Metabolic Age Calculator
Lean mass comes first, from body fat percentage, and then BMR from the Katch–McArdle equation: 370 + 21.6 × lean mass. Next, the Mifflin–St Jeor average is traced across ages at the same sex, height and weight, and the age where that curve meets your figure is the answer. Mifflin drops 5 kcal per year of age, so running 5 kcal above the average buys you one year. A 35-year-old man of 175 cm and 70 kg at 18% body fat carries 57.4 kg of lean mass and a BMR of 1,610 kcal, while the average for that body at 35 is 1,624 kcal — a metabolic age of 37.8.
Your numbers
Metabolic age
37.8yr
BMR
1,610kcal
Difference
2.8yr
Lean mass
57.4kg
What it means
A metabolic age of 37.8 is 2.8 years above your actual age. Almost all of that gap comes from lean mass — one percentage point of body fat moves it by about 3 years.
Formula
Metabolic age = Age + (avgBmr − BMR) ÷ 5
"Metabolic age" is a consumer idea invented by body-composition scales, not a clinical measure. The only thing it really reads is lean mass: in the example above, one percentage point of body fat shifts the answer by three years. Your own figure also comes from Katch–McArdle while the reference curve is Mifflin–St Jeor, and the two do not line up exactly at an average body composition — so read the direction and size of the gap rather than the year itself.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Sex (M1 / F0)Where the formula splits by sex — enter 1 for male, 0 for female. | 1 | 0 ~ 1 |
| Age (yr)Age in completed years — it shifts by one depending on the birthday. | 35 | 0 ~ 100 |
| Body fat (%)The share of body weight that is fat. | 18 | 0 ~ 70 |
| Height (cm) (cm)Height in centimetres; converted to metres inside the formula. | 175 | 0 ~ 250 |
| Weight (kg) (kg)The second weight you are comparing against. | 70 | 0 ~ 250 |
Step by step
Quick reference table
Results when only Body fat (%) changes and everything else stays put.
| Body fat (%) | Metabolic age (yr) | BMR (kcal) | Difference (yr) |
|---|---|---|---|
| 9 | 18 | 1,746 | -17 |
| 13.5 | 24.2 | 1,678 | -10.8 |
| 18 | 37.8 | 1,610 | 2.8 |
| 27 | 65 | 1,474 | 30 |
| 36 | 85 | 1,338 | 50 |
What each result means
| Result | At default values |
|---|---|
| Metabolic age (yr)The age whose average BMR matches yours — a consumer-scale idea, not a clinical one. | 37.8 |
| BMR (kcal)The calories you burn in a day doing nothing at all. | 1,610 |
| Difference (yr)The gap between two values — a quantity, not a rate. | 2.8 |
| Lean mass (kg)Everything that is not fat — muscle, bone, water. | 57.4 |
Common mistakes
"Metabolic age" is a consumer idea invented by body-composition scales, not a clinical measure. The only thing it really reads is lean mass: in the example above, one percentage point of body fat shifts the answer by three years. Your own figure also comes from Katch–McArdle while the reference curve is Mifflin–St Jeor, and the two do not line up exactly at an average body composition — so read the direction and size of the gap rather than the year itself.
Glossary
- Sex (M1 / F0)
- Where the formula splits by sex — enter 1 for male, 0 for female.
- Age
- Age in completed years — it shifts by one depending on the birthday.
- Body fat
- The share of body weight that is fat.
- Height (cm)
- Height in centimetres; converted to metres inside the formula.
- Weight (kg)
- The second weight you are comparing against.
- Metabolic age
- The age whose average BMR matches yours — a consumer-scale idea, not a clinical one.
- BMR
- The calories you burn in a day doing nothing at all.
- Difference
- The gap between two values — a quantity, not a rate.
- Lean mass
- Everything that is not fat — muscle, bone, water.
Frequently asked questions
Q. How is Metabolic Age Calculator calculated?
Metabolic age = Age + (avgBmr − BMR) ÷ 5 — Lean mass comes first, from body fat percentage, and then BMR from the Katch–McArdle equation: 370 + 21.6 × lean mass. Next, the Mifflin–St Jeor average is traced across ages at the same sex, height and weight, and the age where that curve meets your figure is the answer. Mifflin drops 5 kcal per year of age, so running 5 kcal above the average buys you one year. A 35-year-old man of 175 cm and 70 kg at 18% body fat carries 57.4 kg of lean mass and a BMR of 1,610 kcal, while the average for that body at 35 is 1,624 kcal — a metabolic age of 37.8.
Q. Can you walk through an example?
With Sex (M1 / F0) 1, Age 35yr, Body fat 18%, Height (cm) 175cm, Weight (kg) 70kg, the answer is Metabolic age 37.8yr.
Q. What do I need to enter?
Enter Sex (M1 / F0), Age, Body fat, Height (cm), Weight (kg). The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Body fat (%) moves the answer to Body fat (%) 9 → Metabolic age (yr) 18 and Body fat (%) 36 → Metabolic age (yr) 85. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
"Metabolic age" is a consumer idea invented by body-composition scales, not a clinical measure. The only thing it really reads is lean mass: in the example above, one percentage point of body fat shifts the answer by three years. Your own figure also comes from Katch–McArdle while the reference curve is Mifflin–St Jeor, and the two do not line up exactly at an average body composition — so read the direction and size of the gap rather than the year itself.
Related calculators
These calculators help you understand health figures; they do not replace diagnosis or prescription. See a clinician if you have symptoms.