Metabolic age compares your resting metabolic rate with the average resting metabolic rate of people the same age and sex. It is the kind of number provided by smart scales and body composition monitors, and it can serve as a useful, non-invasive snapshot of how your lifestyle and body composition are tracking.
This calculator estimates your BMR using two respected methods: Mifflin-St Jeor when body-fat input is skipped, and Katch-McArdle when you enter a body-fat percentage. It then compares that BMR to a reference average BMR curve to derive an approximate metabolic age.
Use it to monitor progress during fat-loss or muscle-building programs, or simply to understand why two people of the same age can have very different resting energy needs. Remember that metabolic age is an estimate for guidance, not a medical diagnosis.
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How the Metabolic Age Calculation Works
Metabolic age is a comparison metric. Your basal metabolic rate (BMR) is first estimated, and then that number is compared with a reference average BMR for your chronological age and sex. If your BMR matches the average 35-year-old, your metabolic age is about 35; if it matches the average 25-year-old, your metabolic age is about 25.
This calculator uses two standard BMR equations:
- Mifflin-St Jeor equation — used when body fat is not entered. Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5. Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161.
- Katch-McArdle formula — used when body fat is entered. BMR = 370 + 21.6 × lean body mass(kg), where lean body mass = weight(kg) × (1 − body fat percentage / 100). Because this equation uses lean mass rather than total weight, it generally gives a more meaningful metabolic age for people with unusual body compositions.
Once BMR is known, the calculator compares it to a reference BMR curve. The reference values used here assume an average adult body size of 88 kg and 175 cm for men, and 72 kg and 163 cm for women, with BMR declining about 5 kcal per year after early adulthood. This is a simplification, but it produces a stable and intuitive scale for everyday tracking.
Step-by-Step Guide
- Select your biological sex and enter your chronological age in years.
- Enter your height and weight. Use the dropdowns to switch between centimeters/inches and kilograms/pounds.
- Optional but recommended: enter your body fat percentage. You can use one of the quick-fill presets. If you skip it, the calculator falls back to the Mifflin-St Jeor equation.
- Press Calculate Metabolic Age. The result screen will show your estimated BMR, the reference BMR for your age, the difference in kcal/day, and your metabolic age.
- Use Reset to clear all values before entering a different profile.
Worked Examples
Example 1: 40-year-old woman, 165 cm, 68 kg, 28% body fat
Lean body mass = 68 × (1 − 0.28) = 48.96 kg. BMR = 370 + 21.6 × 48.96 = 1428 kcal/day. The reference female average BMR at age 40 is about 1378 kcal/day, so her metabolic age is about 30 — about 10 years younger than her chronological age.
Example 2: 55-year-old man, 178 cm, 92 kg, 30% body fat
Lean body mass = 92 × 0.70 = 64.4 kg. BMR = 370 + 21.6 × 64.4 = 1761 kcal/day. The reference male average BMR at age 55 is about 1704 kcal/day, so his metabolic age is about 43.5.
Reference Average BMR Values
The table below shows the approximate reference BMR values used by this calculator for an average male body (88 kg, 175 cm) and average female body (72 kg, 163 cm). These are rounded estimates, not medical norms.
| Age | Men (kcal/day) | Women (kcal/day) |
|---|---|---|
| 20 | 1,879 | 1,478 |
| 25 | 1,854 | 1,453 |
| 30 | 1,829 | 1,428 |
| 35 | 1,804 | 1,403 |
| 40 | 1,779 | 1,378 |
| 45 | 1,754 | 1,353 |
| 50 | 1,729 | 1,328 |
| 55 | 1,704 | 1,303 |
| 60 | 1,679 | 1,278 |
| 65 | 1,654 | 1,253 |
| 70 | 1,629 | 1,228 |
Limitations & Accuracy Notes
Metabolic age is not a medical measurement. BMR equations have a typical error range of roughly 5–10% when applied to groups, and the error can be larger for an individual. Direct measurement using indirect calorimetry remains the gold standard.
Because absolute BMR increases with body size, the weight-based Mifflin-St Jeor method can make a heavier person appear metabolically younger even if their body fat is high. Entering body fat percentage helps correct this because the Katch-McArdle formula focuses on metabolically active lean mass. If you do not know your body fat percentage, treat the result as a rough guide only.
Smart scales and gym devices often produce different metabolic ages because they rely on proprietary bioelectrical impedance algorithms and different reference populations. Use one consistent method when tracking changes over time.
Frequently Asked Questions
What does a lower metabolic age mean? It means your estimated resting calorie burn is closer to what is typical for a younger adult of the same sex. This often accompanies higher lean mass, lower body fat, or both, but it is not a guarantee of overall health.
How can I improve my metabolic age? The most reliable strategies are resistance training to build and preserve muscle, adequate protein intake, consistent sleep, and regular movement. As lean mass increases, BMR tends to rise, which usually shifts metabolic age downward.
Why did my metabolic age go up after losing weight? Rapid weight loss can reduce both fat and lean mass. Because BMR is closely tied to metabolically active tissue, losing muscle can lower BMR and push metabolic age upward. This is why body-fat percentage matters for interpretation.
Is metabolic age a medical diagnosis? No. It is an educational estimate. Only a qualified health professional can evaluate your metabolic health using clinical tests, medical history, and laboratory results.