Your body is constantly using energy, even when you're resting. The amount of energy needed to keep essential functions running, such as breathing, circulation and cell repair, is known as your basal metabolic rate (BMR). In fact, BMR accounts for the largest proportion of the calories your body uses each day.

Your maintenance calorie needs are higher than your BMR because they also include the energy you use through everyday movement, exercise and digestion.

While BMR is only one part of the picture, understanding how much energy your body uses at rest can be a helpful starting point during maintenance. Our calorie calculator can give you an estimate of your maintenance calorie needs based on a few simple details about you.

What does BMR mean?

BMR stands for basal metabolic rate. Put simply, it's the amount of energy (calories) your body needs to carry out its most basic functions while you're completely at rest. This includes things like breathing, regulating your body temperature, circulating blood, and keeping your organs working properly.

For most people, BMR accounts for around 60% to 75% of the total energy they use each day. The rest comes from digesting food and from physical activity.

Your BMR is not something you can choose or control directly. It reflects your body size, composition, age, and sex, along with factors that are largely outside your influence.

BMR vs RMR

When researching metabolism, you may come across two similar terms: basal metabolic rate (BMR) and resting metabolic rate (RMR). Both refer to the amount of energy your body uses to carry out essential functions, such as breathing, circulation and cell repair, but they're measured slightly differently.

BMR is measured under strict conditions:

• after an overnight fast
• following a full night’s rest
• in a temperature-controlled room
• before any physical activity

Because those conditions are difficult to arrange outside a laboratory, true BMR is rarely measured directly.

RMR, on the other hand, is measured under more practical, real-world conditions, making it easier to assess in clinical settings. As a result, RMR is usually slightly higher than BMR because your body is not quite as fully at rest.

For most people, however, the difference is small and unlikely to have a meaningful impact on day-to-day decisions. That's why the terms are often used interchangeably, and why estimates from calorie calculators like ours can still provide a helpful guide when you're working out your maintenance calorie needs.

How is BMR calculated?

The most common way to estimate BMR is an equation called the Mifflin-St Jeor formula, which is what our calculator uses. A systematic review found it to be among the most reliable of the equations commonly used in clinical practice. It estimates your BMR from four variables.

• Weight: Body weight is the single largest influence on BMR. A larger body has more tissue to maintain, so it uses more energy at rest. This is why BMR estimates tend to fall as weight decreases.
• Height: Height also contributes to BMR, as taller people generally have a greater surface area and larger organs, both of which increase resting energy use.
• Age: BMR tends to decline gradually with age, partly because muscle mass typically decreases over time.
• Sex: Men usually have a higher BMR than women of the same height, weight, and age. This mainly reflects differences in average body composition, as men tend to carry proportionally more muscle mass.
• Body composition: The standard equation does not account for body composition directly, but it matters. Two people of the same weight, height, age, and sex can have meaningfully different BMRs if one carries considerably more muscle than the other.

Together, these factors provide an estimate of your BMR, but your actual energy needs can vary from the figure calculated.

BMR during the maintenance phase

When you reach a weight you're happy with, it's normal for your energy needs to change. A smaller body generally requires less energy to function, which means your calorie requirements during maintenance may be different from when you first started your weight loss journey.

You may also notice that maintenance feels different from losing weight for another reason. After significant weight loss, some people experience what's known as metabolic adaptation, where the body uses slightly less energy at rest than might be expected based on body size alone. This is a normal response and can be one of the reasons maintaining weight loss can sometimes feel more challenging than losing it.

Understanding your BMR can help provide useful context, but it's important to see it as just one piece of the puzzle rather than a number you need to focus on too closely. Your energy levels, hunger and fullness cues, sleep quality, physical activity, and ability to maintain healthy habits over the long term are all equally important indicators of how well your maintenance plan is working.

It's also worth remembering that calorie tracking isn't right for everyone. Some people find it helpful, while others find it stressful or difficult to sustain. If tracking calories starts to feel overwhelming or becomes something you think about more than you'd like, you're not doing anything wrong. There are many different ways to approach weight maintenance, and our clinicians can help you find a strategy that feels balanced, realistic, and sustainable for you.

"Your BMR can be a useful tool for understanding how much energy your body needs at rest, but it's not a number you need to manage obsessively. To maintain your weight successfully, it’s much more important to focus on sustainable habits, such as staying active, eating a balanced diet, and responding to your body's changing needs over time. When those habits are in place, the numbers tend to take care of themselves."

Niya Mansuri, Weight Loss Expert at myBMI

How accurate is BMR?

The figure our calculator gives you is an estimate and not a measurement. Equations like Mifflin-St Jeor are built from population averages, so they describe a typical person of your height, weight, and sex rather than you specifically.

Research has found that estimated BMR can differ meaningfully from measured BMR. Accuracy also tends to be lower in people living with obesity than in those who are not, whichever equation is used. Several factors contribute to that gap:

• Body composition: muscle uses more energy at rest than fat tissue, and fat-free mass is one of the strongest predictors of measured BMR. This means that people with more muscle than average will tend to have a higher BMR than an equation predicts.
• Genetics: resting metabolic rate varies between individuals for reasons that are not fully understood.
• Hormones: thyroid function has a substantial effect on resting energy use.
• Health conditions and medication: a range of conditions and treatments can raise or lower BMR.
• Recent weight change: BMR may sit lower than predicted following significant weight loss.

If you want to measure your resting energy use more precisely, indirect calorimetry is a clinical method that can be used. It involves analysing the oxygen you use and the carbon dioxide you produce while at rest, and it is available through some clinical and sports science services.

For most people, though, that level of precision is not necessary. An estimate tends to be enough for general orientation.

Next steps

Your BMR is a useful starting point for understanding how your body uses energy, and it can help explain why maintaining a weight may feel different from losing it. However, it’s not a rule to follow.

Maintenance works best when it is built on habits you can keep up, rather than just a number you are trying to hit. That means:

• eating in a way that feels sustainable
• keeping some form of regular movement in your week
• getting support when you need it

You may also find our BMI calculator useful alongside this one, as the two give you different pieces of the same picture.

If you are moving into maintenance, or thinking ahead to it, our clinicians can talk you through what that might look like for you.

Sources:

• Basal metabolic rate. ScienceDirect. [Accessed 26 August 2026]
• An evaluation of basal metabolic rate among healthy individuals - a cross-sectional study. (2023) Springer Nature. [Accessed 26 August 2026]
• Comparison of Predictive Equations for Resting Metabolic Rate in Healthy Nonobese and Obese Adults: A Systematic Review. (2005) Journal of the American Dietetic Association. [Accessed 26 August 2026]
• Metabolic Consequences of Weight Reduction. (2021) National Library of Medicine. [Accessed 26 August 2026]
• Calories and weight loss. (2025) NHS. [Accessed 26 August 2026]
• Comparative analysis of basal metabolic rate measurement methods in overweight and obese individuals: A retrospective study. (2024) National Library of Medicine. [Accessed 26 August 2026]
• Bias and accuracy of resting metabolic rate equations in non-obese and obese adults. (2013) Clinical Nutrition. [Accessed 26 August 2026]