Methodology

Data Sources

Percentile data are drawn from peer-reviewed scientific studies and large-scale population health surveys. Primary sources include:

All sources are listed on the references index.

Population Context

Several metrics, including BMI and body fat percentage, use data from the US population (NHANES). NHANES is one of the largest, most rigorously collected, and most frequently updated population health datasets available, with measured (not self-reported) values and nationally representative sampling.

However, the US has one of the highest obesity rates among major countries. This means US percentile distributions skew higher than they would in most other populations. Being at the 50th percentile in the US does not mean a value is healthy, it means half the US population is higher.

Where this matters, context is added directly on the metric pages:

  • BMI: WHO category bands are overlaid on charts so bars visually land in Underweight/Normal/Overweight/Obese zones. An international comparator note references mean BMI by country from the NCD-RisC (Lancet, 2024).
  • Body fat percentage: A caveat notes that Americans carry 5 to 10 percentage points more body fat than European populations at the same age and BMI.
  • Blood pressure: Systolic and diastolic BP percentiles are from NHANES 2001-2008. Age-standardized mean BP and hypertension prevalence vary substantially across countries (NCD-RisC, Lancet 2021), so the upper percentiles may not generalize globally. Each overview page notes this caveat.
  • 30-Second Chair Stand: Normative data is from US community-dwelling older adults (Rikli & Jones, n=7,183). A German study (n=1,657, ages 65-75) found lower scores in comparable age groups, partly attributed to higher body weight.

Where international comparator data exists, it is linked on each metric's reference page.

Percentile Calculation

Five percentile points are reported for each metric: 5th, 25th, 50th (median), 75th, and 95th. Depending on the source study, these values are derived by one of the following methods:

  • Directly reported, the source study publishes the exact percentile values we display.
  • LMS curve fitting, the source provides L (skewness), M (median), and S (coefficient of variation) parameters by age. Percentiles are computed at each age point using the standard LMS formula. Used for lean mass index and appendicular lean mass index (Kelly et al. 2009).
  • Normal distribution, calculated from reported means and standard deviations assuming a normal distribution. Used where the source reports summary statistics but not full percentile tables.
  • Percentile proxy, when P5, P25, P75, or P95 is not available, we substitute the nearest reported value (e.g. P10 for P5, P90 for P95, P20 for P25, P80 for P75). Documented per metric.
  • Interpolation, when the nearest reported percentiles bracket the target (e.g. P20 and P30 are available but not P25), we interpolate: P25 = (P20 + P30) / 2. Used for Tomkinson 2017/2018 youth norms.
  • Calculator-derived, where the study authors publish an interactive calculator exposing the full distribution, we read quartile values directly from it. Used for the powerlifting lifts (van den Hoek 2024, via thestrengthinitiative.com).
  • Category-boundary estimate, for tests scored in discrete performance categories (e.g. push-up norms by Fitness Category), we map category boundaries to approximate percentile positions.
  • Derived from microdata, where no published percentile table exists for the target population, we derive percentiles directly from publicly available raw survey data. Currently used for waist-to-height ratio (NHANES). See below.

Each metric's reference page documents which method applies. When multiple studies report data for the same demographic, priority is given to larger sample sizes and more recent publication dates.

Derived Datasets

For some metrics, no published study provides age- and sex-stratified percentile tables derived from a large general population sample. Where a suitable public microdata source exists, percentiles are derived directly from that data and the method is documented here.

Waist-to-Height Ratio — NHANES

No published paper provides sex- and age-stratified WHtR percentile tables for US adults. The CDC NHANES body measures examination collects waist circumference (BMXWAIST) and standing height (BMXHT) for all participants. Percentiles were derived as follows:

  • Data: Three NHANES releases were used: 2015–2016 (BMX_I / DEMO_I), 2017–March 2020 (P_BMX / P_DEMO), and 2021–2023 (BMX_L / DEMO_L). Adults aged 20 and over with complete waist and height measurements were included after filtering biologically impossible values (n ≈ 18,700).
  • Weights: MEC examination weights were applied: WTMEC2YR for 2015–2016 and 2021–2023; WTMECPRP for the 2017–March 2020 release. Each cycle's weight was divided by 3 before pooling to give equal contribution across cycles. No CDC-prescribed pooling method exists for this combination of releases.
  • Pandemic discontinuity: CDC advises caution when combining the 2021–2023 cycle with earlier cycles due to a 1.5-year fieldwork gap. Cycle-specific medians were compared against the pooled table and no material discontinuity was observed.
  • Quantiles: Weighted empirical quantiles (P5, P25, P50, P75, P95) were computed for each sex × 5-year age bracket using linear interpolation on the weighted cumulative distribution. Survey-design standard errors were not computed.

This is an internally derived dataset, not a peer-reviewed publication. The derivation method and its limitations are summarised on the reference page.

Age Brackets

Age brackets match the source study's native groupings to preserve data fidelity. Most metrics use decade brackets (20-29, 30-39, ... 80+), while some use 5-year brackets (20-24, 25-29, ...) when the source data supports finer granularity. The number of brackets varies by metric depending on the age range covered by the primary study.

For trend curves showing how metrics change with age, we use the midpoint of each bracket (e.g., 24.5 for a 20-29 bracket, 22 for a 20-24 bracket).

Percentile Approximation

Source studies rarely report all five of our target percentiles (P5, P25, P50, P75, P95) directly. The approximation method varies by source:

  • Tomkinson 2017/2018 (youth norms) reports P10, P20, P30, P70, P80, and P90. P25 is interpolated as (P20 + P30) / 2; P75 as (P70 + P80) / 2. P5 uses P10 as a proxy; P95 uses P90.
  • van den Hoek 2024 (powerlifting) reports deciles P10 to P90. P5 uses P10 as a proxy; P95 uses P90. P25 and P75 are read from the authors' online calculator, which exposes the full distribution.
  • Other sources, where P25 or P75 is absent and no interpolation bracket exists, we use the nearest available percentile as a proxy. Each metric page notes which percentiles are approximated.

Rating System

A five-tier rating system is used based on percentile rankings:

Rating Percentile Interpretation
Excellent 95th Top 5% of the population
Above Average 75th Better than 75% of people
Average 50th Median value
Below Average 25th Better than 25% of people
Poor 5th Bottom 5% of the population

Note: For some metrics like resting heart rate and reaction time, lower values are better. In these cases, the 5th percentile represents "Excellent" and the 95th percentile represents "Poor".

Limitations

  • Population representation: Several metrics use US-only data (NHANES), and the US has unusually high obesity rates. Percentiles for body composition metrics will skew higher than in most other countries. See Population Context above for details and the specific caveats we add to affected pages.
  • Measurement methods: Different studies may use slightly different protocols. For example, VO2 max can be measured directly via metabolic cart or estimated from submaximal tests.
  • Selection bias: Clinical and fitness registry data may over-represent healthier, more active individuals who seek testing.
  • Temporal changes: Population fitness levels change over time. The most recent available data are used, but some studies may be several years old.
  • Special-population norms: Some metrics are norms for a specific subpopulation rather than the general public. The powerlifting lifts (squat, bench press, deadlift) are based on drug-tested competitive powerlifters, a highly trained group whose values are substantially higher than recreational gym-goers. Where this applies, we add a prominent caveat on the metric page and note it on the reference page.

Updates

Data are updated as new large-scale studies become available. Each metric page shows the publication year and sample size of its sources.