How Many Calories Do You Need to Lose Weight?

The Science of Weight Loss: Calories In vs. Calories Out

Weight loss comes down to one simple truth: you must consume fewer calories than your body burns over time. Your body uses energy (calories) for three things: maintaining basic bodily functions (breathing, digestion, circulation) — your Basal Metabolic Rate; digesting food — the thermic effect of food; and physical activity and daily movement — your activity expenditure.

The sum of all these is your Total Daily Energy Expenditure (TDEE). If you eat below your TDEE, you create a calorie deficit, and your body draws on stored energy (primarily body fat) to make up the shortfall. Over time, this deficit produces fat loss. Understanding your personal TDEE is the first step to choosing a calorie target that works for you.

What is BMR and How to Calculate It

Your Basal Metabolic Rate (BMR) is the number of calories your body burns at rest — just to keep you alive. It includes the energy cost of breathing, circulation, cell production, brain function, and maintaining body temperature. BMR is the largest component of TDEE for most sedentary people.

The most widely used formula is the Mifflin-St Jeor equation:

For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5

For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

Example: A 35-year-old woman, 165 cm tall, weighing 75 kg. BMR = (10 × 75) + (6.25 × 165) − (5 × 35) − 161 = 750 + 1,031 − 175 − 161 = 1,445 kcal. This woman burns 1,445 calories per day at rest, before accounting for any activity.

From BMR to TDEE: Adding Activity

Your TDEE is your BMR multiplied by an activity factor. This factor reflects how active you are:

Sedentary (little or no exercise): × 1.2

Lightly active (exercise 1–3 days per week): × 1.375

Moderately active (exercise 3–5 days per week): × 1.55

Very active (exercise 6–7 days per week): × 1.725

Continuing the example: the 75 kg woman with a BMR of 1,445 kcal who exercises 3–5 days per week (moderately active) has a TDEE of 1,445 × 1.55 = 2,240 kcal per day. She burns approximately 2,240 calories per day through a combination of rest metabolism and activity.

How Many Calories Should You Cut to Lose Weight?

A calorie deficit is the difference between your TDEE and your chosen eating target. The size of the deficit determines how fast you lose weight — but larger deficits are not always better.

A deficit of 300–500 calories per day is considered moderate and sustainable. This produces weight loss of approximately 0.3–0.5 kg per week, or 1.5–2 kg per month. This pace is sustainable for most people because it is aggressive enough to produce visible results while not so extreme that it causes constant hunger, fatigue, or nutrient deficiencies.

A smaller deficit of 100–200 calories per day may feel more sustainable but produces very slow results (0.1–0.2 kg per week) that are easily undone by a single high-calorie day. Conversely, a deficit of 750–1,000+ calories per day accelerates weight loss but significantly increases the risk of muscle loss, nutrient deficiencies, extreme hunger, and metabolic adaptation — making the diet unsustainable and counterproductive long-term.

The NHS recommends a deficit of no more than 600 calories per day for safe, sustainable weight loss. This translates to a maximum daily reduction of about 600 kcal below TDEE, meaning no fewer than approximately 1,400 kcal per day for most women and 1,600 kcal per day for most men.

Calorie Targets for Different Weight Loss Goals

Using the example above: the woman with a TDEE of 2,240 kcal could aim for these targets:

1,740 kcal per day (500 calorie deficit) → 0.5 kg per week → 2 kg per month

1,890 kcal per day (350 calorie deficit) → 0.35 kg per week → 1.4 kg per month

1,990 kcal per day (250 calorie deficit) → 0.25 kg per week → 1 kg per month

Faster weight loss is tempting, but a deficit larger than 500 kcal per day should only be pursued under medical supervision. For sustainable weight loss that you can maintain long-term, aim for the middle ground: a 300–500 calorie deficit that produces 0.3–0.5 kg of weight loss per week.

Quick Reference: Common UK Weight Loss Targets

These examples are not prescriptions, but they help you sanity-check a calorie target before building a meal plan. The right number depends on your TDEE, current weight, activity level, and how aggressive you want the deficit to feel.

How Long Will It Take to Lose Weight?

Weight loss is not linear. You will not lose exactly the same amount every week — water retention, menstrual cycle (for women), food volume in your stomach, and hormonal fluctuations cause normal weekly variations of ±1–2 kg.

To see your true progress, weigh yourself once per week (same day, same time, ideally after waking) and look at the trend over 4 weeks rather than day-to-day changes. Calculate your average weight for weeks 1–4, then your average for weeks 5–8, and so on. A downward trend over multiple weeks indicates genuine fat loss.

To estimate how long weight loss will take: divide the target weight loss in kilograms by your weekly loss rate. For example, a 20 kg weight loss target at 0.5 kg per week = 40 weeks (~9 months). A 20 kg loss at 0.3 kg per week = 67 weeks (~15 months). Patience and consistency matter more than speed.

Factors That Affect How Many Calories You Need

BMR and TDEE are not static — they vary based on several factors:

• Age: BMR decreases by approximately 2% per decade after age 20 due to loss of muscle mass.
• Sex: Women typically have a lower BMR than men of the same age and weight due to differences in muscle mass.
• Muscle mass: More muscle burns more calories at rest. This is why strength training supports weight loss.
• Hormones: Thyroid function, menstrual cycle phase, and stress hormones all influence metabolic rate.
• Genetics: Some people naturally have a faster or slower metabolism than average for their age, sex, and size.
• Environment: Living in a cold climate increases your BMR slightly as your body works to maintain warmth.

Metabolic Adaptation and the Plateau

After several weeks of consistent dieting, many people experience a weight loss plateau — the scale stops moving despite maintaining their calorie deficit. This is largely due to metabolic adaptation: as you lose weight, your BMR decreases (you are smaller and require fewer calories), so your TDEE also decreases.

The solution is straightforward: recalculate your TDEE based on your current weight and reduce your calorie target by 50–100 kcal. As you lose weight, plan to reduce your target roughly every 6–8 weeks. This keeps your deficit meaningful while preventing the extreme hunger that can come from trying to maintain the same deficit as you shrink.

Using This Information to Choose Your Calorie Target

Now that you understand how to calculate BMR, TDEE, and ideal deficits, here is how to choose your calorie target:

1. Calculate your BMR using the Mifflin-St Jeor formula above.

2. Estimate your activity factor honestly (sedentary, lightly active, moderately active, very active).

3. Multiply BMR by activity factor to get your TDEE.

4. Subtract 300–500 kcal to find your calorie target.

5. Aim to stay within 100–200 kcal of this target, not obsessing over precision.

If the maths feels overwhelming, use our free meal plan generator to set your calorie target directly — the tool handles all the calculations for you.