Vital Capacity Calculator

The Vital Capacity Calculator estimates predicted vital capacity based on age, sex, and height using standard regression equations. Vital capacity (VC) is the maximum volume of air that can be exhaled after a maximal inhalation, representing the total usable lung volume. It is one of the fundamental measurements in pulmonary function testing and is essential for diagnosing and monitoring restrictive lung diseases.

The prediction equations used are based on the ERS/ECSC standards: Males: VC = (0.0576 x Height_cm) - (0.026 x Age) - 4.34 liters. Females: VC = (0.0443 x Height_cm) - (0.026 x Age) - 2.89 liters. VC increases with height (larger thoracic cavity and lung size) and decreases with age (loss of elastic recoil, chest wall stiffening, respiratory muscle weakening).

Vital capacity comprises three component volumes: tidal volume (normal breathing volume, approximately 500 mL), inspiratory reserve volume (additional air inspired beyond tidal, approximately 3000 mL in males), and expiratory reserve volume (additional air expelled beyond tidal, approximately 1200 mL in males). VC does not include residual volume, the air remaining after maximal expiration.

Measured VC below 80% of predicted suggests restrictive physiology. Mild restriction: 60-79%. Moderate: 40-59%. Severe: below 40%. Causes of restriction include pulmonary fibrosis (idiopathic, drug-induced, radiation), pleural disease (effusion, thickening), chest wall deformity (kyphoscoliosis), neuromuscular disease (ALS, myasthenia gravis, diaphragm paralysis), and obesity hypoventilation.

In clinical practice, FVC (forced vital capacity from spirometry) often substitutes for VC, though relaxed (slow) VC may be 100-200 mL larger because forced expiration can cause small airway collapse. In obstructive disease, slow VC may significantly exceed FVC, and the difference reflects air trapping severity.

Serial VC measurements are particularly valuable in neuromuscular diseases: progressive VC decline triggers critical management decisions including non-invasive ventilation initiation (typically when VC falls below 50% predicted or below 1.0 L) and timing for tracheostomy or hospice discussions. Bedside VC measurement requires only a simple spirometer.