Question 1

What is BMR (Basal Metabolic Rate)?

Accepted Answer

BMR, or Basal Metabolic Rate, is the number of calories your body burns at complete rest to maintain essential vital functions — breathing, heartbeat, circulation, cell repair, temperature regulation, and organ function. It represents the minimum energy your body needs to survive if you stayed in bed all day. BMR accounts for roughly 60-75% of your total daily calorie expenditure, making it the most important factor in understanding your energy needs.

Question 2

Which BMR formula should I use — Mifflin-St Jeor, Harris-Benedict, or Katch-McArdle?

Accepted Answer

For most people, the Mifflin-St Jeor equation is the gold standard — validated by multiple clinical studies, it is the most accurate for sedentary to moderately active adults. Harris-Benedict (revised 1984) is slightly older and tends to overestimate by 5-15%. Katch-McArdle is the most accurate formula for athletes and lean individuals who know their body fat percentage, as it accounts for lean mass directly rather than using height and weight as proxies. If unsure, use Mifflin-St Jeor.

Question 3

How do I calculate TDEE from BMR?

Accepted Answer

TDEE (Total Daily Energy Expenditure) is your BMR multiplied by an activity factor: Sedentary (desk job, no exercise) = BMR x 1.2; Lightly active (light exercise 1-3 days/week) = BMR x 1.375; Moderately active (moderate exercise 3-5 days/week) = BMR x 1.55; Very active (hard exercise 6-7 days/week) = BMR x 1.725; Extra active (athlete or physical job plus daily training) = BMR x 1.9. For weight loss, eat 300-500 kcal below TDEE; for gain, eat 250-500 kcal above TDEE.

Question 4

What is the difference between BMR and RMR?

Accepted Answer

BMR (Basal Metabolic Rate) is measured under strict conditions — completely at rest, in a fasting state, in a temperature-neutral environment, after a full night's sleep. RMR (Resting Metabolic Rate) is measured at rest but without the strict fasting and sleep requirements, making it slightly higher (typically 10-20% above BMR). In practice, most online calculators and apps calculate RMR while labeling it BMR. The difference is small for everyday calorie planning purposes.

Question 5

How does age affect BMR?

Accepted Answer

BMR naturally declines with age — typically 1-2% per decade after age 30. This is primarily due to sarcopenia (loss of muscle mass), which reduces metabolically active tissue. A 50-year-old has roughly 5-10% lower BMR than their 30-year-old self even at the same weight. This explains why many people gain weight gradually with age despite eating the same amount. Resistance training to preserve muscle mass is the most effective strategy to maintain BMR as you age.

Question 6

Does muscle mass affect BMR?

Accepted Answer

Yes significantly. Muscle tissue burns approximately 6 kcal per pound per day at rest, while fat tissue burns only about 2 kcal per pound per day. A person with more lean muscle mass has a substantially higher BMR than someone of the same weight with more body fat. This is why the Katch-McArdle formula (which uses lean body mass) is the most accurate for athletes. Strength training that builds muscle is one of the few proven ways to permanently increase BMR.

Question 7

Should I eat at my BMR or at my TDEE?

Accepted Answer

You should base food intake on your TDEE, not your BMR. BMR is the absolute minimum — eating at BMR level is appropriate only under strict medical supervision. Consistently eating below BMR can cause muscle loss, metabolic adaptation, nutritional deficiencies, and hormonal disruption. A healthy calorie deficit for weight loss is 300-500 kcal below TDEE, which leads to approximately 0.3-0.5 kg (0.7-1 lb) of fat loss per week.

Question 8

How accurate are online BMR calculators?

Accepted Answer

The Mifflin-St Jeor equation has a prediction error of approximately 10% for most people. The only truly accurate measurement requires indirect calorimetry — measuring oxygen consumption and CO2 production in a clinical lab setting. For practical purposes, use the Mifflin-St Jeor result as your starting point, track your actual weight change over 3-4 weeks while eating at calculated TDEE, then adjust your calorie intake up or down by 100-200 kcal based on real-world results.

Question 9

Does BMR change with weight loss?

Accepted Answer

Yes — as you lose weight, your BMR decreases because you have less body mass to maintain. This is known as metabolic adaptation. Additionally, sustained calorie restriction triggers further adaptive thermogenesis — your body becomes more efficient and burns even fewer calories than the formula predicts. This is why weight loss often plateaus. Recalculate your BMR and TDEE every 5-10 kg of weight change, and consider diet breaks or refeeds to partially reverse metabolic adaptation.

Question 10

Is my data private when I use this BMR calculator?

Accepted Answer

Yes, completely. All BMR calculations run entirely within your browser. No personal data — age, weight, height, or body fat percentage — is transmitted to any server, stored in a database, or shared with any third party. The calculator works fully offline once the page loads. You can safely enter your real measurements without any privacy concerns.

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How to Use This Calculator

Step 1: Pick your units: metric (kg, cm) or imperial (lb, ft/in)

Step 2: Enter your age, gender, weight, height

Step 3: See your BMR instantly with 3 top formulas

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