Calorie Deficit Calculator
What is a calorie deficit?
A calorie deficit occurs when you consume fewer calories than your body burns throughout the day. Your body requires energy for everything it does, from breathing and circulating blood to walking and exercising. This total energy requirement is known as your Total Daily Energy Expenditure (TDEE).
When you consistently eat fewer calories than your TDEE, your body turns to stored energy (primarily body fat) to make up the difference. This is the fundamental mechanism behind all weight loss, regardless of which diet you follow. Whether it's low-carb, keto, intermittent fasting, or simply eating smaller portions, every effective weight loss approach works because it creates a calorie deficit.
For example, if your body burns 2,500 calories per day and you eat 2,000 calories, you’re in a 500-calorie deficit. Over a week, that totals 3,500 calories—roughly equivalent to half a kilogram (about one pound) of body fat.
How does our calorie deficit calculator work?
Our calculator uses the Mifflin-St Jeor equation, which research has consistently shown to be the most accurate formula for estimating calorie needs in healthy adults. Here’s the three-step process it follows:
Step 1: Calculate your Basal Metabolic Rate (BMR)
Your BMR is the number of calories your body needs at complete rest—simply to keep your organs functioning, maintain body temperature, and support cellular processes. It typically accounts for 60–75% of your total daily calorie expenditure.
Mifflin-St Jeor Equations
Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
Step 2: Determine your Total Daily Energy Expenditure (TDEE)
Your TDEE represents the total number of calories you burn each day, including all physical activity. We calculate it by multiplying your BMR by an activity factor that reflects your lifestyle:
Step 3: Set your calorie deficit
Once you know your TDEE, you subtract a set number of calories to create your deficit. The size of your deficit determines how quickly you’ll lose weight:
We recommend starting with a moderate deficit of 500 calories per day. This strikes the best balance between noticeable progress and long-term sustainability.
Understanding your results
Basal Metabolic Rate (BMR)
Your BMR is the foundation of your calorie calculations. It represents the minimum energy your body needs to function at rest. Factors that influence your BMR include your age, biological sex, weight, and height. Generally, men have a higher BMR than women due to greater muscle mass, and BMR naturally decreases with age.
Total Daily Energy Expenditure (TDEE)
Your TDEE is the total number of calories you burn in a day, factoring in all physical activity. This is the most important number for weight management—eating below your TDEE leads to weight loss, eating at your TDEE maintains your weight, and eating above it leads to weight gain.
Body Mass Index (BMI)
BMI is a simple screening tool that compares your weight to your height. While it doesn’t directly measure body fat, it provides a useful general indication of whether you’re in a healthy weight range. The standard categories are: underweight (below 18.5), healthy weight (18.5–24.9), overweight (25–29.9), and obese (30 and above).
Minimum safe calorie intake
Our calculator will never recommend a calorie intake below established safety thresholds. The NHS and most health organisations recommend that women eat no fewer than 1,200 calories per day and men eat no fewer than 1,500 calories per day without medical supervision. Eating below these levels can lead to nutritional deficiencies, muscle loss, and other health complications.
Tips for maintaining a healthy calorie deficit
Start with a moderate deficit
Jumping straight into an aggressive deficit is a common mistake. Start with 500 calories below your TDEE and adjust based on your progress after 2–3 weeks.
Prioritise protein intake
Aim for 1.6–2.2g of protein per kilogram of body weight. Protein helps preserve muscle mass during weight loss and keeps you feeling fuller for longer.
Track your progress weekly
Weigh yourself at the same time each week under the same conditions. Daily fluctuations are normal—it’s the weekly trend that matters.
Don’t skip meals
Skipping meals often leads to overeating later in the day. Spread your calories across regular meals and snacks to maintain steady energy levels.
Stay hydrated
Drinking enough water supports your metabolism and helps distinguish genuine hunger from thirst. Aim for 6–8 glasses of water per day as recommended by the NHS.
Be patient and consistent
Sustainable weight loss takes time. Losing 0.5–1 kg per week may not feel dramatic, but over 6 months that’s 13–26 kg—a life-changing amount.
When to consider medical weight loss
For some people, a calorie deficit through diet and exercise alone may not deliver the results they need. If your BMI is 30 or above (or 27 and above with weight-related health conditions), you may be eligible for prescription weight loss medication.
Clinically proven GLP-1 medications work by reducing appetite and helping you feel full sooner, making it easier to maintain a healthy calorie deficit. When combined with lifestyle changes, patients can lose up to 20% of their body weight.
Doctor-led treatment
Every prescription reviewed by UK-registered doctors
Clinically proven medications
GLP-1 treatments backed by extensive clinical trials
Personalised support
Ongoing care and guidance throughout your journey
Frequently asked questions
Calorie deficit
A moderate calorie deficit of 500 calories per day is widely recommended by health professionals. This typically results in a weight loss of about 0.5 kg (1 lb) per week, which is considered a safe and sustainable rate. Deficits greater than 750 calories per day are generally not recommended without medical supervision.
To lose 1 kg per week, you need a weekly deficit of approximately 7,700 calories, which works out to about 1,100 calories per day below your TDEE. However, this is a relatively aggressive deficit. For most people, aiming for 0.5 kg per week (a 500 calorie daily deficit) is safer and more sustainable long-term.
A 1,000 calorie daily deficit is considered aggressive and may not be safe for everyone. It can lead to muscle loss, nutritional deficiencies, fatigue, and metabolic slowdown. The NHS recommends that women should not eat fewer than 1,200 calories per day and men should not eat fewer than 1,500 calories per day without medical supervision.
With a moderate 500-calorie daily deficit, you can expect to lose about 0.5 kg (1 lb) per week. This means losing 5 kg would take roughly 10 weeks. However, weight loss is rarely linear—you may lose more initially (often water weight) and then the rate may slow. Consistency over time is more important than speed.
This depends on your approach. If your calorie target already accounts for your activity level (as our calculator does via the TDEE), you generally don’t need to eat back exercise calories. However, if you do additional exercise beyond your selected activity level, eating back a portion (about 50–75%) of those calories can help maintain energy and prevent excessive deficit.
Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest—just to keep your organs functioning, maintain body temperature, and support basic cellular processes. It typically accounts for 60–75% of your total daily calorie burn. Knowing your BMR helps you understand the minimum energy your body needs and ensures you don’t eat below a safe threshold.
Track your weight weekly (at the same time, under the same conditions) rather than daily, as weight naturally fluctuates. A consistent downward trend over 2–4 weeks indicates your deficit is working. Other signs include clothes fitting more loosely, improved energy levels, and better sleep. If you’re not seeing results after 3–4 weeks, you may need to reassess your calorie intake or activity level.
Yes, prolonged or extreme calorie restriction can cause metabolic adaptation, where your body becomes more efficient at using energy and burns fewer calories. This is why moderate deficits (500 cal/day) are preferred over extreme ones. Maintaining adequate protein intake, resistance training, and taking periodic diet breaks can help minimise metabolic slowdown.