)
1.5 lbs/week	750 cal	~6 lbs	Those with significant weight to lose
2 lbs/week	1,000 cal	~8 lbs	Only with medical guidance

Your body composition also plays a role. People with more body fat to lose can generally tolerate larger deficits without losing muscle. As you get leaner, a smaller, more conservative deficit becomes important to preserve lean tissue and maintain hormonal balance.

The Mifflin-St Jeor Equation Explained

This calculator uses the Mifflin-St Jeor equation, widely considered the most accurate formula for estimating Basal Metabolic Rate (BMR) in healthy adults. Developed by MD Mifflin and ST St Jeor in 1990, it replaced the older Harris-Benedict equation as the preferred method.

Mifflin-St Jeor Formulas:

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

BMR represents the calories your body needs to perform its most basic life-sustaining functions: breathing, circulation, cell production, nutrient processing, and temperature regulation. It accounts for about 60 to 75 percent of your total daily energy expenditure. The remaining calories are burned through physical activity and the thermic effect of food (the energy required to digest what you eat).

The equation accounts for the key variables that influence metabolic rate. Weight has the largest impact because more body mass requires more energy to maintain. Height matters because taller individuals have greater surface area and more tissue. Age is a factor because metabolism naturally slows as you get older, primarily due to loss of lean muscle mass. The gender adjustment reflects the difference in average body composition between men and women.

Activity Levels and TDEE

Your Total Daily Energy Expenditure (TDEE) is calculated by multiplying your BMR by an activity factor. This gives you a more complete picture of how many calories you actually burn each day, including movement and exercise.

Level	Multiplier	Description
Sedentary	1.2	Desk job, little to no exercise
Lightly Active	1.375	Light exercise 1–3 days per week
Moderately Active	1.55	Moderate exercise 3–5 days per week
Very Active	1.725	Hard exercise 6–7 days per week
Extra Active	1.9	Very intense exercise and physical job

Choosing the right activity level is critical for accurate results. Most people overestimate their activity level, which leads to an inflated TDEE and a smaller actual deficit than intended. If you have a desk job and exercise three times a week for 30 to 45 minutes, you are likely "lightly active" rather than "moderately active." When in doubt, select a lower activity level and adjust upward if you find yourself losing weight too quickly.

Non-exercise activity thermogenesis (NEAT) — the calories burned through fidgeting, walking around the office, taking stairs, and other non-deliberate movements — can vary by as much as 2,000 calories per day between individuals. This is one reason why two people with identical stats can have very different calorie needs.

Safe Rates of Weight Loss

While it may be tempting to create a large deficit for faster results, safety should always come first. Health organizations generally recommend losing no more than 1 to 2 pounds per week for sustainable, healthy weight loss.

Minimum calorie intake for women: 1,200 calories per day. Going below this level without medical supervision increases the risk of nutrient deficiencies, gallstones, and hormonal disruption.
Minimum calorie intake for men: 1,500 calories per day. Men have higher lean mass on average and require more energy to maintain basic body functions.
Rapid weight loss risks: Muscle loss, hair thinning, weakened immune function, menstrual irregularities, fatigue, irritability, and potential for disordered eating patterns.
Exceptions: People with a very high body fat percentage may safely lose weight faster under medical supervision because their bodies have larger energy reserves to draw from.

Remember that the scale does not tell the whole story. Body composition changes — gaining muscle while losing fat — can sometimes mask progress. Taking measurements, progress photos, and monitoring how your clothes fit are often more reliable indicators of change than the number on the scale alone.

Metabolic Adaptation

One of the biggest challenges during extended weight loss is metabolic adaptation, sometimes called "adaptive thermogenesis." This is your body's natural response to prolonged calorie restriction, and understanding it can help you plan more effectively.

When you reduce your calorie intake over time, your body responds by becoming more energy-efficient. Your BMR decreases not just because you weigh less, but also because your body actively downregulates metabolic processes to conserve energy. Thyroid hormone production may decrease, NEAT tends to drop unconsciously, and your muscles may become more mechanically efficient during exercise.

Research from studies like the well-known "Biggest Loser" study has shown that metabolic adaptation can persist for years after significant weight loss. However, there are strategies to minimize its impact:

Diet breaks: Periodically returning to maintenance calories for 1 to 2 weeks can help restore hormonal balance and reduce adaptation.
Refeed days: Strategic higher-calorie days, particularly with increased carbohydrates, can temporarily boost leptin levels and metabolic rate.
Resistance training: Maintaining or building muscle through strength training keeps your metabolic rate higher than it would be otherwise.
Adequate protein: Consuming enough protein (0.7 to 1 gram per pound of body weight) helps preserve lean tissue and has a higher thermic effect than carbohydrates or fat.

Nutrition During a Deficit

When you are eating fewer calories, making each calorie count becomes even more important. Nutrient density — the amount of vitamins, minerals, and beneficial compounds per calorie — should be a priority.

