What is BMR (Basal Metabolic Rate)?
Your Basal Metabolic Rate (BMR) is the number of calories your body needs to sustain its core physiological functions — heartbeat, breathing, brain activity, cell repair, hormone production — while you are completely at rest and in a fasted state. Think of it as your body's idle fuel consumption, the minimum energy needed just to keep you alive.
BMR accounts for roughly 60–75% of all calories most people burn daily. Understanding your BMR is the foundation of any effective nutrition plan, whether your goal is weight loss, weight gain, or maintenance. It is also a key metric used by NHS dietitians, sports scientists, and nutritionists when planning calorie-controlled diets.
Two main formulas are used to estimate BMR. This calculator supports both:
From BMR to TDEE — Activity Multipliers
Your BMR is multiplied by an activity factor to estimate your Total Daily Energy Expenditure (TDEE) — the real-world calories you burn each day. The activity factors used in this calculator are the same as those recommended in NHS nutritional guidance:
| Activity Level | Description | Multiplier |
|---|
| Sedentary | Desk job, little or no exercise | × 1.20 |
| Lightly Active | Light exercise 1–3 days per week | × 1.375 |
| Moderately Active | Moderate exercise 3–5 days per week | × 1.55 |
| Very Active | Hard exercise 6–7 days per week | × 1.725 |
| Extra Active | Twice-daily training or physical job | × 1.90 |
Using Your BMR for Weight Loss
To lose weight safely, you create a calorie deficit relative to your TDEE — not your BMR. Eating at exactly your BMR for extended periods can cause muscle loss, metabolic adaptation, fatigue, and nutrient deficiencies. NHS guidelines recommend a deficit of 300–500 kcal per day below TDEE for a gradual, sustainable rate of fat loss (approximately 0.3–0.5 kg per week).
For example, if your TDEE at moderate activity is 2,200 kcal/day, a 500 kcal deficit means eating 1,700 kcal/day — a manageable target that keeps you well above your BMR and preserves muscle mass.
How to Use This BMR Calculator
1
Choose Your Units and Formula
Select Metric (kg/cm) or Imperial (lbs/ft). Choose the Mifflin-St Jeor formula for the most accurate modern result, or Harris-Benedict if you want a comparison. Both are widely used by NHS dietitians and sports nutritionists.
2
Enter Gender, Age, Activity, Height and Weight
All five inputs are required. Your age and gender influence the formula constants significantly — especially for women, whose BMR is notably lower than men's at equivalent height and weight due to differences in lean muscle mass.
3
Enable Pregnancy Mode (if applicable)
Tick the pregnancy checkbox and select your trimester. The NHS recommends no extra calories in the first trimester, +300 kcal/day in the second, and +450 kcal/day in the third. These are automatically added to your TDEE totals.
4
Read Your BMR and TDEE Breakdown
Your BMR is shown prominently. Below it, the TDEE for all five activity levels is listed so you can find the right calorie target regardless of how active you are that week. The calorie deficit section shows your exact weight-loss targets.
BMR Calculator During Pregnancy
During pregnancy, your BMR rises as your body works harder to support the developing foetus, increases blood volume, and maintains the placenta and uterine tissue. The rise is minimal in the first trimester but becomes significant in the second and third. The NHS advises pregnant women not to "eat for two" — rather, to make a modest, targeted calorie increase aligned with trimester. This calculator implements those NHS/WHO trimester adjustments automatically when pregnancy mode is enabled.
BMR for Teenagers and Kids
This calculator is suitable for anyone aged 2 and above. Children and teenagers have higher BMR per unit of body weight than adults because they are in active growth phases. Adolescents and teens should never use the output as a basis for severe calorie restriction — any weight management plan for under-18s must be supervised by a paediatric healthcare provider. The calorie deficit guidance shown in the results does not apply to users under 18.