Every diet that has ever worked — keto, intermittent fasting, Slimming World, Weight Watchers, cutting carbs — has worked for exactly one reason: it created a calorie deficit. Understanding this single concept puts you ahead of 90% of people who start a weight loss plan in the UK every January.
This guide explains what a calorie deficit is, how to calculate yours accurately, and how to sustain it without misery.
What a Calorie Deficit Actually Is
A calorie deficit occurs when you consume fewer calories than your body expends in a given period. Your body requires a continuous energy supply to keep you alive — breathing, pumping blood, regulating temperature, digesting food, moving. This total energy requirement is your Total Daily Energy Expenditure (TDEE).
When calories in are lower than TDEE, your body makes up the shortfall by breaking down stored energy — primarily body fat. This is fat loss. When calories in exceed TDEE, the surplus is stored — primarily as body fat. This is fat gain.
According to the NHS guidance on understanding calories, the average adult in the UK requires approximately 2,000 calories per day (women) and 2,500 (men), though individual requirements vary significantly based on age, body size, and activity level.
How to Calculate Your Calorie Deficit
Step 1: Estimate your maintenance calories (TDEE)
The most practical method without lab testing:
| Activity level | Calculation |
|---|---|
| Sedentary (desk job, little exercise) | Bodyweight in kg × 26–28 |
| Lightly active (1–2 sessions/week) | Bodyweight in kg × 29–31 |
| Moderately active (3–4 sessions/week) | Bodyweight in kg × 32–34 |
| Very active (5+ sessions/week) | Bodyweight in kg × 35–38 |
Example: 80kg person, moderately active → 80 × 33 = ~2,640 calories/day to maintain weight.
Step 2: Set your deficit
A deficit of 300–500 calories per day is the evidence-based sustainable range:
- 300 cal deficit → ~0.3kg loss per week. Slower but very manageable. Good for people close to their goal or those who struggle with hunger.
- 500 cal deficit → ~0.5kg loss per week. The standard recommendation. Produces meaningful progress without excessive restriction.
- 700+ cal deficit → faster initial loss but increases muscle loss, hunger, and the likelihood of giving up.
Using the example above: 2,640 – 500 = 2,140 calories/day to lose ~0.5kg/week.
Step 3: Track accurately
The NHS Eatwell Guide provides a useful framework for food balance, but for calorie tracking you need a food scale and an app. Nutracheck is the best UK option — it has the most accurate UK supermarket database. MyFitnessPal also works well.
Portion estimation without weighing is notoriously inaccurate — studies consistently show people underestimate intake by 20–40%.
Why Calorie Deficits Stop Working (And What to Do)
Fat loss slows over time even on the same calorie intake. This is not your metabolism "breaking" — it's normal physiological adaptation:
Your body gets lighter. A smaller body requires fewer calories to maintain. As you lose weight, your TDEE decreases. Recalculate every 4–6 weeks.
NEAT drops. Non-exercise activity thermogenesis — the calories burned through fidgeting, walking, general movement — decreases when you're in a prolonged deficit. You move less without realising it.
Hunger hormones increase. Ghrelin (the hunger hormone) rises during calorie restriction. This is temporary but real. Higher protein intake significantly blunts this effect.
The fix: every 4–6 weeks, recalculate your TDEE based on your current weight and adjust your calorie target accordingly. A diet break of 2 weeks at maintenance calories every 2–3 months of dieting also helps reset hunger hormones and improve adherence.
Calorie Deficit Without Calorie Counting
Tracking every meal is the most accurate approach but not the only one. These methods create a deficit without explicit counting:
Protein-first eating. Build every meal around a protein source (chicken, eggs, fish, Greek yoghurt, cottage cheese). Protein is the most filling macronutrient. Eating enough of it naturally reduces overall intake.
Volume eating. Fill half your plate with vegetables at every meal. Low calorie density, high fibre, high water content — keeps you full on fewer calories.
Cutting liquid calories. Alcohol, sugary drinks, and fruit juices add significant calories with no satiety benefit. Replacing these with water or zero-calorie drinks is one of the highest-leverage single changes you can make.
Eating to a window. Intermittent fasting (typically 16:8 — eating within an 8-hour window) works for fat loss primarily because it reduces the number of hours available to eat, naturally reducing intake. According to the British Nutrition Foundation, a consistent eating pattern also supports metabolic health.
Common Calorie Deficit Mistakes
Eating back exercise calories. Fitness trackers wildly overestimate calories burned during exercise. If you log a 400-calorie gym session and eat it back, you've likely wiped out your deficit and sometimes exceeded it.
Weekend deviation. A 500-calorie daily deficit from Monday to Friday (2,500 weekly) can be completely erased by 2,500 calories of excess on Saturday and Sunday. Fat loss happens over weekly energy balance, not daily.
Not accounting for cooking oils and condiments. A tablespoon of olive oil is 120 calories. Peanut butter eaten from the jar adds up fast. These are the invisible calories that explain plateaus.
Losing weight too fast. Deficits above 750 calories per day significantly increase muscle loss. You end up lighter but with a higher body fat percentage — and a slower metabolism going forward.
How Milo Handles This For You
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Frequently Asked Questions
How big should my calorie deficit be to lose weight in the UK?
A deficit of 300–500 calories per day is the recommended sustainable range. This produces 0.3–0.5kg of fat loss per week — fast enough to see progress, small enough to preserve muscle and manage hunger without feeling deprived.
How do I calculate my calorie deficit?
Estimate your Total Daily Energy Expenditure (TDEE) using your bodyweight and activity level, then subtract 300–500 calories. For a moderately active 75kg adult, this is roughly 75 × 33 = 2,475 maintenance calories, minus 400 = 2,075 calories per day to lose weight.
Why has my weight loss stalled on a calorie deficit?
As you lose weight, your maintenance calorie requirement decreases. Recalculate your TDEE based on your current bodyweight every 4–6 weeks and adjust your calorie target downward. NEAT also drops during prolonged restriction — increasing daily steps helps counteract this.
Can I lose weight without counting calories?
Yes. Protein-first eating, volume eating with vegetables, cutting liquid calories, and eating within a time window all create a calorie deficit without explicit tracking. These methods are less precise but more sustainable for people who find calorie counting stressful.
What is the minimum calories a woman should eat to lose weight?
Avoid going below 1,400 calories per day for extended periods. Very low calorie diets accelerate muscle loss, impair hormonal function, and are difficult to sustain. A moderate deficit of 300–500 calories below your TDEE is more effective long-term.
Related guides:
- Fat Loss Basics UK — The full picture on losing fat sustainably
- Weight Loss Meal Plan UK — A practical weekly eating plan
- Exercise for Weight Loss UK — What training actually moves the needle
- About UK Weight Loss Guide — How this site works