A well-structured deficit should include adequate amounts of all three macronutrients, with particular attention to protein and micronutrient needs:

Macronutrient	Recommended Intake	Why It Matters
Protein	0.7–1 g per lb of body weight	Preserves muscle, increases satiety, highest thermic effect
Fat	20–35% of total calories	Hormone production, vitamin absorption, cell function
Carbohydrates	Remaining calories	Energy for exercise, brain function, fiber intake

Focus on whole, minimally processed foods: lean meats, fish, eggs, legumes, vegetables, fruits, whole grains, nuts, and seeds. These foods provide more satiety per calorie than processed alternatives and help ensure you're meeting your micronutrient needs. Consider a daily multivitamin as insurance when eating at a significant deficit, and pay special attention to iron, calcium, vitamin D, and B vitamins.

Common Mistakes and Tips

Even with a solid understanding of calorie deficits, there are common pitfalls that can derail your progress. Being aware of these can help you stay on track and achieve your goals more efficiently.

Underestimating calorie intake: Studies consistently show that people underestimate how much they eat by 20 to 50 percent. Cooking oils, sauces, dressings, and beverages are the most commonly overlooked sources. Use a food scale for accuracy, especially when starting out.
Overestimating exercise calories: Fitness trackers and gym machines often overestimate calorie burn by 15 to 30 percent. Don't rely on these numbers to justify eating more.
All-or-nothing thinking: One bad meal or even a bad week does not ruin your progress. Weight loss is a long-term trend, not a daily event. Consistency over time matters far more than perfection on any single day.
Ignoring sleep and stress: Poor sleep and chronic stress elevate cortisol levels, which can increase water retention, cravings, and fat storage — particularly around the midsection. Aim for 7 to 9 hours of sleep per night.
Skipping strength training: Cardio alone during a deficit leads to more muscle loss than combining it with resistance training. Lifting weights 2 to 4 times per week helps preserve the lean mass that keeps your metabolism running efficiently.
Not adjusting over time: As you lose weight, your TDEE decreases. Recalculate every 10 to 15 pounds lost to ensure your deficit is still appropriate. What worked at 200 pounds will not produce the same results at 170.

The most effective deficit is one you can maintain consistently. Choose a moderate approach, prioritize protein and whole foods, stay active, sleep well, and be patient. Weight loss is a marathon, not a sprint, and the habits you build during the process matter as much as the number on the scale.

Frequently Asked Questions

What is a calorie deficit and how does it cause weight loss?

A calorie deficit occurs when you consume fewer calories than your body burns. Your body needs a certain number of calories each day to maintain basic functions and fuel your activity. When you eat less than that amount, your body turns to stored energy — primarily body fat — to make up the difference. Since one pound of body fat stores roughly 3,500 calories, creating a consistent daily deficit gradually reduces your fat stores over time.

How much of a calorie deficit is safe?

Most health professionals recommend a deficit of 500 to 1,000 calories per day, which translates to losing about 1 to 2 pounds per week. Very aggressive deficits can lead to muscle loss, nutrient deficiencies, fatigue, and metabolic slowdown. Women should generally not consume fewer than 1,200 calories per day, and men should not go below 1,500 calories per day without medical supervision.

Why has my weight loss stalled even though I'm still in a deficit?

Weight loss plateaus are completely normal and happen for several reasons. As you lose weight, your body requires fewer calories to maintain itself, so the deficit that worked initially may no longer be sufficient. Your metabolism can also adapt to prolonged calorie restriction by becoming more efficient. Water retention, hormonal fluctuations, and increased muscle mass from exercise can all mask fat loss on the scale. Try recalculating your TDEE at your new weight, varying your calorie intake, or adjusting your activity level.

Should I eat back the calories I burn through exercise?

It depends on your goals and how your deficit is structured. If your calorie target already accounts for your activity level through the TDEE calculation, you generally don't need to eat back exercise calories. However, if you perform an unusually intense workout on top of your normal activity, eating back a portion of those calories can help prevent excessive fatigue and muscle loss. A common guideline is to eat back about half of estimated exercise calories, since calorie burn estimates from fitness trackers tend to overestimate.

What is the Mifflin-St Jeor equation and why is it used?

The Mifflin-St Jeor equation is a formula developed in 1990 to estimate your Basal Metabolic Rate (BMR) — the number of calories your body burns at rest. It uses your weight, height, age, and gender to produce an estimate. Research has shown it to be one of the most accurate predictive equations for BMR in healthy adults, which is why it's widely recommended by dietitians and health organizations over older formulas like the Harris-Benedict equation.

Can I lose weight without counting calories?

Yes, many people lose weight successfully without tracking every calorie. Strategies like eating more whole foods, increasing protein and fiber intake, reducing processed food consumption, practicing mindful eating, and controlling portion sizes can naturally lead to a calorie deficit. However, calorie counting provides a more precise framework, especially if you've hit a plateau or want to reach a specific goal within a defined time frame.

How does muscle mass affect my calorie needs?

Muscle tissue is more metabolically active than fat tissue, meaning it burns more calories at rest. People with more muscle mass tend to have a higher BMR and TDEE. This is one reason why strength training is recommended during weight loss — it helps preserve muscle, keeping your metabolism higher. Losing muscle during a deficit can lower your metabolic rate and make it harder to continue losing weight.

Additional Resources

NIH: Adult Overweight & Obesity Information Mayo Clinic: Counting Calories for Weight Loss Harvard T.H. Chan: Healthy Weight Guide

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