BMI & BMR Calculator

How this calculator works

BMI — what it measures and what it doesn't

BMI = weight in kilograms divided by the square of height in metres. The formula is straightforward: a person 170 cm tall weighing 70 kg has BMI 70 ÷ 1.7² = 24.2.

BMI was designed in the 1830s by a Belgian statistician as a population-level tool, not an individual diagnostic. It correlates with body fat for most adults but breaks down for: very muscular people (lifters, athletes — who show as overweight or obese while having low body fat), older adults losing muscle mass (whose BMI looks "healthy" while body fat is high), pregnant or breastfeeding women, and people under 18. BMI is a starting point, not the verdict.

The Asian-Indian BMI thresholds

Population studies across India, Pakistan, Bangladesh and Sri Lanka have consistently shown that South Asians develop type 2 diabetes, cardiovascular disease and metabolic syndrome at lower BMI than Europeans. The reason: South Asian body composition typically carries more visceral fat (around organs) and less subcutaneous fat (under skin) at any given BMI. Visceral fat drives insulin resistance and inflammation more than subcutaneous fat does.

The ICMR and the Asia-Pacific Working Group recommend these thresholds for Indians:

If your BMI is 24, the standard WHO chart classifies you as normal. The Asian-Indian threshold classifies you as overweight. The clinical implication is real: Indians with BMI 23-25 have measurably higher rates of insulin resistance and dyslipidemia than Europeans at the same BMI. The threshold isn't arbitrary — it reflects actual disease risk.

BMR — the resting calorie burn

Basal Metabolic Rate is the calories your body burns at complete rest, just to maintain breathing, circulation, body temperature, and basic cellular function. BMR accounts for 60-75% of your total daily calorie expenditure for most people.

This calculator uses the Mifflin-St Jeor equation, which is the most accurate published formula for healthy adults — outperforming the older Harris-Benedict equation by 5-10% in validation studies:

For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161

Example: a 35-year-old woman, 165 cm, 65 kg: BMR = 650 + 1031.25 − 175 − 161 = 1,345 calories per day.

From BMR to TDEE

Total Daily Energy Expenditure adds the calories burned through movement and digestion. The standard activity multipliers:

• Sedentary (desk job, no formal exercise): BMR × 1.2
• Lightly active (1-3 days light exercise per week): BMR × 1.375
• Moderately active (3-5 days moderate exercise): BMR × 1.55
• Very active (6-7 days intense exercise or physical job): BMR × 1.725
• Extremely active (intense daily exercise plus physical job): BMR × 1.9

For the 35-year-old woman above with a desk job and 3 yoga sessions a week, TDEE is 1,345 × 1.375 ≈ 1,850 calories per day. That's her maintenance: eating this amount keeps her weight stable. To lose weight, eat less; to gain, eat more.

Realistic calorie deficits

A deficit of 500 calories per day produces about 0.5 kg loss per week (since 1 kg of body fat is roughly 7,700 calories). Most clinicians recommend not exceeding 750 calories per day deficit because larger deficits trigger metabolic adaptation — the body lowers BMR to conserve energy, plateauing weight loss and making the diet harder to sustain.

For someone with TDEE 2,200, a sustainable cutting target is 1,700-1,900 calories per day. Below 1,200 calories for women and 1,500 for men is generally considered too low for long-term sustainability without medical supervision.

What BMI misses — waist circumference matters more

For Indians especially, waist circumference is a better predictor of metabolic disease than BMI. The thresholds for Indian adults:

• Men: waist above 90 cm (35.4 inches) indicates abdominal obesity
• Women: waist above 80 cm (31.5 inches) indicates abdominal obesity

Someone with BMI 22 (normal even on Indian thresholds) but waist 95 cm has visceral fat accumulation that puts them at higher diabetes and cardiovascular risk than someone with BMI 26 but waist 85 cm. The "thin outside, fat inside" phenotype — common in South Asia — is missed entirely by BMI.

Worked example

30-year-old man, 175 cm, 75 kg, sedentary office worker:

• BMI = 75 ÷ 1.75² = 24.5
• Standard WHO: Normal
• Asian-Indian: Overweight (above 23 threshold)
• BMR (Mifflin-St Jeor) = 750 + 1093.75 − 150 + 5 = 1,699 calories
• TDEE at sedentary 1.2: 2,039 calories per day for maintenance
• To lose 0.5 kg per week: target 1,539 calories

40-year-old woman, 160 cm, 58 kg, moderately active (yoga 4 times a week):

• BMI = 58 ÷ 1.6² = 22.7
• Both standards: Normal weight
• BMR = 580 + 1000 − 200 − 161 = 1,219 calories
• TDEE at moderate 1.55: 1,889 calories per day for maintenance
• To lose 0.5 kg per week: target 1,389 calories

55-year-old man, 168 cm, 82 kg, lightly active:

• BMI = 82 ÷ 1.68² = 29.1
• Standard WHO: Overweight
• Asian-Indian: Obese Class I (above 25)
• BMR = 820 + 1050 − 275 + 5 = 1,600 calories
• TDEE at light 1.375: 2,200 calories per day for maintenance
• To lose 0.7 kg per week: target 1,500-1,600 calories with strength training to preserve muscle