Last updated: June 2025 | 8 min read
Losing weight comes down to one principle: burn more calories than you consume. This gap is called a calorie deficit and knowing yours is the first step to losing 1kg per week safely and sustainably.
In this guide, you’ll learn exactly how calorie deficits work, how to calculate yours, and how to apply it in real life using NHS-aligned guidelines.
Table of Contents
What Is a Calorie Deficit?
A calorie deficit occurs when you consume fewer calories than your body burns in a day. When this happens, your body draws on stored body fat as its alternative fuel source which is what causes you to lose weight over time.
There are two numbers you need to understand before you can calculate your deficit:
Basal Metabolic Rate (BMR) the number of calories your body burns at complete rest, just to keep you alive. This accounts for roughly 60–75% of your total daily calorie burn and is driven by your age, height, weight, and sex.
Total Daily Energy Expenditure (TDEE) your BMR multiplied by an activity factor. This is the total number of calories your body actually burns on a typical day, including all movement and exercise.
Your TDEE is your maintenance calorie level. Eat at your TDEE and your weight stays the same. Eat below it and you lose weight. Eat above it and you gain.
How to Calculate Your Calorie Deficit
The most accurate way to estimate BMR is the Mifflin–St Jeor equation, which is the formula used by most NHS-aligned dietary tools:
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
Once you have your BMR, multiply it by your activity level:
| Activity Level | Description | Multiplier |
|---|---|---|
| Sedentary | Little or no exercise, desk job | × 1.2 |
| Lightly Active | Light exercise 1–3 days/week | × 1.375 |
| Moderately Active | Moderate exercise 3–5 days/week | × 1.55 |
| Very Active | Hard exercise 6–7 days/week | × 1.725 |
| Extra Active | Very hard exercise or physical job | × 1.9 |
The result is your TDEE your personal maintenance calorie number.
Example: A 35-year-old woman, 165cm, 75kg, moderately active:
- BMR = (10 × 75) + (6.25 × 165) − (5 × 35) − 161 = 1,476 kcal
- TDEE = 1,476 × 1.55 = 2,288 kcal/day
- To lose 0.5kg/week: eat 2,288 − 500 = 1,788 kcal/day
- To lose 1kg/week: eat 2,288 − 1,100 = 1,188 kcal/day
💡 Use our free Calorie Deficit Calculator to get your personalised numbers instantly no maths required.
The 7,700 Calorie Rule for Losing 1kg Per Week
One kilogram of body fat contains approximately 7,700 kilocalories of stored energy. To lose 1kg of fat in a week, you need to create a total weekly deficit of 7,700 kcal.
The maths:
- 7,700 kcal ÷ 7 days = 1,100 kcal deficit per day
So if your TDEE is 2,500 kcal/day, you would need to eat around 1,400 kcal/day to lose approximately 1kg per week.
Important caveats:
The 7,700 rule is an estimate. Real weight loss is messier it involves water retention, glycogen depletion in the first week, and gradual metabolic adaptation. You may see a larger drop in week one (mostly water), followed by more consistent fat loss from week two onwards.
As you lose weight, your BMR decreases because your body is lighter. This is the most common reason people hit a weight loss plateau — their TDEE has fallen, shrinking their deficit without them realising. Recalculate your numbers every 4–6 weeks to stay on track.
Is Losing 1kg Per Week Safe?
Losing 1kg (2.2 lbs) per week is at the upper end of what health professionals generally recommend.
The NHS advises aiming for 0.5–1kg per week as a safe and sustainable rate for most adults. Faster weight loss is possible but increases the risk of:
- Loss of muscle mass alongside fat
- Nutritional deficiencies (iron, vitamin D, B12, calcium)
- Persistent fatigue and poor concentration
- Hormonal disruption, especially in women
- Rebound weight gain after stopping the diet
Who should aim for 0.5kg/week instead:
- People already close to a healthy BMI
- Anyone with a history of disordered eating
- Those managing a chronic health condition
- Anyone whose 1kg/week target would require eating below 1,200 kcal/day (women) or 1,500 kcal/day (men)
⚠️ Warning signs your deficit is too aggressive: persistent fatigue, hair loss, feeling cold all the time, irritability, irregular periods, or obsessive thoughts about food. If you experience these, increase your calorie intake and speak to your GP.
