What Is BMR and How Does This Calculator Work? 2026
If you have ever wondered how many calories your body burns just by existing, that number is your Basal Metabolic Rate, or BMR. It is the energy your body needs every single day simply to keep you alive - powering your heart, lungs, brain, liver and every other organ while you are completely at rest. Think of it as the running cost of being human, before you even get out of bed.
Your BMR typically accounts for between 60% and 75% of the total calories you burn in a day, which surprises most people when they first hear it. Even if you spent the entire day lying on the sofa doing absolutely nothing, your body would still burn a significant number of calories just to maintain its basic functions. That is your BMR at work.
Our BMR calculator 2026 uses the Mifflin-St Jeor equation, which is widely regarded as the most accurate formula for estimating metabolic rate in modern populations. I will explain the maths behind it in the next section, but in simple terms, it takes four pieces of information about you - your weight, height, age and gender - and produces a calorie figure that represents your resting energy expenditure.
Here is how to use the calculator. First, enter your weight in kilograms. If you only know your weight in stones, multiply the number of stones by 6.35 to convert to kilograms. Next, enter your height in centimetres. If you measure yourself in feet and inches, multiply feet by 30.48 and add inches multiplied by 2.54. Then enter your age in years and select your biological sex. Finally, choose the activity level that best describes a typical week for you, and the calculator will work out both your BMR and your Total Daily Energy Expenditure (TDEE).
The results you get include your BMR in kilocalories per day, which is the headline figure. You will also see your TDEE, which adjusts your BMR to account for how active you are. Beyond that, the calculator shows you how many calories you would need to eat each day to lose roughly half a kilogram per week or to gain half a kilogram per week. These are practical targets based on the well-established principle that a 500-kcal daily deficit or surplus leads to approximately 0.5 kg of weight change over seven days.
I have also included your BMR broken down per hour, which is interesting to think about, and your weekly BMR total. These extra figures can help you appreciate just how much energy your body uses over time, even without any physical activity factored in.
One thing worth noting is that BMR calculators, including this one, provide estimates rather than exact measurements. The only way to measure your true BMR with laboratory precision is through indirect calorimetry, a clinical test that analyses the gases you breathe in and out. However, the Mifflin-St Jeor equation gets within about 10% of measured values for most people, which makes it more than accurate enough for everyday nutritional planning.
Whether you are trying to lose weight, build muscle, or simply understand your body better, knowing your BMR is the essential starting point. It gives you a science-backed baseline from which to make informed decisions about how much to eat and how active to be.
The Mifflin-St Jeor Equation Explained 2026
The Mifflin-St Jeor equation was published in 1990 by a team of researchers led by Mark Mifflin and Sachiko St Jeor at the University of Nevada. Their goal was to develop a BMR prediction formula that was more accurate for contemporary populations than the older Harris-Benedict equation, which had been the standard since 1919. After testing hundreds of subjects, they succeeded, and their formula has since been validated by numerous studies and endorsed by organisations including the British Nutrition Foundation.
Here are the two versions of the formula. For men, the equation is: BMR = (10 multiplied by weight in kg) plus (6.25 multiplied by height in cm) minus (5 multiplied by age in years) plus 5. For women, it is exactly the same calculation but with minus 161 at the end instead of plus 5. That final constant is the only difference between the male and female versions, and it accounts for the fact that women tend to have a lower metabolic rate than men of the same size and age, largely due to differences in average body composition.
Let me walk through a worked example. Suppose you are a 35-year-old man who weighs 80 kg and stands 180 cm tall. Plugging those numbers into the formula: BMR = (10 times 80) plus (6.25 times 180) minus (5 times 35) plus 5. That gives us 800 plus 1,125 minus 175 plus 5, which equals 1,755 kcal per day. So your body would burn roughly 1,755 calories a day just to stay alive at complete rest.
Now let us do the same for a 28-year-old woman who weighs 65 kg and is 165 cm tall. Her BMR = (10 times 65) plus (6.25 times 165) minus (5 times 28) minus 161. That works out to 650 plus 1,031.25 minus 140 minus 161, which equals 1,380 kcal per day.
Each component of the formula tells us something important about how metabolism works. The weight term (10 times weight) has the largest coefficient, which makes sense because heavier bodies require more energy to sustain. The height term (6.25 times height) adds calories for taller people, reflecting the greater body surface area and longer circulatory system. The age term (minus 5 times age) reduces the estimate as you get older, accounting for the natural decline in metabolic rate that comes with ageing, primarily driven by the gradual loss of lean muscle mass. And the gender constant (plus 5 for men, minus 161 for women) captures average physiological differences.
You might wonder why this formula replaced the Harris-Benedict equation, which had served well for decades. The answer is straightforward: the Harris-Benedict equation was developed using a sample of people from the early 1900s, whose body compositions, diets and lifestyles were quite different from those of people today. When researchers compared both formulas against actual metabolic measurements taken in a laboratory, the Mifflin-St Jeor equation consistently produced more accurate results. A landmark review by the American Dietetic Association in 2005 confirmed this, and the formula has been the gold standard ever since.
It is worth mentioning that no equation based solely on weight, height, age and gender can perfectly predict every individual's metabolic rate. Factors like muscle-to-fat ratio, genetics, thyroid function and even ambient temperature all play a role. But for the vast majority of healthy adults, the Mifflin-St Jeor equation provides a reliable and practical estimate that you can use as the foundation for your nutritional planning in 2026.
Understanding TDEE and Activity Levels 2026
Your BMR tells you what your body burns at rest, but unless you spend all day in bed, you burn considerably more than that. The total number of calories you burn in a day, factoring in all your movement and exercise, is called your Total Daily Energy Expenditure, or TDEE. Working out your TDEE is arguably even more useful than knowing your BMR alone, because it is the number you actually need to compare against your food intake when managing your weight.
