If you've ever wondered why one person can eat more than another without gaining weight, or why weight loss won't budge no matter how hard you try, the answer is often found in your basal metabolic rate. It's the baseline number everything else is built on.
When I started my own journey of losing nearly 40 kg, I made exactly the mistake most people make at the start: I guessed. I picked up a calorie target I'd heard somewhere was "a good number for losing weight", and I never once stopped to think about what it was actually based on. It wasn't tied to my size, my age or my everyday life in any way at all. Only once I understood what BMR and TDEE actually mean did the whole thing become a plan instead of guesswork.
What does BMR mean?
BMR stands for Basal Metabolic Rate. It tells you how many calories your body burns at complete rest, just to keep you alive. Your heart, lungs and brain, along with cell renewal, all require energy even if you don't move a muscle.
For a typical adult, BMR falls somewhere between 1,400 and 2,000 kcal. It's the number of calories you'd burn if you lay on the sofa all day without moving at all.
This is worth pausing on for a second, because it reveals something rather important: the vast majority of your daily burn happens without you doing anything at all. The calories burned at the gym turn out to be a surprisingly small slice of the whole day. Plenty of people, myself included, back in the day, imagine it works the other way round, and try to burn off with exercise what actually gets decided in the kitchen.
How is BMR calculated?
There are several formulas. RestinCal uses the Mifflin-St Jeor equation, one of the commonly used ways to estimate resting energy expenditure. An individual's result can still differ from a measured value:
- Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (yr) + 5
- Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (yr) − 161
For example, a 30-year-old man weighing 75 kg and 175 cm tall:
10 × 75 + 6.25 × 175 − 5 × 30 + 5 = 1,724 kcal/day
That's the baseline. But it doesn't yet tell you how much you actually need.
BMR vs. TDEE, what's the difference?
BMR is only the starting point. In real life you need TDEE (Total Daily Energy Expenditure), which accounts for your activity:
| Activity level | Multiplier | Example |
|---|---|---|
| Desk job, no exercise | × 1.2 | Office work with no hobbies |
| Light exercise 1–3 days/wk | × 1.375 | A couple of walks a week |
| Moderate exercise 3–5 days/wk | × 1.55 | Gym a few times a week |
| Heavy exercise 6–7 days/wk | × 1.725 | Athlete level |
The same 30-year-old man with moderate activity: 1,724 × 1.55 = approx. 2,672 kcal/day. That's how much he needs to maintain his weight.
What do you do with this?
- Weight loss: Eat 300–500 kcal below your TDEE. This produces about 0.3–0.5 kg of loss per week, slow enough to preserve muscle.
- Maintenance: Eat at your TDEE.
- Muscle gain: Eat 200–300 kcal above your TDEE, with enough protein.
The numbers are always estimates, the formula doesn't know your body fat percentage, your hormone levels or how well you sleep. But it gives a solid starting point you can fine-tune based on real-world tracking.
Why the calculator only gives you a starting point
This is the point where it's worth being honest: the formula doesn't really know anything about you. It doesn't know your body fat percentage, your hormones, how well you sleep, or how much you move during the day without even noticing. Two people of exactly the same size can burn different amounts simply because one is naturally restless and fidgets all day, while the other sits still.
The biggest source of error, though, isn't the BMR formula itself, it's the activity multiplier. It's a rough bracket, and most of us overestimate our own activity by quite a margin. A couple of gym sessions a week doesn't make anyone "heavily active", however it might feel.
That's why the number the calculator gives you is best treated as a hypothesis, not a truth. Here's how I did it myself:
- Work out your TDEE and pick the lower multiplier if you're torn between two options. Guessing low is safer than guessing high.
- Eat at that level for 2–3 weeks and log everything honestly, including cooking oils, coffee milks and tastes while cooking.
- Look at what the scale says as a weekly average, not on a single morning. The daily swings are mostly water and tell you nothing.
- Adjust by 100–200 kcal at a time in the direction you want, then give it another couple of weeks.
After two or three weeks you'll have your own, real number in your hands, not the formula's guess. That's considerably more valuable information than any calculator can give you. This is also the point where honest logging decides everything: if you only record part of what you eat, you'll be adjusting the number based on false information and then wondering why nothing works.
The most common misconceptions
- "BMR slows down dramatically when dieting." Energy expenditure generally falls as body weight decreases because a smaller body requires less energy. The size of that change cannot be expressed as one percentage for everyone, so it makes sense to update the estimate as weight changes.
- "I have a slow metabolism." Rarely the real explanation. In studies, the variation between people of the same size is surprisingly small. Most often the problem is underestimating calories, not metabolism.
- "The activity multiplier is accurate." It isn't. Most people overestimate their activity. If you're unsure, pick a lower multiplier and watch your actual weight change over a few weeks.
- "I need to find the one correct number." You don't. The number changes along the way: as the weight comes off, your burn drops too, because a smaller body is lighter to move around. That isn't failure, it's perfectly normal physics, and it's exactly why the number is worth rechecking every few months.
The number is a tool, not a verdict
Plenty of people are disappointed the first time they see their own TDEE. It can look small, and it's a quick leap from there to "I have a slow metabolism" or to feeling the whole thing is somehow unfair.
For me, that number turned out to be pure relief. It ended the guessing. Once I knew roughly what I burned and logged honestly what I ate, all the mystery drained out of weight loss. It became an ordinary calculation you adjust as needed, not a project built on mood or lucky breaks. That predictability is exactly why it lasted the full two years.
So BMR isn't a verdict on what you're capable of. It's a starting point that tells you where you're setting off from. What you build on top of it is entirely up to you.
Sources and further reading
Reviewed 6 August 2026