The FEV1/FVC ratio is the single most important number in a spirometry report — it is the first screen that separates a normal breathing pattern from an obstructive one. This guide explains what the ratio measures, why 70% is used as a cutoff (and why that number is imperfect), and what a low or borderline result usually means for what comes next.
What FEV1/FVC represents and where it comes from
During a forced spirometry maneuver, a person inhales fully and then exhales as hard and fast as possible into a sensor. Two numbers matter most: FEV1, the volume exhaled in the first second, and FVC, the total volume exhaled by the end of the breath. Dividing FEV1 by FVC and expressing it as a percentage produces the FEV1/FVC ratio — historically called the Tiffeneau index, after French physiologist Robert Tiffeneau, and later refined by Pinelli to use FVC rather than a slow vital capacity maneuver.
In healthy lungs, most of the air comes out fast — a healthy adult typically exhales 75-85% of their vital capacity within the first second. When the airways are narrowed or prone to collapsing during exhalation, that first second captures a smaller share of the total volume, and the ratio falls.
Obstructive vs restrictive: the two broad patterns
A low FEV1/FVC ratio (below 70%, or below the LLN) points to an obstructive pattern — airways that are narrowed by inflammation, mucus, smooth-muscle constriction, or loss of elastic recoil. COPD, asthma, and bronchiectasis are the most common causes.
A normal-or-high FEV1/FVC ratio combined with a reduced FVC can point toward a restrictive pattern — the lungs (or the chest wall, or the muscles that move them) cannot expand to a normal volume, so both FEV1 and FVC fall roughly together and their ratio stays normal or even rises slightly. Interstitial lung disease, chest-wall deformity, neuromuscular weakness, and severe obesity are common causes. Confirming restriction requires comparing FVC — and ideally total lung capacity (TLC) — to predicted values based on age, sex, height, and race-appropriate reference equations, which this calculator does not collect or compute.
Limits of the fixed ratio and what comes next
This calculator uses the fixed 70% cutoff endorsed by GOLD (the Global Initiative for Chronic Obstructive Lung Disease) because it is simple and widely recognized. But FEV1/FVC naturally declines with age even in healthy lungs, so a single fixed cutoff systematically over-diagnoses obstruction in older adults and under-diagnoses it in younger adults. The alternative — the lower limit of normal (LLN), the 5th percentile of a matched healthy reference population — adjusts for age, sex, and height, but requires reference equations (such as the Global Lung Function Initiative 2012 set) that this calculator does not implement.
Whatever the ratio shows, it is a screening result, not a diagnosis. An abnormal or borderline FEV1/FVC should prompt full pulmonary function testing (lung volumes and diffusing capacity), bronchodilator reversibility testing, and evaluation by a qualified clinician who can weigh symptoms, exposures, and imaging alongside the numbers.