Body Mass Index is mass in kilograms divided by height in metres squared. It's a crude population screen, not a diagnosis. WHO categories: under 18.5 underweight, 18.5-24.9 normal, 25-29.9 overweight, 30+ obese.

What is the Mifflin-St Jeor equation?

A widely used estimate of resting energy expenditure: BMR = 10·mass + 6.25·height − 5·age + s, where s = +5 for males and −161 for females. More accurate than the older Harris-Benedict in modern populations.

Total Daily Energy Expenditure: BMR multiplied by an activity factor (1.2 sedentary to 1.9 very active). It estimates daily kilocalories required to maintain current mass.

Are these numbers diagnostic?

No. BMI doesn't distinguish lean mass from fat mass, and BMR is an estimate that can be ±10% off for any individual. Use them for HSC homeostasis and nutrition discussion, not medical advice.

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NSW · HSCBiology

BMI and BMR calculator

Compute BMI from mass and height, plus BMR from the Mifflin-St Jeor equation, plus TDEE for a chosen activity level. Useful for HSC Biology Module 8 (non-infectious disease) and human-biology contexts.

Formula

BMI = mass(kg) / height(m)²

BMR = 10·mass + 6.25·height(cm) − 5·age + s

where s = +5 for males and −161 for females (Mifflin-St Jeor 1990).

TDEE = BMR × activity factor

Inputs

Biological sexAge (years)Height (cm)Body mass (kg)Activity level

BMI = mass / height². BMR uses the Mifflin-St Jeor equation. TDEE = BMR × activity factor.

Result

BMI

22.86 kg/m²

Normal weight

BMR (Mifflin-St Jeor)

1714 kcal/day

BMR = 10·m + 6.25·h − 5·age + 5

Estimated TDEE

2356 kcal/day (BMR × 1.375)

Worked example

A 17-year-old male is 175 cm tall and weighs 70 kg, lightly active (activity factor 1.375).

BMI = 70 / 1.75² = 70 / 3.0625 ≈ 22.86 kg/m² — normal weight.

BMR = 10 × 70 + 6.25 × 175 − 5 × 17 + 5 = 700 + 1093.75 − 85 + 5 = 1713.75 kcal/day.

TDEE = 1713.75 × 1.375 ≈ 2356 kcal/day.

How this calculator works

The calculator converts height from cm to m for BMI, applies the Mifflin-St Jeor equation for BMR, and multiplies BMR by the selected activity factor to estimate TDEE. BMI categories follow the WHO classification (under 18.5, 18.5-24.9, 25-29.9, 30+).

Common questions

What is BMI?: Body Mass Index is mass in kilograms divided by height in metres squared. It's a crude population screen, not a diagnosis. WHO categories: under 18.5 underweight, 18.5-24.9 normal, 25-29.9 overweight, 30+ obese.
What is the Mifflin-St Jeor equation?: A widely used estimate of resting energy expenditure: BMR = 10·mass + 6.25·height − 5·age + s, where s = +5 for males and −161 for females. More accurate than the older Harris-Benedict in modern populations.
What is TDEE?: Total Daily Energy Expenditure: BMR multiplied by an activity factor (1.2 sedentary to 1.9 very active). It estimates daily kilocalories required to maintain current mass.
Are these numbers diagnostic?: No. BMI doesn't distinguish lean mass from fat mass, and BMR is an estimate that can be ±10% off for any individual. Use them for HSC homeostasis and nutrition discussion, not medical advice.