Skeletal muscle index
ASM ÷ height² (kg/m²), with optional sex-specific cut-offs
Skeletal muscle index
ASM is the sum of lean muscle in both arms and both legs from a body-composition report (BIA or DXA). This tool divides that mass by height squared—same definition most research papers use when they write "kg/m²" for appendicular muscle.
How the index is defined
Researchers and device manuals usually define skeletal muscle index (in this appendicular sense) as appendicular skeletal muscle mass in kilograms divided by standing height in metres, squared. Units end up as kg/m²—superficially like BMI, but the numerator is limb muscle, not total body weight.
SMI = ASMkg ÷ h² (h in metres)
Example: ASM 30.0 kg at 1.75 m tall → 30 ÷ (1.75 × 1.75) ≈ 9.8 kg/m². If you only know height in centimetres, divide by 100 first.
US height input on this page is converted to metres for you (total inches × 0.0254). Muscle mass entered in pounds is converted to kilograms before the division.
EWGSOP2-style cut-offs used in the calculator
The 2019 European Working Group on Sarcopenia in Older People (EWGSOP2) paper gives < 7.0 kg/m² for men and < 5.5 kg/m² for women as indicative low appendicular muscle mass when paired with other clinical findings. Those thresholds assume ASM from standardised BIA or DXA pipelines—not a bathroom scale body-fat guess.
| Sex (comparison mode) | Cut-off on this page | Plain-language note |
|---|---|---|
| Male | < 7.0 kg/m² | Flagged as below the published male threshold; not an automatic sarcopenia label. |
| Female | < 5.5 kg/m² | Same idea for the female threshold—context from gait, grip, and history still matters. |
Worth keeping in mind
- Some Asian consensus documents adjust cut-offs or add calf circumference and chair-stand tests. If your clinic follows one of those systems, defer to their handout.
- Oedema, metal implants, and very different hydration states can shift BIA-derived ASM more than people expect.
- Athletes with large arms and legs can sit above the cut-offs yet still be injured; frail individuals sometimes sit near the line while feeling weak for other reasons.
- For a complementary height-and-weight screen, try the BMI calculator—it answers a different question entirely.
Quick guide
1. What do I type in?
- Grab appendicular skeletal muscle mass (ASM) from a BIA or DXA print-out—arms and legs combined, not whole-body lean mass.
- Enter height and ASM in either metric or US units, then hit Calculate (or wait a beat; it updates after you stop typing).
- Choose male or female if you want the usual 7.0 / 5.5 kg/m² comparison lines; leave sex on Skip if you only need the raw index.
2. What formula runs under the hood?
SMI = ASM ÷ height², with height in metres and ASM in kilograms. The result is always expressed as kg/m².
US mode converts feet and inches to metres and pounds to kilograms before the same division runs.
3. How seriously should I take the colour band?
Clinicians rarely diagnose sarcopenia from a single index. EWGSOP2, for example, pairs low muscle quantity with strength or performance tests.
Different machines and software builds do not always agree to the last decimal. Treat this page as a double-check, not a replacement for the lab or physician who ordered the test.
4. Why use a browser-only tool?
- No account, no server round-trip: numbers stay in your tab.
- Handles mixed US height with metric muscle mass if you flip modes instead of juggling converters elsewhere.
- Copy button hands you a plain-text line you can paste into notes or email.
5. Typical situations
- Comparing your printed SMI to what you get when you recompute from ASM and height.
- Students or staff learning how body-composition indices are normalised to height.
- Rough tracking after resistance training when repeat scans exist—always same device vendor when possible.