Always consult your GP or a registered dietitian before starting a significant calorie restriction programme, particularly if you have underlying health conditions.
Diet vs Exercise: Which Creates a Better Deficit?
The short answer: both, combined, produce the best results.
A calorie deficit can be created by eating less, moving more, or a combination. Research consistently shows that combining dietary changes with physical activity leads to better long-term weight loss and improved body composition compared to diet alone.
Why exercise matters even though “you can’t out-exercise a bad diet”:
- A 45-minute brisk walk burns roughly 200–300 kcal giving you more food budget while maintaining the same deficit
- Resistance training preserves muscle mass during a deficit, keeping your metabolism higher
- Exercise improves sleep quality, reduces stress, and helps regulate appetite hormones
Step-by-step approach:
- Calculate your TDEE using the formula above or our calculator
- Set your daily calorie target (TDEE minus 500–1,100 kcal depending on your goal)
- Track your food intake for the first 2–4 weeks using an app like MyFitnessPal or Cronometer studies show self-monitoring significantly improves outcomes
- Add exercise to increase your daily calorie burn and give yourself more dietary flexibility
- Recalculate every 4–6 weeks as your weight changes
Macronutrients on a Calorie Deficit
While total calories drive fat loss, what you eat affects how you feel, how much muscle you preserve, and how sustainable the diet is.
| Macronutrient | Recommended Split | Calories per Gram | Why It Matters |
|---|---|---|---|
| Protein | 30–35% of calories | 4 kcal/g | Preserves muscle, reduces hunger |
| Carbohydrates | 35–45% of calories | 4 kcal/g | Main energy source, sustains training |
| Fats | 20–30% of calories | 9 kcal/g | Hormonal health, fat-soluble vitamins |
The single most important macronutrient on a deficit is protein. Aim for 1.6–2.2g of protein per kg of body weight per day to minimise muscle loss. Good UK-friendly sources include:
- Chicken breast, turkey, lean beef
- Eggs and low-fat dairy (Greek yoghurt, cottage cheese)
- Tinned fish (tuna, salmon, sardines)
- Legumes (lentils, chickpeas, kidney beans)
- Quorn and tofu for plant-based options
See also: Protein Intake Calculator find your personalised daily protein target
Common Calorie Deficit Mistakes
Overestimating how many calories you burn through exercise Gym machines and fitness trackers overestimate calorie burn by 20–50% on average. Avoid “eating back” all your exercise calories or you risk eliminating your deficit entirely.
Underestimating how many calories you eat Studies show people underestimate their actual calorie intake by 20–40% on average. The most common culprits are cooking oils, sauces, dressings, alcohol, and snacks eaten “on autopilot.” Weighing food on kitchen scales rather than estimating portions — is the single most effective fix.
Forgetting liquid calories A daily latte, two glasses of wine, and a fruit juice can easily add 500–700 kcal without you noticing. Everything you drink counts.
Being strict Monday–Friday and loose on weekends A 1,000 kcal daily deficit through the week is undone by an extra 2,000 kcal over the weekend. Weight loss is determined by your weekly average, not individual days.
Cutting calories too aggressively at the start Extreme deficits trigger metabolic adaptation your body reduces its energy expenditure to compensate. A moderate, consistent deficit beats an aggressive one that you cannot maintain.
Not eating enough protein Low protein during a deficit leads to muscle loss, a slower resting metabolism, increased hunger, and slower long-term fat loss. Protein is not optional.
How to Maintain Your Deficit Long-Term
The hardest part of weight loss is not losing the weight it is keeping it off. Research from the American Journal of Clinical Nutrition consistently shows that gradual, sustainable weight loss leads to significantly better long-term outcomes than rapid loss.
