BMR Calculator — Calculate Your Basal Metabolic Rate

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BMR calculator: find your basal metabolic rate

Our BMR calculator estimates the number of calories your body needs to maintain essential functions — such as breathing, circulation, and cell repair — while completely at rest. Enter your age, sex, height, and weight to get your basal metabolic rate in seconds, along with a TDEE (total daily energy expenditure) estimate based on your activity level. Understanding these figures can help you make more informed decisions about your diet, calorie intake, and overall health.

BMR & TDEE Calculator

Calculate your Basal Metabolic Rate and Total Daily Energy Expenditure

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What does your BMR calculator result actually mean?

Your BMR calculator result tells you the minimum number of calories your body requires each day simply to stay alive — before any movement, exercise, or daily activity is factored in. Think of it as your body's engine idling: even when you are asleep, your organs, muscles, and brain are continuously consuming energy. For most adults, BMR accounts for roughly 60–75% of total daily calorie burn.

The figure your BMR calculator returns is expressed in kilocalories (kcal), the same unit you see on food packaging. If your result is, say, 1,600 kcal, that is the floor — the absolute minimum your body needs. Eating consistently below your BMR over time can slow your metabolism, cause muscle loss, and deprive vital organs of the energy they need, which is why very low-calorie diets should only be followed under medical supervision.

How our BMR calculator works: the formula explained

Our BMR calculator uses the Mifflin–St Jeor equation, which is widely regarded by dietitians and researchers as the most accurate formula for estimating basal metabolic rate in the general population. The equation takes four variables — your sex, age, height in centimetres, and weight in kilograms — and produces a calorie estimate tailored to you. An older formula called the Harris–Benedict equation is still used in some tools, but research suggests Mifflin–St Jeor predicts measured resting energy expenditure more closely in most adults.

  • For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
  • For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
  • TDEE is then calculated by multiplying your BMR by an activity factor ranging from 1.2 (sedentary) to 1.9 (very active)
  • Results are estimates — individual variation due to genetics, body composition, and hormones means your true BMR may differ by up to 10–15%
  • Body composition matters: muscle tissue burns more calories at rest than fat tissue, so two people with identical weight can have noticeably different BMRs

How to use your BMR calculator results to reach your goals

Once you have your BMR and TDEE figures, you have a practical starting point for adjusting your calorie intake depending on whether you want to lose weight, gain muscle, or simply maintain your current weight. Most health guidelines suggest a moderate calorie deficit of 500 kcal per day below TDEE for gradual, sustainable weight loss of approximately 0.5 kg per week — though individual results will vary.

Matching your intake to your goal

To lose weight gradually, aim to eat roughly 500 kcal below your TDEE while keeping your intake above your BMR. Crash diets that push calories far below BMR may cause short-term weight loss but can also reduce lean muscle mass and lower your metabolic rate over time, making long-term maintenance harder. A registered dietitian can help you set a realistic and safe target.

To maintain weight, aligning your daily calorie intake with your TDEE is the goal. To build muscle or support an active training programme, you may need to eat slightly above TDEE — often called a 'calorie surplus' — combined with adequate protein intake. Your BMR calculator result gives you the baseline from which all of these adjustments are made, making it a genuinely useful first step in any nutrition plan.

Factors that can affect your BMR calculator estimate

While the Mifflin–St Jeor equation is highly regarded, it is important to remember that a BMR calculator produces an estimate, not a laboratory measurement. Several biological and lifestyle factors can cause your actual resting calorie burn to sit higher or lower than the figure shown. Being aware of these can help you interpret your result more accurately and adjust your approach if the numbers do not seem to match your real-world experience.

If you have a thyroid condition, are pregnant or breastfeeding, are recovering from illness or surgery, or are taking certain medications, your actual energy needs may differ significantly from what any online BMR calculator can predict. In these cases, speaking with your GP or a registered dietitian is strongly recommended before making dietary changes based solely on a calculated figure.

Frequently Asked Questions

What is a BMR calculator and how does it work?
A BMR calculator estimates the number of calories your body burns at complete rest — the energy needed to keep your heart beating, lungs breathing, and organs functioning. It uses your age, sex, height, and weight in a validated formula (usually Mifflin–St Jeor) to produce a personalised kilocalorie figure.
Is a BMR calculator accurate?
A BMR calculator provides a close estimate for most healthy adults, but it is not as precise as clinical measurements such as indirect calorimetry. Results can vary by up to 10–15% from your true resting metabolic rate due to factors like body composition, genetics, and hormonal health.
What is the difference between BMR and TDEE?
BMR (basal metabolic rate) is the calories your body burns at rest, while TDEE (total daily energy expenditure) adds the calories burned through daily movement and exercise. Your BMR calculator result is multiplied by an activity factor to give your TDEE, which is the more useful figure for planning your daily calorie intake.
How do I use my BMR calculator result to lose weight?
To lose weight gradually, aim to eat roughly 500 kcal below your TDEE — but always keep your intake above your BMR result to protect muscle mass and organ function. A deficit of this size may support a loss of around 0.5 kg per week on average, though results vary between individuals.
Does a BMR calculator work for women differently than men?
Yes — the Mifflin–St Jeor formula used in our BMR calculator includes a sex-specific constant, as women typically have a lower basal metabolic rate than men of the same age, height, and weight, largely due to differences in average muscle mass and hormonal profile.
Can I use a BMR calculator if I have a thyroid condition?
You can use a BMR calculator as a rough guide, but thyroid disorders — both hypothyroidism and hyperthyroidism — directly affect your resting metabolic rate and can make standard formula estimates significantly inaccurate. Speak to your GP or an endocrinologist for personalised advice.
What is a normal BMR for a woman in the UK?
There is no single 'normal' figure, as BMR varies with age, height, and weight, but many adult women in the UK have a basal metabolic rate somewhere between 1,200 and 1,600 kcal per day. Use our BMR calculator to get an estimate specific to your measurements.
Does BMR decrease with age?
Yes — basal metabolic rate tends to decline gradually from around age 30 onwards, partly because muscle mass naturally decreases with age (a process called sarcopenia) and partly due to hormonal changes. Our BMR calculator accounts for age in its formula, so your result will reflect this.

Sources

  1. Calories: how to achieve a healthy weight NHS [accessed 26 August 2026]
  2. Obesity: identification, assessment and management (CG189) NICE [accessed 26 August 2026]
  3. A new predictive equation for resting energy expenditure in healthy individuals (Mifflin et al., 1990) American Journal of Clinical Nutrition / PubMed [accessed 26 August 2026]
  4. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults (Frankenfield et al., 2005) Journal of the American Dietetic Association / PubMed [accessed 26 August 2026]
  5. Energy requirements and dietary reference values British Nutrition Foundation [accessed 26 August 2026]
  6. Dietary reference values for food energy and nutrients for the United Kingdom Scientific Advisory Committee on Nutrition (SACN) [accessed 26 August 2026]
  7. Physical activity guidelines for adults aged 19 to 64 NHS [accessed 26 August 2026]
  8. Resting metabolic rate: best ways to measure it — and raise it too Harvard Health Publishing [accessed 26 August 2026]
  9. Sarcopenia: age-related muscle loss and its impact on metabolic rate PubMed / National Library of Medicine [accessed 26 August 2026]
  10. Thyroid disease and metabolism British Thyroid Foundation [accessed 26 August 2026]

This calculator provides an estimate for general informational purposes only and is not medical advice. Always consult a qualified healthcare professional before making changes to your diet or starting any weight loss treatment.