FEV1/FVC Ratio Calculator

How it is calculated

The tool divides two spirometry volumes, both in litres: FEV1, the air blown out in the first second of a forced breath, and FVC, the total air blown out in the whole forced breath. The formula is ratio = FEV1 ÷ FVC × 100, shown as a percentage with one decimal. It then applies a single fixed cut-off: a ratio below 70% gets a note that the pattern is suggestive of obstruction, and 70% or above gets a note that it is within the normal range. The cut-off is applied to the value as displayed (one decimal), so a shown 70.0% is never marked as below 70. Both volumes must be above zero, FEV1 cannot be larger than FVC, and FVC cannot exceed 20 L; otherwise there is no result. The tool does not use age, sex, height or reference equations, which clinicians use to set a personal lower limit of normal, and it is not a diagnosis.

Common mistakes

• Entering the percent-of-predicted values from the report instead of litres. The fields want measured volumes in litres, not the "% pred" column.
• Swapping FEV1 and FVC. FEV1 cannot be larger than FVC, so the tool shows no result when the two are the wrong way round; swap them back.
• Entering millilitres, such as 2100 and 3600. The tool shows no result because FVC above 20 L is rejected, so enter litres (2.1 and 3.6).
• Taking 70% as a sharp line between healthy and ill. A single fixed cut-off ignores age and sex, and a value close to 70% needs a clinician's judgement, ideally after a repeat test.
• Using the pre-bronchodilator and post-bronchodilator numbers interchangeably. They are separate measurements; use the pair that the report names for the question you are asking.