The standard approach to estimating TDEE is to multiply your BMR by an activity factor. These factors were originally developed from research by the Food and Agriculture Organization of the United Nations and have been refined over the years. Our calculator uses five widely accepted levels.
Sedentary, with a multiplier of 1.2, describes someone who does little or no structured exercise and spends most of the day sitting - at a desk, in a car, or on the sofa. This is the reality for a large proportion of the UK working population. If your BMR is 1,700 kcal, your TDEE at this level would be about 2,040 kcal.
Lightly active, multiplied by 1.375, applies if you exercise lightly one to three days per week. This might include a brisk walk, a gentle swim or a yoga class a couple of times a week. Using the same 1,700-kcal BMR, your TDEE would be approximately 2,338 kcal.
Moderately active, at 1.55, fits someone who exercises at moderate intensity three to five days per week. Think jogging, cycling, gym sessions or recreational sports. Your TDEE in this case would be around 2,635 kcal.
Very active, multiplied by 1.725, suits people who train hard six or seven days a week. This could be competitive athletes, serious gym-goers or those with physically demanding hobbies. With a 1,700-kcal BMR, that gives a TDEE of roughly 2,933 kcal.
Extra active, at 1.9, is reserved for those with extremely intense daily exercise or a physically demanding job such as construction, farming or professional sport. The TDEE here would be about 3,230 kcal.
Choosing the right activity level is one of the most common sources of error when people use a BMR calculator. I find that most people overestimate how active they are. Walking to the shops once a day and doing the housework does not make you "moderately active" in the context of these multipliers. Be honest with yourself. If in doubt, choose the level below what you think you are, and adjust upwards later if you find you are losing weight faster than expected.
It also helps to understand that TDEE is not a fixed number. It changes from day to day depending on how much you move. On a day when you go for a long run, your TDEE will be higher than on a rest day. The multiplier gives you an average across a typical week, which is why consistency matters more than any single day.
The NHS Eatwell Guide does not use the term TDEE explicitly, but its general calorie guidance aligns closely with TDEE estimates. For instance, the NHS states that the average man needs about 2,500 kcal per day and the average woman about 2,000 kcal per day, which corresponds roughly to a BMR multiplied by a factor between 1.4 and 1.6 for most adults. Knowing your personal TDEE gives you a far more tailored number than these broad averages.
In 2026, with so many activity trackers and smartwatches available, you can cross-reference the TDEE from this calculator with the calorie-burn data from your wearable device. If the numbers are in the same ballpark, you can be fairly confident that your estimate is on the right track.
Using BMR for Weight Management Goals 2026
Once you know your BMR and TDEE, you have the two most important numbers for making informed decisions about your diet and weight. Whether your goal in 2026 is to lose body fat, gain muscle or simply maintain your current weight, the principle is the same: calories in versus calories out.
To maintain your current weight, you need to eat roughly the same number of calories as your TDEE. If your TDEE is 2,400 kcal per day and you consistently eat about 2,400 kcal, your weight should remain stable over time. In practice, small day-to-day fluctuations are normal due to water retention, meal timing and other factors, but the trend over weeks and months will hold steady.
To lose weight, you need to create a calorie deficit by eating fewer calories than your TDEE. The most commonly recommended rate of weight loss is 0.5 kg per week, which requires a daily deficit of approximately 500 kcal. So if your TDEE is 2,400 kcal, you would aim for about 1,900 kcal per day. This pace of weight loss is endorsed by the NHS as safe and sustainable for most adults. Faster weight loss is possible with larger deficits, but it often leads to muscle loss, nutritional deficiencies and is harder to maintain in the long run.
To gain weight, particularly lean muscle mass, you need a calorie surplus. Eating about 500 kcal above your TDEE each day should result in roughly 0.5 kg of weight gain per week. However, to ensure that the weight you gain is predominantly muscle rather than fat, you need to combine the surplus with regular resistance training. The British Nutrition Foundation recommends that people aiming to build muscle focus on protein-rich foods alongside their overall calorie increase.
There is an important boundary to be aware of. You should generally avoid eating fewer calories than your BMR for extended periods. Your BMR represents the bare minimum your body needs to function, and chronically undereating below that level can lead to fatigue, nutrient deficiencies, hormonal disruption and a slowed metabolism as your body tries to conserve energy. This is sometimes called "metabolic adaptation" and it can make further weight loss increasingly difficult.
I would also encourage you to think about the quality of the calories you eat, not just the quantity. Two thousand calories from nutrient-dense whole foods - lean protein, vegetables, whole grains, healthy fats - will support your health far better than 2,000 calories from processed snacks and sugary drinks, even though the calorie count is identical. The NHS Eatwell Guide provides excellent, evidence-based advice on building a balanced diet.
Tracking your intake does not have to be obsessive. Many people find it useful to track carefully for a few weeks to build awareness of portion sizes and calorie content, then transition to a more intuitive approach once they have a good feel for the numbers. Apps and food diaries can help during the tracking phase, and the BMR and TDEE figures from this calculator give you the targets to work towards.
It is also worth revisiting your BMR calculation periodically. As your weight, age or activity level changes, so will your calorie needs. If you lose 10 kg, your BMR will be lower than it was before, and your TDEE will drop accordingly. Recalculating every few months ensures that your targets stay accurate and your progress continues.
Finally, remember that these numbers are guides, not absolute rules. Every body is different, and real-world results will vary. If you have specific health concerns, a history of disordered eating, or medical conditions that affect your metabolism, I would strongly recommend speaking with a registered dietitian or your GP before making significant changes to your calorie intake. The NHS website at nhs.uk offers free resources and can help you find professional support in your area.