Weight loss comes down to one fundamental principle: consuming fewer calories than your body burns. This is called a calorie deficit, and it is the mechanism behind every effective fat loss approach — regardless of whether you follow keto, intermittent fasting, or a balanced diet.
But knowing how to create a deficit, and how large that deficit should be, makes the difference between sustainable progress and frustration. This guide walks you through everything you need to know.
What is a calorie deficit?
A calorie deficit occurs when your energy intake (calories from food and drink) is lower than your Total Daily Energy Expenditure (TDEE) — the total number of calories your body burns each day through basic metabolism, digestion, and physical activity.
When this deficit is sustained over time, your body turns to stored body fat as an alternative fuel source, resulting in fat loss. A deficit of approximately 3,500 calories is generally associated with losing around 0.45 kg (1 lb) of fat, though this varies between individuals.
How to find your TDEE
Before creating a deficit, you need to know your maintenance calories — the amount you need to eat to keep your current weight stable. This is your TDEE.
The most practical way to estimate TDEE is to use a validated formula like the Mifflin-St Jeor equation (which our calculator uses), then multiply by an activity factor. The result gives you a good starting estimate, which you can refine based on real-world results over 2–3 weeks.
| Activity Level | Description | Multiplier |
|---|---|---|
| Sedentary | Desk job, little or no exercise | × 1.2 |
| Lightly active | 1–3 workouts per week | × 1.375 |
| Moderately active | 3–5 workouts per week | × 1.55 |
| Very active | 6–7 workouts per week | × 1.725 |
| Extremely active | Physical job + daily training | × 1.9 |
How large should your calorie deficit be?
The size of your deficit determines the speed of fat loss — but larger is not always better. Here is how the main approaches compare:
Small deficit (200–300 kcal/day)
Results in slow but very sustainable fat loss of roughly 0.2–0.3 kg per week. Best suited to people already close to their goal weight, athletes who need to maintain performance, or anyone who has struggled to adhere to larger deficits in the past.
Moderate deficit (400–500 kcal/day)
The most widely recommended approach, producing around 0.4–0.5 kg of fat loss per week. This is the default used by most evidence-based guidelines and is what our calculator applies for the "fat loss" goal. It balances meaningful results with day-to-day adherence.
Large deficit (700+ kcal/day)
Leads to faster initial weight loss but comes with significant trade-offs: increased muscle loss, greater hunger and fatigue, higher risk of nutrient deficiencies, and a stronger metabolic adaptation response that can stall progress. Large deficits are generally not recommended unless supervised by a healthcare professional.
The role of protein in a calorie deficit
When you eat in a deficit, your body loses both fat and some lean muscle mass. Adequate protein intake is the single most important nutritional factor for minimising muscle loss during fat loss.
The current evidence-based recommendation for people eating in a calorie deficit is 1.6–2.4 grams of protein per kilogram of body weight per day. Higher end of this range is appropriate when the deficit is larger or when resistance training is part of the programme.
Protein also has a higher thermic effect than carbohydrates or fat (meaning your body burns more calories digesting it), and it is the most satiating macronutrient — making it easier to stay within your calorie target.
Common mistakes that slow weight loss
Underestimating portion sizes
Studies consistently show that people underestimate their calorie intake by 20–40%. Using a food scale for at least the first few weeks can dramatically improve accuracy and reveal hidden calorie sources.
Choosing too aggressive a deficit
Cutting calories too hard leads to extreme hunger, muscle breakdown, and metabolic adaptation. You may lose weight faster initially, but the chances of rebounding are much higher.
Not accounting for liquid calories
Drinks — including fruit juice, smoothies, coffee with milk, and alcohol — can add hundreds of calories per day without contributing much to satiety.
Ignoring activity level changes
When people eat less, they often move less unconsciously (a phenomenon called adaptive thermogenesis). This reduces TDEE, meaning the deficit shrinks over time even if food intake stays the same. Maintaining activity levels and periodically recalculating your targets helps counteract this.
Expecting linear progress
Weight naturally fluctuates by 1–3 kg day-to-day due to water retention, glycogen, and digestive content. Looking at weekly averages rather than daily weigh-ins gives a much more accurate picture of true fat loss progress.
Putting it all together
A practical approach to fat loss through calorie management:
- Use the Calories & Macros calculator to estimate your TDEE and get a starting calorie target.
- Track your food intake consistently for 2–3 weeks using an app like Cronometer or MyFitnessPal.
- Weigh yourself daily and calculate the weekly average to assess actual rate of loss.
- If losing more than 0.75 kg/week, increase calories by 100–200 kcal; if not losing, reduce by 100–200 kcal.
- Prioritise protein at every meal, resistance training 2–4 times per week, and adequate sleep.
Fat loss doesn't have to be complicated. A moderate deficit, sufficient protein, and consistent tracking are the three pillars that produce reliable, sustainable results for most people.