What is metabolic age?
Metabolic age is a way of expressing your resting metabolism relative to population averages by age. If your metabolism burns more energy than is typical for your chronological age, your metabolic age is lower (younger); if it burns less, your metabolic age is higher (older).
The concept became popular through smart scales, which estimate it from body composition. It isn't a precise clinical metric, but it's a useful, motivating way to see the payoff from building muscle and reducing body fat.
How this calculator works
Because muscle is far more metabolically active than fat, this calculator bases your metabolic age on your lean body mass. It uses your body fat percentage to work out your lean mass, then estimates your resting metabolic rate with the Katch–McArdle equation, which is driven by lean mass rather than total weight.
It then compares your metabolic rate to the expected rate for people of different ages who carry an average amount of body fat, and reports the age that best matches your metabolism. That's why a lean, muscular person scores a younger metabolic age than someone of the same weight with higher body fat.
Worked example: a 40-year-old, 80 kg man at 15% body fat has about 68 kg of lean mass and a Katch–McArdle BMR near 1,840 kcal — higher than the average 40-year-old, which maps to a metabolic age in the low 30s.
What affects your metabolic age?
The single biggest lever is the amount of lean muscle you carry. More muscle raises your resting metabolic rate and lowers your metabolic age, while higher body fat does the opposite.
- Muscle mass — more lean tissue means a higher resting metabolism.
- Body fat percentage — lower body fat for your age lowers your metabolic age.
- Physical activity and training history — regular resistance training builds and preserves muscle.
- Age — metabolism naturally declines over time, largely because muscle is lost if it isn't trained.
- Sex — men typically carry more muscle and have higher resting metabolic rates than women.
Average body fat percentage by age
The calculator compares you against typical body fat levels for your age and sex. The table below shows approximate averages used as a reference — being leaner than these figures generally produces a younger metabolic age.
| Age | Men | Women |
|---|---|---|
| 20–29 | ~18% | ~26% |
| 30–39 | ~20% | ~28% |
| 40–49 | ~22% | ~30% |
| 50–59 | ~23% | ~31% |
| 60+ | ~25% | ~33% |
How to lower your metabolic age
The reliable way to reduce your metabolic age is to improve your body composition: build and maintain muscle through progressive resistance training, eat enough protein, and gradually reduce excess body fat. Because muscle is metabolically active tissue, gaining it raises your resting energy expenditure and pulls your metabolic age down over time. Staying active, sleeping well and managing stress all support the process.
How accurate is metabolic age?
Treat your metabolic age as an educational estimate rather than a diagnosis. It depends heavily on the accuracy of your body fat measurement, and different tools use different reference data, so the exact number will vary between calculators and devices. What's most useful is the trend: as you build muscle and lose fat, your estimated metabolic age should fall. It is not a medical assessment and should not replace advice from a healthcare professional.