The answer is different for every person — but the calculation is the same. To lose weight, you need to eat fewer calories than your body burns. How many fewer depends on your total daily energy expenditure (TDEE) and how fast you want to lose. This guide explains the maths in plain terms and tells you exactly how to get your number.
Step 1: Find Your TDEE
Your TDEE (Total Daily Energy Expenditure) is the total number of calories your body burns in a day, including exercise. It's the number you need to beat. Eat below it and you lose weight. Eat above it and you gain.
TDEE is calculated from your BMR (Basal Metabolic Rate — calories your body burns at rest) multiplied by an activity factor. The Mifflin-St Jeor equation is the most accurate formula for this:
- 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
Multiply by your activity level: sedentary (1.2), lightly active (1.375), moderately active (1.55), very active (1.725). The result is your TDEE.
You don't need to do this by hand. Our TDEE calculator does the full calculation in under 30 seconds — input your age, weight, height, and activity level and it returns your maintenance calories plus specific deficit targets.
Step 2: Choose Your Deficit
A calorie deficit is the difference between your TDEE and what you actually eat. The size of the deficit determines how fast you lose weight:
- Small deficit (250–300 kcal/day): Approximately 0.25kg weight loss per week. Very sustainable. Minimal muscle loss. Recommended for people close to their goal or those new to dieting.
- Moderate deficit (500 kcal/day): Approximately 0.5kg per week. The standard recommendation for consistent fat loss. Manageable for most people with a structured meal plan.
- Larger deficit (750–1,000 kcal/day): Approximately 0.75–1kg per week. Appropriate only for people with substantial weight to lose (BMI 30+). Requires careful protein intake to limit muscle loss.
The 3,500 kcal per pound rule (approximately 7,700 kcal per kg of fat) is the underlying maths. A 500 kcal/day deficit creates a 3,500 kcal weekly deficit — equating to roughly 0.5kg of fat per week. In practice results vary (water weight fluctuates, metabolic adaptation occurs), but this is the reliable baseline to plan from.
How to Calculate Your Target Calories
Once you know your TDEE, the maths is:
Target calories = TDEE − chosen deficit
If your TDEE is 2,300 kcal and you want a 500 kcal/day deficit, your target intake is 1,800 kcal. That's the number you plan your meals around. Use our calorie deficit calculator to see your options — it shows you deficit tiers from gentle to aggressive and gives projected weight loss timelines for each.
The Minimum Calorie Floor
Going below certain thresholds is counterproductive and medically inadvisable:
- Women: Do not go below 1,200 kcal/day without medical supervision
- Men: Do not go below 1,500 kcal/day without medical supervision
Very low calorie intakes (below these floors) increase muscle loss, suppress metabolism, impair recovery, and are associated with nutrient deficiencies. Most people don't need to go near these limits — a 500 kcal deficit from a normal TDEE (2,000–2,800 for most UK adults) leaves plenty of room to eat satisfying meals.
Why Protein Matters More Than Just Calories
When you're in a calorie deficit, your body can break down muscle for energy if protein intake is too low. To lose fat — not muscle — you need adequate protein throughout your diet. The evidence-based recommendation during a deficit is 1.6–2.4g of protein per kg of bodyweight per day. This is higher than standard dietary guidelines because standard guidelines are designed for weight maintenance, not body composition in a deficit.
For a 70kg person, that means 112–168g of protein per day. Hitting that target while staying within your calorie budget is where meal planning earns its keep. Our macro calculator will break out the exact protein, carbohydrate, and fat targets based on your calorie goal and bodyweight.
Common Mistakes That Stall Weight Loss
- Underestimating portions. Research consistently shows people underestimate calorie intake by 20–40%. Use a kitchen scale for the first month. You'll be surprised how inaccurate eye measurement is.
- Not accounting for liquid calories. Lattes, juices, alcohol, and flavoured drinks add up fast. A daily flat white (200 kcal) wipes out 40% of a 500 kcal deficit.
- Using a static calorie target. As your weight decreases, your TDEE decreases. Recalculate every 4–6 weeks as you lose weight. Sticking to the same intake you started with leads to a plateau.
- Too aggressive a deficit at the start. A 1,000 kcal/day deficit feels fine for a week and miserable after three. The diet you can sustain for 12 weeks beats the perfect deficit you quit after two.
Getting Your Exact Number
The fastest way to get a precise, personalised calorie target is to run the full calculation with your own data. Start with our TDEE calculator to find your maintenance calories, then use the calorie deficit calculator to model different deficit scenarios and see projected weight loss timelines. Once you have your target, our meal planner will generate a full week of meals that hit it automatically, with a shopping list ready to go.