If you're trying to lose fat, you’ve likely heard one of two things: “It’s all about calories in versus calories out,” or “It’s your hormones, not your calories.” Both statements contain truth, but also oversimplifications. In this post, we’ll unpack how to calculate your daily calorie needs for fat loss and clarify the key factors that actually move the needle.

Why Calorie Balance Still Matters

At its core, fat loss comes down to an energy equation: to lose weight, you must burn more calories than you consume. This is called a caloric deficit. Without it, fat loss won’t happen.

That said, how your body responds to a calorie deficit depends on your physiology, hormones, stress levels, sleep, and the quality of the food you eat. For example, high levels of cortisol from chronic stress can promote fat storage, especially around the abdomen. Poor sleep can dysregulate hunger hormones like ghrelin and leptin, making you crave more food and feel less satisfied. Hormonal imbalances, such as low thyroid function, insulin resistance, or fluctuating oestrogen, can all slow metabolic rate and impair fat loss. Even the nutrient quality of your meals matters: whole, protein-rich foods keep you fuller and fuel your body better than ultra-processed options, even if the calories are the same. So while calories matter, context matters just as much, if not more, for long-term success.

The Two Main Schools of Thought

1. The Calorie Balance Model

This view argues that body weight is governed by energy balance, regardless of where those calories come from. If you eat more calories than you burn, you gain fat. If you eat fewer, you lose fat.

2. The Insulin-Carbohydrate Model

This model focuses on hormonal regulation, particularly the role of insulin. The idea is that high-carb or high-glycaemic diets spike insulin, which promotes fat storage and prevents fat burning. Advocates believe fat loss depends more on hormonal control than just calorie math.

What’s the truth? Both matter. Research shows that calorie balance is essential for fat loss, but hormones like insulin play a critical role in how calories are processed and stored. Elevated insulin levels, especially when chronically high due to a high-sugar or high-refined-carb diet, can impair fat mobilisation and increase fat storage. This means some individuals may need to manage both calorie intake and insulin response for effective fat loss. That’s why a holistic approach, one that considers both metabolic input and hormonal output, works best.

Step 1: Understand Your Energy Needs

To calculate how many calories you need for fat loss, you need to know two numbers:

1. Basal Metabolic Rate (BMR) how many calories your body needs at rest, just to maintain basic functions like breathing and circulation.

2. Total Daily Energy Expenditure (TDEE) your BMR plus calories burned through movement, digestion, and activity.

Step 2: Calculate Your BMR

There are several formulas used to estimate Basal Metabolic Rate (BMR). The most common include:

• Mifflin-St Jeor Equation widely considered the most accurate for modern lifestyles.

• Harris-Benedict Equation an older formula that tends to slightly overestimate BMR.

• Katch-McArdle Formula best used if you know your body fat percentage, as it bases the calculation on lean body mass.

For simplicity and accuracy, we recommend the Mifflin-St Jeor formula:

Use the Mifflin-St Jeor formula, considered one of the most accurate for estimating BMR:

BMR for women = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161

Example: A 35-year-old woman who is 165 cm tall and weighs 70 kg:

BMR = (10 × 70) + (6.25 × 165) – (5 × 35) – 161
= 700 + 1031.25 – 175 – 161
= 1395.25 calories/day

Step 3: Adjust for Activity to Find TDEE

Multiply your BMR by an activity factor:

• Sedentary (little or no exercise): × 1.2

• Lightly active (light exercise/sports 1-3 days/week): × 1.375

• Moderately active (moderate exercise 3-5 days/week): × 1.55

• Very active (hard exercise 6-7 days/week): × 1.725

• Super active (very intense training or physical job): × 1.9

Continuing the example, if she’s moderately active: TDEE = 1395.25 × 1.55 = ~2163 calories/day

Step 4: Create a Calorie Deficit

To lose fat, reduce your TDEE by 15–25% depending on your goals, timeline, and metabolic health.

• Conservative deficit (15%): ~1838 calories/day

• Moderate deficit (20%): ~1730 calories/day

• Aggressive deficit (25%): ~1622 calories/day

Note: Bigger deficits can lead to faster weight loss but may increase the risk of muscle loss, fatigue, and metabolic adaptation, especially in women.

Step 5: A Real-Life Alternative – Track, Observe, Adjust

If you don’t want to rely on formulas, try this:

1. Track your food intake (using an app or food journal) for 2 weeks. Be meticulous—track everything you eat and drink, including small bites, snacks, and condiments. For accuracy, weigh your food using a kitchen scale and log the exact quantities. This level of precision is essential to truly understand your intake and establish a reliable baseline.

2. Weigh yourself consistently under the same conditions (e.g. morning, post-toilet).

3. If your weight is stable, that intake = your maintenance calories.

4. To lose fat, reduce that number by 15–25% and monitor progress.

This approach works well if you already have a consistent eating pattern.

Final Thoughts

Calculating calories for fat loss isn’t about being perfect, it’s about being informed. Understand your body’s energy needs, apply a sensible deficit, and adjust based on results. Remember, your physiology is unique. If fat loss stalls, it doesn’t mean you’re broken. It means it’s time to tweak, not quit.

One of the most important, and often overlooked, factors in long-term fat loss success is accountability and support. Whether it’s working with a coach, joining a group programme, or checking in with a friend, having someone in your corner can make the difference between giving up and real change. The process of trial, error, and refinement is much easier, and more enjoyable, when you’re not doing it alone.

References

• Hall, K. D., Heymsfield, S. B., Kemnitz, J. W., Klein, S., Schoeller, D. A., & Speakman, J. R. (2012). "Energy balance and its components: implications for body weight regulation." The American Journal of Clinical Nutrition, 95(4), 989–994. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3319009/

• Ludwig, D. S., & Ebbeling, C. B. (2018). "The carbohydrate-insulin model of obesity: beyond 'calories in, calories out'." JAMA Internal Medicine, 178(8), 1098–1103. https://pubmed.ncbi.nlm.nih.gov/29971405/

• Müller, M. J., Enderle, J., Bosy-Westphal, A., et al. (2016). "Beyond the body mass index: tracking body composition in the path to prevention and treatment of obesity." Obesity Reviews, 17(3), 285–300. https://pubmed.ncbi.nlm.nih.gov/26693811/

• Weigle, D. S., Breen, P. A., Matthys, C. C., et al. (2005). "A high-protein diet induces sustained reductions in appetite, ad libitum caloric intake, and body weight despite compensatory changes in diurnal plasma leptin and ghrelin concentrations." The American Journal of Clinical Nutrition, 82(1), 41–48. https://pubmed.ncbi.nlm.nih.gov/16002805/

• National Health and Nutrition Examination Survey (NHANES) 2013–2014. "Average protein consumption for women ages 20 and older." U.S. Department of Agriculture, Agricultural Research Service. https://www.cdc.gov/nchs/nhanes/index.htm

• National Diet and Nutrition Survey (NDNS), UK. Public Health England and Food Standards Agency. (2020). "Protein intake by age and gender in the UK population." https://www.gov.uk/government/statistics/ndns-results-from-years-9-to-11-2016-to-2019