PFT Calculator (Pulmonary Function Test)
Calculate predicted spirometry reference values (FVC, FEV1, FEV1/FVC ratio) based on patient age, biological sex, and height. Compare measured spirometry results against predicted values and Lower Limits of Normal (LLN).
How to Use the PFT Calculator (Pulmonary Function Test)
- Select patient biological sex and enter age in years (5–100).
- Enter patient height in centimeters or select Feet & Inches.
- Optionally enter measured Forced Vital Capacity (FVC in Liters) and Forced Expiratory Volume in 1 Second (FEV1 in Liters) from your spirometry test report.
- Click "Calculate PFT Values" to calculate predicted values, % predicted ratios, and Lower Limit of Normal (LLN) cutoffs.
- Use the quick sample data buttons to pre-fill test clinical scenarios.
Standard Spirometry Reference Equations
Understanding the Science & Clinical Context
Pulmonary Function Tests (PFTs) quantify lung volume, maximal airflow rates, and gas exchange capacity. Spirometry measures Forced Vital Capacity (FVC)—the maximum volume of air forcefully exhaled following full inspiration—and Forced Expiratory Volume in 1 Second (FEV1). Expected normal values vary primarily with demographic factors (age, height, and biological sex). An FEV1/FVC ratio falling below the Lower Limit of Normal (LLN) serves as the key diagnostic marker for obstructive ventilatory defects.
Published Clinical & Scientific References
- Quanjer PH, et al. Peak expiratory flow: official statement of the European Respiratory Society. Eur Respir J Suppl. 1993;16:9-40.
- Zapletal A, et al. Lung function in children and adolescents: methods, reference values. Prog Respir Res. 1977.
Frequently Asked Questions
What is a normal FEV1/FVC ratio?
In healthy adults, the normal FEV1/FVC ratio generally ranges between 70% and 85%. Values below the Lower Limit of Normal (LLN) or below 70% may indicate obstructive airway impairment such as asthma or COPD.
What does % Predicted mean in PFT results?
% Predicted compares an individual’s measured spirometry value against the statistical mean predicted value for a healthy individual of the same age, biological sex, and height. Values between 80% and 120% of predicted are typically considered within normal limits.
What is the Lower Limit of Normal (LLN)?
The Lower Limit of Normal (LLN) represents the 5th percentile of the reference population distribution. It is clinically preferred over fixed 70% cutoffs because it prevents over-diagnosis of obstruction in older adults.
Can this PFT calculator diagnose asthma or COPD?
No. This calculator provides educational reference estimates based on published statistical equations. Diagnosing asthma, COPD, or restrictive lung disease requires comprehensive medical evaluation, clinical history, physical examination, and physician interpretation.
Related Guides & Articles
Understanding Spirometry & PFT Results: FVC, FEV1, and LLN Explained
A complete guide to interpreting Pulmonary Function Test (PFT) spirometry values, predicted ranges, FEV1/FVC ratios, and Lower Limit of Normal (LLN).
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