Practical strategies that work:
Take planned diet breaks. A 1–2 week maintenance phase every 8–12 weeks can help reduce hormonal adaptation, restore energy, and prevent burnout. You are not “falling off the wagon” you are being strategic.
Prioritise high-volume, low-calorie foods. Vegetables, lean protein, legumes, and whole fruit are filling and nutritious relative to their calorie count. These foods make a calorie deficit feel far more manageable than cutting the same calories from processed foods.
Add resistance training. Lifting weights 2–3 times per week helps preserve muscle during a deficit, which keeps your resting metabolism higher and improves your body composition regardless of the number on the scales.
Monitor your weight sensibly. Weigh yourself weekly, at the same time (ideally morning, after the toilet), and track a 4-week rolling average rather than reacting to daily fluctuations. Weight naturally varies by 1–2kg day to day due to water and food.
Address the non-food factors. Poor sleep raises ghrelin (the hunger hormone) and reduces leptin (the satiety hormone), making calorie restriction far harder. Stress has a similar effect. Improving sleep and managing stress are legitimate, evidence-based parts of any weight loss programme.
Frequently Asked Questions
How many calories should I cut to lose 1kg per week?
You need a daily deficit of approximately 1,100 kcal below your TDEE to lose 1kg per week. However, this is only achievable if your TDEE is high enough that eating at a 1,100 kcal deficit keeps you above the safe minimum (1,200 kcal/day for women, 1,500 kcal/day for men). Use our calculator above to check whether this goal is appropriate for your body.
Is a 1,000 calorie deficit too much?
For most people, yes a 500–750 kcal deficit offers a better balance of fat loss speed and sustainability. A 1,000–1,100 kcal deficit is only appropriate if your TDEE is 2,500+ kcal/day and you will still eat above safe minimum thresholds.
Why am I not losing weight on a calorie deficit?
The most common causes are: underestimating calorie intake (especially oils, sauces, and drinks), overestimating exercise calorie burn, water retention masking real fat loss, or metabolic adaptation after weeks of restriction. Try tracking more carefully for 2 weeks and recalculating your TDEE.
Can I lose 1kg a week without exercise?
Yes weight loss is primarily driven by diet. However, creating a 1,100 kcal/day deficit through food alone is more restrictive and harder to sustain. Exercise gives you dietary flexibility, preserves muscle, and has well-documented benefits for mental health and long-term weight maintenance.
What is the minimum safe calorie intake?
Most health authorities, including the NHS, recommend a minimum of 1,200 kcal/day for women and 1,500 kcal/day for men. Going below these levels regularly risks nutritional deficiencies, muscle loss, and metabolic damage. A registered dietitian can set safe personalised targets.
Should I eat more on workout days?
Some people find “calorie cycling” (eating more on training days, less on rest days) helpful for energy and compliance. If you try this, ensure your weekly average still hits your target deficit. A simple starting point: eat 200–300 kcal more on training days.
How long will it take to lose 5kg?
At 1kg/week, allow 6–8 weeks rather than exactly 5, to account for the larger water weight drop in week one and natural weekly fluctuations.
Why do I hit a weight loss plateau?
As you lose weight, your BMR decreases. If you do not recalculate your calorie targets regularly, your deficit gradually shrinks and weight loss stalls. Recalculating every 4–6 weeks is the most effective way to prevent this.
References
- NHS: Should you lose weight fast?
- Mifflin MD, et al. (1990) — A new predictive equation for resting energy expenditure. American Journal of Clinical Nutrition, 51(2), 241–247
- Hall KD, et al. (2012) — Quantification of the effect of energy imbalance on body weight. The Lancet, 378(9793), 826–837
- NIDDK: Healthy Eating and Physical Activity
- World Health Organisation: Obesity and Overweight Fact Sheet
This article is for informational purposes only and does not substitute professional medical advice. Always consult your GP before making significant changes to your diet.