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.

Spirometry % Predicted Calculator (GLI-2012)

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.

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.

Notes and limitations

The equations reproduce adult predicted values within a few percent of the full GLI-2012 model, which is adequate for screening and teaching. The true GLI-2012 reference uses age-spline LMS lookup tables that yield slightly different lower limits of normal, and a restrictive pattern on spirometry always needs static lung volumes to confirm. Use a certified spirometry system for any diagnostic report.