What does percent predicted mean?

Percent predicted is the measured value divided by the predicted value for a healthy person of the same sex, age, height, and ethnicity, times 100. A value near 100% is typical; values well below show reduced function relative to the reference population.

How is an obstructive pattern identified?

Obstruction is suggested when the FEV1/FVC ratio falls below the lower limit of normal. Severity is then graded from FEV1 percent predicted following ATS/ERS: 80% or above is mild, 50 to 79% moderate, 30 to 49% severe, and below 30% very severe.

What suggests a restrictive pattern?

When the FEV1/FVC ratio is preserved but FVC is below 80% predicted, a restrictive pattern is possible. Spirometry alone cannot confirm restriction — static lung volumes such as total lung capacity are needed, which is why the tool labels it as a pattern to confirm.

Why does ethnic group change the predicted value?

Average lung volumes differ between populations of the same height and age. GLI-2012 provides separate reference modules, and this tool applies comparable multipliers, so omitting or mis-selecting ethnicity can shift percent predicted by several percent.

Are these values suitable for a formal diagnosis?

No. The tool uses GLI-comparable regression equations and an approximate z-score for education and screening. Formal diagnosis requires a certified spirometer applying the full GLI-2012 LMS reference and a clinician's interpretation of quality-assured manoeuvres.

What is the Spirometry % Predicted Calculator (GLI-2012)?

Enter FEV1, FVC, sex, age, height, and ethnic group to compute percent predicted and an approximate z-score using GLI-comparable reference equations, with ATS/ERS obstruction and restriction pattern classification. It runs free in your browser on Gera Tools, with nothing uploaded.

Spirometry % Predicted Calculator (GLI-2012)

Name: Spirometry % Predicted Calculator (GLI-2012)
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Spirometry is only interpretable against a reference population: a 3.2 litre FEV1 is normal for one person and markedly reduced for another. This calculator computes percent predicted and an approximate z-score, then classifies the result as a normal, obstructive, or restrictive pattern.

Key spirometry values explained

FEV1 (Forced Expiratory Volume in 1 second) is the volume of air a person can forcibly exhale in the first second of a maximal breath-out from full inflation. It is the central measure of airflow limitation — in obstructive disease the airways narrow and slow delivery, so FEV1 falls disproportionately.

FVC (Forced Vital Capacity) is the total volume of air that can be forcibly exhaled from full inflation to maximum exhalation. It reflects the overall lung volume accessible in a forced manoeuvre.

FEV1/FVC ratio (sometimes called the Tiffeneau-Pinelli index) is the fraction of the vital capacity expelled in the first second. In healthy adults this is typically around 0.75–0.80. A ratio below the lower limit of normal (LLN) signals obstruction — the lungs hold air normally but cannot expel it fast enough.

How it works

Predicted FEV1 and FVC are computed from sex, age, and height using GLI-comparable regression equations, then adjusted by an ethnic multiplier:

predicted FEV1 / FVC = regression(sex, age, height) × ethnic factor
% predicted          = measured / predicted × 100
FEV1/FVC ratio       = FEV1 / FVC × 100
pattern:
  ratio < LLN                 →  obstruction (grade by FEV1 % predicted)
  ratio normal, FVC < 80%     →  possible restriction (confirm lung volumes)
  otherwise                   →  normal spirometry

Severity of obstruction follows the ATS/ERS bands on FEV1 percent predicted, and the z-score expresses how many standard deviations the FEV1 sits from its predicted mean.

ATS/ERS obstruction severity grading

When the FEV1/FVC ratio falls below the LLN (indicating obstruction), severity is graded from FEV1 percent predicted:

FEV1 % predicted	Severity grade
80% or above	Mild
50–79%	Moderate
30–49%	Severe
Below 30%	Very severe

This grading is used in GOLD staging for chronic obstructive pulmonary disease (COPD) and guides treatment decisions.

Why ethnic group changes the predicted value

Average lung dimensions differ between populations after accounting for height and age. The GLI-2012 reference provides separate reference modules and ethnic correction factors, reflecting that individuals of African heritage tend to have smaller lung volumes for a given height compared to those of European or North-East Asian heritage, based on large population studies. The reasons involve skeletal structure, trunk-to-limb ratio, and other factors. Applying the wrong ethnic group shifts the predicted values and therefore the percent predicted and LLN — which can lead to misclassification. Where ethnicity is uncertain or mixed, the GLI-2012 global reference without ethnic correction is available.

Reading the z-score

The z-score expresses how many standard deviations the measured value sits from the predicted mean for that individual’s reference group. A z-score of 0 is exactly predicted. The LLN corresponds to a z-score of approximately −1.645 (the 5th percentile). A z-score of −2 means the value is unusually low even for a clinical population, while −0.5 would be within the normal scatter of healthy individuals.

Notes and limitations

The equations reproduce adult predicted values within a few percent of the full GLI-2012 model, adequate for screening and educational use. The true GLI-2012 reference uses age-spline LMS lookup tables with slightly different lower limits of normal. A restrictive pattern suggested by spirometry (preserved ratio, low FVC) always requires static lung volumes — particularly total lung capacity (TLC) — to confirm, since air trapping in obstruction can also reduce FVC. Use a certified spirometry system and trained personnel for any diagnostic report.