Understanding the Numbers: TDEE, BMR, and Calorie Deficit
What Is BMR (Basal Metabolic Rate)?
Your BMR is the number of calories your body burns at complete rest—just keeping you alive. This includes breathing, circulating blood, maintaining body temperature, and cellular repair. BMR accounts for 60-75% of your total daily calorie burn.
BMR calculation uses the Mifflin-St Jeor equation (most accurate for modern populations):
- Men: (10 × weight in kg) + (6.25 × height in cm) - (5 × age) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) - (5 × age) - 161
What Is TDEE (Total Daily Energy Expenditure)?
TDEE is your total calorie burn including all activity: walking, exercise, fidgeting, digestion. Calculate it by multiplying BMR by an activity factor:
| Activity Level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.2 | Desk job, no exercise, mostly sitting |
| Lightly Active | 1.375 | Light exercise 1-3 days/week, some walking |
| Moderately Active | 1.55 | Moderate exercise 3-5 days/week, active job |
| Very Active | 1.725 | Hard exercise 6-7 days/week, physical job |
| Extremely Active | 1.9 | Physical job + intense daily training |
🎯 Most Common Mistake: Overestimating Activity
Nearly everyone overestimates their activity level. Going to the gym 3× per week doesn't make you "very active" if you sit at a desk the rest of the time. Start with Lightly Active unless you have a genuinely physical job or train intensely 5+ days per week.
What Is a Calorie Deficit?
To lose weight, you must eat fewer calories than your TDEE. This forces your body to burn stored fat for energy. The deficit size determines how fast you lose weight:
- Small deficit (250-500 cal/day): Lose 0.25-0.5kg/week. Easiest to stick to.
- Moderate deficit (500-750 cal/day): Lose 0.5-0.75kg/week. Recommended for most.
- Large deficit (750-1000 cal/day): Lose 0.75-1kg/week. Challenging, higher muscle loss risk.
⚠️ Minimum Calorie Warning
Never eat below 1,200 calories/day (women) or 1,500 calories/day (men) without medical supervision. Extreme deficits cause muscle loss, metabolic slowdown, nutrient deficiencies, and unsustainable hunger.
How Fast Should You Lose Weight?
The NHS recommends losing 0.5-1kg per week for sustainable, healthy weight loss. Faster isn't better—here's why:
Slow Weight Loss (0.25-0.5kg/week)
- ✓ Easiest to stick to long-term
- ✓ Minimal hunger and cravings
- ✓ Preserves muscle mass
- ✓ Better energy and mood
- ✗ Takes longer to reach goal
Moderate Weight Loss (0.5-0.75kg/week)
- ✓ Good balance of speed and sustainability
- ✓ Noticeable weekly progress
- ✓ NHS-recommended rate
- ✗ Requires consistent tracking
- ✗ Some hunger expected
Fast Weight Loss (0.75-1kg/week)
- ✓ Rapid visible results
- ✓ Can be motivating short-term
- ✗ Very challenging to sustain
- ✗ Significant hunger and fatigue
- ✗ Higher muscle loss risk
- ✗ Greater rebound weight gain risk
💡 Recommendation: Start Slow
Begin with a moderate 0.5kg/week goal. If you feel miserable after 2 weeks, slow down. If you feel great and want faster results, you can increase the deficit slightly. Sustainability beats speed every time.
How Accurate Is a Calorie Calculator?
Calorie calculators provide estimates, not guarantees. Your actual TDEE can vary by ±10-20% due to:
- Genetics: Some people have naturally faster or slower metabolisms
- Muscle mass: More muscle = higher BMR (muscle burns more calories than fat)
- NEAT: Non-exercise activity (fidgeting, posture, daily movement) varies hugely between people
- Adaptive thermogenesis: Your metabolism slows slightly during calorie restriction
- Accuracy of tracking: Most people underestimate portion sizes by 20-50%
How to Test Your True TDEE
- Use the calculator above to get your starting estimate
- Eat that calorie target for 2 weeks, tracking carefully
- Weigh yourself daily and calculate weekly average
- Adjust based on results:
- Lost more than expected? Add 100-200 calories
- Lost less than expected? Reduce 100-200 calories
- Weight unchanged? Your estimate was accurate!
- Repeat every 2-4 weeks as your weight changes
Common UK Calorie Targets (Examples)
Here are typical calorie targets for different profiles to give you a baseline:
| Profile | TDEE | Weight Loss Target |
|---|---|---|
| Woman, 30, 70kg, 165cm, sedentary | 1,650 cal | 1,200-1,400 cal |
| Woman, 30, 70kg, 165cm, lightly active | 1,900 cal | 1,400-1,650 cal |
| Man, 35, 85kg, 178cm, sedentary | 2,100 cal | 1,600-1,850 cal |
| Man, 35, 85kg, 178cm, moderately active | 2,650 cal | 2,150-2,400 cal |
| Woman, 45, 90kg, 160cm, sedentary | 1,750 cal | 1,250-1,500 cal |
| Man, 25, 75kg, 183cm, very active | 2,900 cal | 2,400-2,650 cal |
What to Do After You Know Your Calorie Target
1. Track Your Food Intake
Download a calorie counting app (like NutraSafe) and log everything you eat and drink. Use a food scale for accuracy—eyeballing portions is wildly inaccurate.
2. Prioritise Protein
Aim for 1.6-2.2g of protein per kg of body weight daily. Protein preserves muscle during weight loss, keeps you full, and has a high thermic effect (burns calories during digestion).
3. Monitor Your Weight Weekly
Weigh yourself daily at the same time (morning, after toilet, before eating) and calculate a weekly average. Daily fluctuations (1-2kg) are normal due to water retention, food in digestive system, and hormones.
4. Adjust Every 2-4 Weeks
As you lose weight, your TDEE decreases (smaller bodies burn fewer calories). Recalculate your target every 2-4 weeks or whenever weight loss stalls for 2+ weeks.
5. Include Strength Training
Lift weights 2-3× per week to preserve muscle mass during weight loss. Muscle keeps your metabolism high and improves body composition (you'll look better at your goal weight).
Frequently Asked Questions
Should I eat back exercise calories?
Generally, no. Your TDEE calculation already includes your typical exercise. Only eat back calories if you do something exceptional (e.g., a 2-hour hike when you're normally sedentary). Even then, only eat back 50-75% as calorie burn estimates are often inflated.
What if I'm not losing weight at my calculated calories?
If you've tracked accurately for 2+ weeks with no weight loss:
- Double-check portion sizes with a food scale
- Log cooking oils, sauces, drinks (these add up)
- Reduce calories by 100-200 and retest for 2 weeks
- Your metabolism may be slower than average—adjust accordingly
Can I have cheat meals?
Yes, but only if it fits your weekly calorie budget. One 3,000-calorie Saturday can undo your entire 500-calorie daily deficit (6 × 500 = 3,000). Either save calories during the week for a bigger Saturday meal, or accept slower weight loss.
How long will it take to lose 10kg?
- At 0.5kg/week: 20 weeks (5 months)
- At 0.75kg/week: 13 weeks (3 months)
- At 1kg/week: 10 weeks (2.5 months)