Calculate predicted FVC, FEV₁, and PEF based on age, sex, height, and ethnicity. Compare measured values to identify obstructive, restrictive, or normal patterns. Includes GOLD COPD staging.
Spirometry is the most common pulmonary function test, measuring how much air you can exhale (Forced Vital Capacity, FVC) and how quickly you can exhale it (Forced Expiratory Volume in 1 second, FEV₁). Comparing measured values to predicted normal values — based on age, sex, height, and ethnicity — is the fundamental method for diagnosing obstructive lung diseases (COPD, asthma) and restrictive lung diseases (pulmonary fibrosis, neuromuscular weakness).
Predicted values are derived from large population studies. The most widely used reference equations include NHANES III (Hankinson 1999) for the US, ECSC/ERS for Europe, and the newer GLI-2012 (Global Lung Initiative) reference that provides multi-ethnic z-scores. Ethnicity-specific correction factors are applied because lung volumes differ significantly across populations — African Americans have approximately 12% lower FVC than Caucasians of the same height and age, while Asians have approximately 6% lower values.
This calculator computes predicted FVC, FEV₁, FEV₁/FVC ratio, and peak expiratory flow (PEF) using the ECSC/ERS reference equations with ethnicity corrections. When measured spirometry values are entered, it calculates percent-predicted values, identifies the ventilatory pattern (obstructive, restrictive, normal, or mixed), and provides GOLD staging for obstructive patterns and ATS severity grading for restrictive patterns — with visual bar charts comparing measured to predicted.
Spirometry interpretation requires comparing measured values to predicted normals that account for age, sex, height, and ethnicity. Manual calculation or reliance on memory for reference values is impractical. This calculator provides instant predicted values, automatic pattern recognition (obstructive/restrictive/normal), GOLD staging, and visual comparison — standardizing the interpretation process for clinicians, respiratory therapists, and students.
Male: FVC = 5.76 × Height(m) - 0.026 × Age - 4.34 | FEV₁ = 4.30 × Height(m) - 0.029 × Age - 2.49 Female: FVC = 4.43 × Height(m) - 0.026 × Age - 2.89 | FEV₁ = 3.95 × Height(m) - 0.025 × Age - 2.60 Ethnicity correction: African American ×0.88, Asian ×0.94, Hispanic ×0.96 Obstructive: FEV₁/FVC < 70% | Restrictive: FVC < 80% predicted with normal ratio
Result: Predicted FVC 4.58 L (83% pred), FEV₁ 3.48 L (63% pred), FEV₁/FVC 57.9% — Obstructive pattern, GOLD 2 Moderate
Predicted FVC = 5.76 × 1.75 - 0.026 × 50 - 4.34 = 4.58 L. Measured FVC 3.8 L = 83% predicted (near normal). Predicted FEV₁ = 4.30 × 1.75 - 0.029 × 50 - 2.49 = 3.48 L. Measured FEV₁ 2.2 L = 63.2% predicted. FEV₁/FVC = 2.2/3.8 = 57.9% (< 70%). Pattern: Obstructive. GOLD 2 (FEV₁ 50-79% predicted).
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Thoracic cage dimensions, trunk-to-leg ratio, and total lung capacity vary across ethnic groups due to genetic and evolutionary factors. African Americans have relatively longer limbs and smaller thoracic volumes for the same height, resulting in lower lung volumes. The GLI-2012 reference uses multi-ethnic equations with ethnicity-specific coefficients to account for this.
The fixed 70% ratio can over-diagnose obstruction in the elderly (normal ratio declines with age) and under-diagnose in young adults. The GLI-2012 and ATS/ERS 2021 guidelines now recommend using the lower limit of normal (LLN, typically the 5th percentile z-score) rather than a fixed cutoff. This calculator uses the fixed 70% for simplicity, which remains standard in GOLD guidelines.
FVC is measured during forced (maximal effort) exhalation — it may be lower than SVC in patients with obstructive disease because forced exhalation causes airway compression. Slow VC is measured during relaxed exhalation. If SVC > FVC, it suggests air trapping from obstruction. True restrictive disease is confirmed by reduced total lung capacity (TLC) on body plethysmography.
Spirometry alone cannot — both show obstruction. However, significant bronchodilator reversibility (>12% and >200 mL improvement in FEV₁ after albuterol) favors asthma. COPD typically shows incomplete reversibility. Clinical history (age of onset, smoking, triggers, variability) is essential for differentiation.
PEF measures the maximum flow during forced exhalation and is effort-dependent. It is useful for asthma monitoring (variability >20% suggests uncontrolled asthma) but is not a substitute for full spirometry. PEF primarily reflects large airway function and can be normal in small airway disease.
Reference equations are derived from non-smoking populations. Smoking accelerates FEV₁ decline from the normal 25-30 mL/year to 50-80 mL/year. The calculator shows predicted values for healthy individuals — the gap between predicted and measured widens with cumulative smoking exposure (pack-years).