🛌 BMR Calculator

Last updated: January 24, 2026
🛌

BMR Calculator

Mifflin-St Jeor Equation

yrs
kg
cm
Please fill in all fields with valid values.
Your Basal Metabolic Rate
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calories / day (at complete rest)
Daily Calorie Needs by Activity
Formula: Mifflin-St Jeor (1990) · TDEE = BMR × activity multiplier

What Actually Happens Inside Your Body While You Sleep

Picture yourself lying completely still in a dark room. No movement, no digestion, no stress — just your body existing. Even in that perfect stillness, your heart is beating, your lungs are expanding, your kidneys are filtering, your neurons are firing. All of that burns calories. A lot of them, actually.

That minimum energy cost just to stay alive is called your Basal Metabolic Rate, or BMR. For most people, it accounts for somewhere between 60% and 75% of everything they burn in an entire day. That means before you've taken a single step, made a single cup of coffee, or typed a single message — your body has already burned more than half its daily calorie budget just keeping the lights on.

The Mifflin-St Jeor Equation: Why This One Wins

Nutrition researchers have been trying to accurately predict BMR since the 1910s. The original Harris-Benedict formula from 1919 was the gold standard for nearly 70 years. Then in 1990, two scientists named Mark Mifflin and Sachiko St Jeor published a new equation after studying 498 people, and their numbers came out significantly more accurate — particularly for people with modern body compositions.

The math works like this: for men, BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5. For women, it's the same but you subtract 161 instead of adding 5. That gender difference exists because men typically carry more muscle mass, and muscle is metabolically expensive tissue — it burns roughly 3 times more calories at rest than fat does.

Multiple validation studies have shown the Mifflin-St Jeor equation hits within about 10% of a lab-measured BMR for most healthy adults. That's not perfect, but it's remarkably good for a formula that only needs four inputs.

Why Your BMR Isn't a Fixed Number

Here's something most calorie-counting apps quietly ignore: your BMR changes constantly. It shifts with your age, your muscle mass, your hormones, even the temperature of the room you're sitting in. Cold environments bump it up slightly because your body burns extra fuel to maintain core temperature. Fever cranks it up dramatically — roughly 7% for every degree Fahrenheit above normal. Being severely sleep-deprived tanks it.

The biggest long-term factor is body composition. Two people can weigh exactly the same and have dramatically different BMRs based on how much of their body is muscle versus fat. A 75kg woman who strength trains seriously might have a BMR 200 calories higher than a 75kg woman who doesn't, simply because she's carrying more lean tissue that needs constant fuel.

Age matters too, but not exactly for the reasons most people think. Your metabolism doesn't slow purely because of some biological clock — it slows largely because people tend to lose muscle mass as they age, and that reduces their metabolic engine. Preserve the muscle, and the slowdown is much gentler than the stereotype suggests.

From BMR to Actual Daily Needs: Where TDEE Comes In

Your BMR is the foundation, but it's not what you should actually be eating. What matters for real life is your Total Daily Energy Expenditure, or TDEE — and to get there, you multiply your BMR by an activity factor.

Someone who sits at a desk all day and doesn't exercise might multiply by 1.2. A teacher who's on their feet for six hours and hits the gym three times a week might be closer to 1.55. A professional athlete or someone with a physically demanding job could be at 1.9 or higher. The calculator above shows your TDEE across all five activity levels, highlighting the one you selected — so you can see the full picture at a glance.

One honest caveat: most people overestimate how active they are. A 45-minute gym session followed by 14 hours of sitting doesn't quite land at "very active." When in doubt, starting at the lower estimate and adjusting based on real results tends to be more effective than starting high and wondering why nothing's working.

What You Can Do With Your BMR Number

Knowing your BMR opens up some genuinely useful decisions. For weight loss, most evidence suggests eating at a deficit of 300–500 calories below your TDEE is the sweet spot — aggressive enough to make real progress, gentle enough that your body doesn't aggressively down-regulate metabolism or start cannibalizing muscle. Going below your actual BMR consistently is where things start to go wrong: your body treats it as a famine signal, and the hormonal response makes maintaining any weight loss extremely difficult long-term.

For muscle gain, eating roughly 200–300 calories above TDEE gives you the surplus needed for muscle protein synthesis without accumulating fat too quickly. For athletic performance or maintenance, matching your TDEE keeps everything stable.

There's also a therapeutic angle. People recovering from illness, eating disorders, or extreme dieting often need to understand their BMR to rebuild a healthy relationship with food — recognizing that even at rest, the body needs a substantial amount of fuel just to function is sometimes a revelation for people who've been chronically under-eating.

