What is FFMI and how is it calculated?

Fat-Free Mass Index (FFMI) is a measure of lean body mass relative to height. It is calculated as Lean Mass (kg) divided by Height (m) squared, where Lean Mass = Weight × (1 − Body Fat %).

What is Normalized FFMI?

Normalized FFMI adjusts FFMI for height using the formula FFMI + 6.1 × (1.8 − Height in meters), making comparisons across body sizes more meaningful.

What is the natural FFMI limit?

Based on the landmark 1995 Kouri et al. study published in the Clinical Journal of Sport Medicine, an FFMI around 25 is considered the natural ceiling for drug-free male athletes. Values above 25 are statistically uncommon without pharmacological assistance.

How accurate is TDEE from the Mifflin-St Jeor equation?

Mifflin-St Jeor is considered one of the most accurate predictive BMR equations for healthy adults. TDEE is BMR multiplied by an activity factor and is a strong starting point that should be adjusted based on real-world weight trends.

Backed by sports-science formulas

TDEE, FFMI, Macro & 1RM Calculators

Instant, science-backed fitness math: TDEE (Mifflin-St Jeor), FFMI, daily macros, and one-rep-max — mobile-first, ad-light, 100% free.

FFMI & Normalized FFMI

Measure lean-mass quality independent of body fat.

Your Result

FFMI

—

Normalized FFMI

—

Lean Mass

— kg

Fat Mass

— kg

Classification Enter your data

—

15 18 20 22 25 32

< 18 · Below average
18 – 20 · Average
20 – 22 · Advanced natural
22 – 25 · Genetic limit
> 25 · Exceptional / likely enhanced

Body Fat % — US Navy Method

A tape measure is all you need. ±3–4% accuracy vs DEXA.

Your Result

Body Fat

—

percent

Body Fat Categories

Category	Men	Women
Essential fat	2–5%	10–13%
Athletes	6–13%	14–20%
Fitness	14–17%	21–24%
Average	18–24%	25–31%
Obese	25%+	32%+

Lean Body Mass

The weight your body carries minus fat — muscle, bone, organs, water.

Your Result

Lean Mass

—

Fat Mass

—

Formula

Lean Mass = Weight × (1 − BodyFat%)

Why it matters: Lean mass is the main input for FFMI. Tracking it over time is a cleaner progress signal than scale weight — you can be losing fat and holding lean mass without the scale moving much.

TDEE & BMR

Mifflin-St Jeor — the gold-standard predictive BMR equation.

Daily Energy

BMR

—

kcal / day at rest

TDEE

—

kcal / day total

Goal Targets

Fat Loss (−500 kcal) —

Maintenance —

Muscle Gain (+300 kcal) —

Macro Distribution Matrix

Pulls your TDEE result and splits it into Protein / Carbs / Fats.

Your Daily Plate

Target calories: — kcal

Macro	%	kcal	grams
Protein	—	—	—
Carbohydrates	—	—	—
Fats	—	—	—

Protein & Carbs = 4 kcal/g · Fats = 9 kcal/g

1RM Calculator

Estimate your one-rep max with Epley & Brzycki — plus a full %1RM table.

Your Estimated Max

Epley

—

Brzycki

—

% of 1RM Reference

Reps	% 1RM	Weight

Frequently Asked Questions

Plain-language answers grounded in peer-reviewed sports science.

What does FFMI actually measure?

Fat-Free Mass Index (FFMI) quantifies lean body mass relative to height. Unlike BMI, FFMI ignores fat mass, making it a far more useful proxy for muscularity in trained populations. The core formula is FFMI = Lean Mass (kg) / Height (m)².

Where does the 25 FFMI "natural limit" come from?

The landmark Kouri et al. (1995) study published in the Clinical Journal of Sport Medicine ("Fat-free mass index in users and nonusers of anabolic-androgenic steroids") examined 157 male athletes and found that the normalized FFMI of drug-free lifters clustered tightly below 25, while users frequently exceeded that ceiling. Modern data largely supports this threshold as the practical upper bound of natural muscularity.

Why normalize FFMI?

Taller athletes naturally hold more absolute lean mass. Normalized FFMI applies the height correction FFMI + 6.1 × (1.8 − Height m) so comparisons across body sizes are fair.

How accurate is the Mifflin-St Jeor TDEE estimate?

The Academy of Nutrition and Dietetics rates Mifflin-St Jeor as the most accurate predictive BMR equation for healthy adults — typically within ~10% of indirect calorimetry. Track 7-day average bodyweight and adjust by ±100–250 kcal as needed.

Which 1RM formula should I trust?

