Skeletal Muscle Index (Estimate)
This is the index used to track muscle lost with age. Take about three quarters of fat-free mass as limb muscle and divide by height squared. Low values show up first as trouble with stairs and walking.
Your numbers
Muscle mass index
12.3
Lean mass
44.6kg
Fat mass
17.4kg
What it means
12.3 sits above the 5.7 cut-off.
Formula
Muscle mass index = Lean mass × 0.75 ÷ (Height (cm) ÷ 100) ²
A proper measurement needs body-composition equipment. This works backwards from body fat, so treat it as a direction rather than a number.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Weight (kg) (kg)The second weight you are comparing against. | 62 | 1 and up |
| Height (cm) (cm)Height in centimetres; converted to metres inside the formula. | 165 | 50 and up |
| Body fat (%)The share of body weight that is fat. | 28 | 1 ~ 60 |
Step by step
Quick reference table
Results when only Weight (kg) (kg) changes and everything else stays put.
| Weight (kg) (kg) | Muscle mass index | Lean mass (kg) | Fat mass (kg) |
|---|---|---|---|
| 31 | 6.15 | 22.3 | 8.7 |
| 46.5 | 9.22 | 33.5 | 13 |
| 62 | 12.3 | 44.6 | 17.4 |
| 93 | 18.45 | 67 | 26 |
| 124 | 24.6 | 89.3 | 34.7 |
What each result means
| Result | At default values |
|---|---|
| Muscle mass indexLimb muscle over height squared — used to spot muscle shortfall. | 12.3 |
| Lean mass (kg)Everything that is not fat — muscle, bone, water. | 44.6 |
| Fat mass (kg)How much of your weight is fat, in kilograms. | 17.4 |
Common mistakes
A proper measurement needs body-composition equipment. This works backwards from body fat, so treat it as a direction rather than a number.
Glossary
- Weight (kg)
- The second weight you are comparing against.
- Height (cm)
- Height in centimetres; converted to metres inside the formula.
- Body fat
- The share of body weight that is fat.
- Muscle mass index
- Limb muscle over height squared — used to spot muscle shortfall.
- Lean mass
- Everything that is not fat — muscle, bone, water.
- Fat mass
- How much of your weight is fat, in kilograms.
Frequently asked questions
Q. How is Skeletal Muscle Index (Estimate) calculated?
Muscle mass index = Lean mass × 0.75 ÷ (Height (cm) ÷ 100) ² — This is the index used to track muscle lost with age. Take about three quarters of fat-free mass as limb muscle and divide by height squared. Low values show up first as trouble with stairs and walking.
Q. Can you walk through an example?
With Weight (kg) 62kg, Height (cm) 165cm, Body fat 28%, the answer is Muscle mass index 12.3.
Q. What do I need to enter?
Enter Weight (kg), Height (cm), Body fat. 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 Weight (kg) (kg) moves the answer to Weight (kg) (kg) 31 → Muscle mass index 6.15 and Weight (kg) (kg) 124 → Muscle mass index 24.6. 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?
A proper measurement needs body-composition equipment. This works backwards from body fat, so treat it as a direction rather than a number.
Related calculators
These calculators help you understand health figures; they do not replace diagnosis or prescription. See a clinician if you have symptoms.