Things BMR Doesn't Capture (And Why That's Okay)

The Mifflin-St Jeor equation doesn't account for thyroid function, which can push BMR up or down by 40% or more in people with thyroid disorders. It doesn't account for certain medications, significant hormonal changes like pregnancy or menopause, or rare metabolic conditions. For most healthy adults it's accurate enough to be genuinely useful as a planning tool.

If you find that eating at your calculated TDEE leads to unexplained weight gain or loss after a month of consistent tracking, that's useful information — it means your actual metabolic rate differs from the prediction, and you can adjust from there. The formula gives you a smart starting point, not an unchangeable verdict.

The most accurate way to measure BMR is indirect calorimetry — a clinical test where you breathe into a specialized mask while a machine measures your oxygen consumption and CO2 output. It costs money and requires a lab visit, which is why equations like Mifflin-St Jeor exist: they give you 90% of the benefit with none of the overhead.

A Quick Note on Hormones and Sex Differences

The male/female split in the equation isn't just about muscle. Testosterone promotes muscle growth and slightly elevates resting metabolism; estrogen has a more complex effect on fat distribution and metabolic efficiency. Women also experience cyclical metabolic shifts across their menstrual cycle — BMR tends to be slightly higher in the luteal phase (after ovulation) than in the follicular phase, by roughly 150–300 calories per day depending on the individual. This is why some women notice genuine hunger spikes in the week before their period — the body is asking for more fuel, and it's not wrong.

For transgender individuals using hormone therapy, the equation may become less accurate over time as body composition shifts, and recalculating periodically makes more sense than treating any single number as permanent.

Start Here, Adjust From Reality

Your BMR number is a starting point for understanding your own biology, not a cage. Human metabolism is adaptive, individual, and constantly responding to inputs you give it. Use this number to build an initial framework, track your real-world results over a few weeks, and then adjust. The combination of a solid estimate and attentive self-observation beats any formula on its own.

FAQ

What is BMR and why does it matter?
BMR stands for Basal Metabolic Rate — it's the number of calories your body burns in a 24-hour period if you were lying completely still and doing nothing. It represents the minimum energy your organs need just to keep you alive: heart pumping, lungs breathing, kidneys filtering, brain functioning. For most people this is 60–75% of their total daily calorie burn, which is why it's the foundation of any accurate nutrition plan.
What makes the Mifflin-St Jeor equation more accurate than older formulas?
The Mifflin-St Jeor equation (published in 1990) was developed using a larger, more representative sample than the original 1919 Harris-Benedict formula. Multiple independent studies have found it predicts measured BMR within about 10% for most healthy adults, outperforming Harris-Benedict in accuracy. The main reason is that modern populations have different average body compositions than people in the early 20th century, and Mifflin-St Jeor was calibrated against contemporary subjects.
Should I eat at my BMR to lose weight?
No — eating at your BMR is typically too low and can backfire. Your body interprets a severe calorie deficit as famine, triggering hormonal responses that slow your metabolism and break down muscle. A safer and more effective approach is to calculate your TDEE (BMR × activity level) and then eat 300–500 calories below that. This creates a meaningful deficit without the metabolic penalty that comes from drastically under-fueling.
Why does muscle mass affect BMR so much?
Muscle tissue is metabolically active — even at rest, each pound of muscle burns roughly 3 times more calories than a pound of fat. Someone with a higher proportion of lean body mass therefore has a higher BMR, even at the same total body weight. This is why strength training can raise your resting metabolism over time, and why people who lose muscle mass through crash dieting often find it harder to maintain weight loss — their metabolic engine has shrunk.
Does BMR change with age?
Yes, but the cause is more nuanced than 'metabolism slows with age.' The main driver is gradual loss of muscle mass (sarcopenia), which starts in your 30s and accelerates without resistance training. Less muscle means a lower BMR. People who maintain muscle mass through regular strength training see a much gentler decline. Hormonal shifts — lower testosterone in men, menopause in women — also contribute, but muscle loss is the dominant factor.
How accurate is the BMR calculator for me personally?
For a typical healthy adult without thyroid disorders or unusual metabolic conditions, the Mifflin-St Jeor equation is accurate within roughly 10%. However, it cannot account for thyroid dysfunction (which can shift BMR by 40% or more), certain medications, pregnancy, or rare metabolic conditions. The best approach is to use the calculator as your starting estimate, then track your actual food intake and weight over 3–4 weeks and adjust your calorie targets based on real results.