Both Epley (w × (1 + reps/30)) and Brzycki (w × 36 / (37 − reps)) perform best at ≤10 reps. Use the average for planning, and always validate with a true heavy single under supervision.

From the FFMI Check Journal

Deep dives on body composition, programming, and the science behind the numbers.

Visit the blog →

Our Story

About FFMI Check

FFMI Check was built by a small group of lifters who got tired of bouncing between half-broken calculators, paywalled apps, and influencer-driven fitness blogs that prioritized newsletter sign-ups over actual science. We wanted one clean place where any athlete in the world could measure what truly matters — lean mass, energy needs, macros, and strength — in under 30 seconds, for free.

Our mission is simple: turn peer-reviewed sports science into instant, beautiful, mobile-first tools. Every formula on this site — Mifflin-St Jeor, Kouri's normalized FFMI, Epley, Brzycki — is implemented exactly as published, with the citations available in plain sight. No proprietary "secret sauce." No upsells.

What we stand for

Built by lifters, for lifters. Every feature is something we wanted ourselves.
Science-first. If a formula isn't peer-reviewed, it isn't on this site.
Free forever. Supported by lightweight, non-intrusive advertising — never by selling your data.
Open in spirit. Our formulas, classifications, and methodology are documented on every page.

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Energy Balance · 8 min read

Mifflin-St Jeor TDEE Calculator: How Accurate Is It, Really?

The Mifflin-St Jeor equation is the most-cited TDEE formula on the web — but how close does it actually land to a metabolic cart? Inside the data.

Open any TDEE calculator on the internet and you will probably be looking at the Mifflin-St Jeor equation. It is the default for a reason: peer-reviewed accuracy, simple inputs, and decades of validation data. But "most accurate predictive equation" is not the same as "perfect" — and if you cut or bulk by the wrong 300 kcal you can stall progress for months. Here is what the research actually shows.

The Equation, Briefly

Mifflin-St Jeor predicts Basal Metabolic Rate (BMR) — the energy your body burns at complete rest:

Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

Total Daily Energy Expenditure (TDEE) is then BMR multiplied by an activity multiplier ranging from 1.2 (sedentary) to 1.725 (six training days per week).

How Accurate Is the Number?

The Academy of Nutrition and Dietetics' systematic review compared every major predictive equation against indirect calorimetry (the gold-standard "metabolic cart") in healthy adults. The headline numbers:

Mifflin-St Jeor: ±10% accuracy in roughly 82% of subjects.
Harris-Benedict (original): ±10% in ~38% of subjects — overshoots BMR by an average of 5%.
Katch-McArdle: ±10% in ~74% — good if you know your actual lean mass, useless if your body fat estimate is off.

Translation: for a 2,500 kcal/day target, Mifflin-St Jeor will usually land within ±250 kcal of your true number.

Where the Equation Breaks Down

The 18% who fall outside the ±10% window tend to share predictable characteristics:

Highly muscular athletes — Mifflin uses only weight, not body composition. A 90 kg bodybuilder and a 90 kg sedentary adult get the same predicted BMR.
Obese subjects (BMI > 30) — Mifflin tends to overestimate BMR; consider using lean-mass-adjusted weight.
Older adults (60+) — Age coefficient flattens out around 60; Mifflin may underestimate BMR in active seniors.
Sub-clinical hypothyroidism — Predicted BMR can be 10–15% higher than measured.

Activity Multipliers Are the Biggest Source of Error

This is the dirty secret of every TDEE calculator: BMR prediction is fairly tight, but the activity multiplier you select can be off by a full level. Most people overestimate. A "moderate" lifter who trains 4×/week and walks 5,000 steps daily is closer to 1.4 than the textbook 1.55. Three-quarters of TDEE errors trace back to this single dropdown.

Pragmatic fix: start with one bracket lower than what feels right, then adjust based on 14-day bodyweight trend.

How to Calibrate Your Personal TDEE

Run your numbers in our TDEE Calculator.
Eat at the predicted maintenance for 14 days, weighing daily and averaging.
Compare your 14-day average to baseline:
- ± 0.5 kg → your TDEE estimate is on target.
- Gained 1+ kg → real TDEE is ~250 kcal lower than predicted.
- Lost 1+ kg → real TDEE is ~250 kcal higher than predicted.
Adjust intake by the offset, repeat.

Two weeks of careful tracking beats any equation in existence.

Bottom Line

Mifflin-St Jeor is the right place to start. It is the most accurate predictive equation available without a $1,200 metabolic cart. But treat the output as a hypothesis, not a verdict — and let two weeks of real-world weight data calibrate it for you.

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