What is Skeletal Muscle Mass Index (SMI)?

SMI is the amount of skeletal muscle in your body normalized to your height squared — the muscle analog of BMI. It's the standard metric used to diagnose sarcopenia (age-related muscle loss) and is strongly predictive of falls, frailty, hospitalization, and mortality in older adults.

What are the SMI cutoffs for sarcopenia?

EWGSOP2 (European consensus, 2018) uses whole-body SMI cutoffs of < 7.0 kg/m² in men and < 5.5 kg/m² in women measured by DXA. For appendicular skeletal muscle mass (ASM/height²), the cutoffs are < 7.0 kg/m² men and < 5.5 kg/m² women. These are thresholds for "low muscle mass" — sarcopenia diagnosis requires both low mass and low strength or performance.

Do I need a DXA scan to measure SMI?

For a clinical diagnosis, yes — DXA or BIA (bioelectrical impedance) gives the cleanest measurement. This calculator estimates SMI using Janssen's validated anthropometric equation based on height, weight, age, and sex, which correlates reasonably well with DXA in most adults. Use it for screening, not diagnosis.

Why does muscle mass matter as I age?

Adults lose about 3-8% of muscle per decade after 30, accelerating after 60. Low muscle mass predicts falls, loss of independence, and worse recovery from illness. It also drags down resting metabolism, insulin sensitivity, and bone density. Maintaining muscle is arguably the single most impactful thing you can do for healthspan past age 50.

How do I build or maintain muscle mass?

Progressive resistance training 2-3 times per week is the primary tool — compound lifts (squats, deadlifts, rows, presses) are most time-efficient. Combine with adequate protein (1.6-2.2 g/kg for those actively training) spread across 3-4 meals. Older adults benefit from the higher end of that protein range because of anabolic resistance.

Is SMI the same as body fat percentage?

No. Body fat percentage tells you what fraction of your body is fat; SMI tells you the absolute amount of muscle relative to your frame. You can have a low body fat percentage and low SMI (thin but under-muscled — "skinny-fat"), or high body fat and high SMI (strong but also carrying weight). They're complementary, not interchangeable.

Skeletal Muscle Mass Index (SMI) Calculator

Estimate skeletal muscle mass normalized to height — the primary metric for sarcopenia screening.

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About Skeletal Muscle Mass Index

Muscle mass is one of the strongest predictors of healthy aging. Low muscle mass — clinically called sarcopenia — is associated with falls, fractures, longer hospital stays, metabolic dysfunction, and higher mortality. SMI is the field standard for quantifying it in a way that's comparable across body sizes.

The Formula (Janssen, 2000)

SM (kg) = 0.407 × weight(kg) + 0.267 × height(cm) − 0.049 × age + 2.55 × sex − 2.898

SMI = SM ÷ height²(m²)

This equation was derived against MRI-measured skeletal muscle in 388 adults and validated in another 343. It explains about 85% of the variance in muscle mass compared to imaging — accurate enough for screening, not for clinical diagnosis.

EWGSOP2 Sarcopenia Cutoffs

Category	Men (kg/m²)	Women (kg/m²)
Low (sarcopenic)	< 7.0	< 5.5
Borderline	7.0 – 8.5	5.5 – 6.2
Average	8.5 – 10.5	6.2 – 8.2
Above average	> 10.5	> 8.2

Mass Is Only Half the Picture

Full sarcopenia diagnosis (EWGSOP2) requires both low muscle mass and low function — grip strength below 27 kg (men) or 16 kg (women), or gait speed under 0.8 m/s. Strength can drop faster than mass with aging, so a low SMI warrants a functional check too.

What Builds Muscle

Progressive resistance training: 2-3 sessions/week, challenging loads, full ROM
Adequate protein: 1.6-2.2 g/kg bodyweight, distributed across meals
Recovery: sleep and training frequency that allows full adaptation
Consistency: beats intensity. Two years of "good enough" beats six weeks of perfect
Creatine monohydrate (3-5 g/day) — one of the best-studied ergogenic aids for older adults

Screening, Not Diagnosis

An anthropometric estimate can't replace DXA or BIA for clinical sarcopenia diagnosis. If your result flags low muscle mass and you're concerned — particularly if you're over 60 or have had recent unintentional weight loss — bring it to a clinician who can arrange proper body composition testing.

Skeletal Muscle Mass Index (SMI) Calculator

About Skeletal Muscle Mass Index

The Formula (Janssen, 2000)

EWGSOP2 Sarcopenia Cutoffs

Mass Is Only Half the Picture

What Builds Muscle

Screening, Not Diagnosis

Frequently Asked Questions

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