The K2X Tool Suite

Resting metabolic rate is estimated using the Mifflin-St Jeor equation. In a systematic review comparing predictive equations against measured resting metabolic rate (indirect calorimetry), Mifflin-St Jeor was found to be the most accurate across nonobese and obese adults, predicting measured RMR within 10% more often than any other equation tested (Frankenfield, Roth-Yousey & Compher, J Am Diet Assoc, 2005).

Protein targets are based on the International Society of Sports Nutrition's position stand on protein and exercise, which recommends 1.4–2.0 g/kg/day for muscle mass maintenance and growth, with intakes toward the higher end (or beyond, in trained individuals) supporting lean mass retention during a calorie deficit (Jäger et al., J Int Soc Sports Nutr, 2017).

These are population-level estimates, not individualized prescriptions — actual metabolic rate varies by as much as 10-15% between individuals of identical age/height/weight/activity level. This tool is a starting point, not a substitute for tracking your own real-world response and adjusting from there — which is exactly what 1:1 coaching is for.

BMI only uses height and total weight — it cannot tell the difference between 20 lb of muscle and 20 lb of fat. A body-composition analysis published in Kyle et al., Clinical Nutrition, 2003 formally decomposed BMI into its fat-free mass and fat mass components across nearly 6,000 adults, demonstrating that two people with an identical BMI can have meaningfully different fat-free mass index and fat mass index values — meaning BMI alone tells you almost nothing about actual body composition.

This isn't just academic. Research on sarcopenic obesity and all-cause mortality has repeatedly shown that BMI misses people who carry “normal” weight with low muscle and high fat mass — a combination that carries elevated health risk that BMI alone cannot detect. Muscle mass itself has also been shown to be a meaningful predictor of clinical outcomes independent of BMI, including in oncology nutrition research using FFMI.

The normalized FFMI formula and scale used in this calculator come from Kouri et al., Clin J Sport Med, 1995, which measured FFMI in 157 male athletes and found drug-free lifters topped out around a normalized FFMI of 25 — a useful, evidence-derived reference point rather than an arbitrary cutoff.

A systematic review across 78 studies (Browning, Hsieh & Ashwell, Nutrition Research Reviews, 2010) found waist-to-height ratio outperformed both waist circumference and BMI alone as a screening tool for cardiovascular disease and diabetes risk, with a boundary value of 0.5 supported across populations. A follow-up analysis in BMC Medicine reinforced the same 0.5 threshold as a simple, globally applicable public health message: keep your waist circumference under half your height.

The 1RM estimate uses the Brzycki formula, which validation research has found produces estimates not significantly different from directly tested 1RM in the bench press when based on a submaximal set of 7-10 reps taken close to failure.

The reference tiers are calibrated against normative strength data from 809,986 competitive powerlifting entries, published in the Journal of Science and Medicine in Sport in 2024 — contemporary, large-sample data rather than decades-old norms. Your exact percentile also depends on age and weight class, which this simplified tool doesn't fully account for.

Max heart rate is estimated using Tanaka, Monahan & Seals, J Am Coll Cardiol, 2001, a meta-analysis of nearly 19,000 subjects that found 208 − (0.7 × age) predicts max heart rate more accurately across the adult age range than the older, still-common “220 minus age” rule of thumb.

Zones are calculated using the Karvonen heart rate reserve method, 1957, which factors in your resting heart rate rather than using max heart rate alone — a more individualized target than flat percentage-of-max approaches.

This estimate uses the non-exercise prediction equation from Jackson et al., Medicine & Science in Sports & Exercise, 1990, developed and cross-validated on over 2,000 subjects using age, sex, BMI, and a standardized self-reported activity rating — no exercise test required. It's a reasonable estimate and useful for tracking change over time, but it is not a substitute for a real treadmill or lab-based VO2max test.

Recommended ranges come from the National Sleep Foundation's expert consensus panel, Sleep Health, 2015, an 18-member multidisciplinary panel representing 12 stakeholder organizations that evaluated the sleep research literature using a formal appropriateness methodology to set age-specific duration recommendations.

This calculator models caffeine elimination using an average half-life of 5 hours, consistent with the pharmacokinetic range described in the NIH StatPearls caffeine pharmacology reference. Individual half-life varies considerably (roughly 1.5-9.5 hours) based on genetics, liver enzyme activity, pregnancy, and other factors — so treat this as a reasonable estimate, not an exact figure for your body.

This conversion uses the formula from the A1C-Derived Average Glucose (ADAG) study, Nathan et al., Diabetes Care, 2008: eAG (mg/dL) = 28.7 × A1c − 46.7. The study combined continuous glucose monitoring with frequent fingerstick checks across 507 subjects to establish this relationship, and it's the same formula used in most lab reports today. Related reading: how resistance training affects these numbers.

Categories follow the 2017 ACC/AHA High Blood Pressure Guideline, Whelton et al., Hypertension, 2018, which lowered the threshold defining hypertension from 140/90 to 130/80 mmHg based on evidence that cardiovascular risk begins rising well below the older cutoff.