This estimates the expected FEV₁ for a person's age, height, and sex — the reference point used to calculate "percent predicted" on a spirometry report.
Measured FEV₁ as a fraction of predicted is how COPD severity is graded.
It expresses a patient's measured value as a percentage of what's expected for someone of the same age, height, and sex, so 70% predicted means a value 30% below the population norm.
Once obstruction is confirmed by the FEV1/FVC ratio, GOLD grades airflow limitation: ≥80% mild, 50–79% moderate, 30–49% severe, and below 30% very severe.
GLI-2012 regression equations. The simplified formula here is an illustrative teaching approximation.
The ratio detects whether obstruction is present; the FEV1 percent predicted then grades how severe that obstruction is.