What this calculator does
Spirometry measures two things about a forced breath out: how much air comes out in the first second, and how much comes out in total. The ratio between them distinguishes a narrowed airway from a small lung.
A healthy adult empties most of their capacity in that first second. An FEV1 of 3.5 L against an FVC of 4.5 L gives 77.8 per cent, while an FEV1 of 2.0 L against the same capacity gives 44.4 per cent, which indicates air is leaving slowly rather than that there is less of it.
The formula
Divide FEV1 by FVC and multiply by 100. Both are read directly from a spirometry trace, in litres.
| Term | Meaning |
|---|---|
| FEV1 | Forced expiratory volume in one second. |
| FVC | Forced vital capacity: the total volume exhaled. |
| Fixed ratio | The convention of treating below 70 per cent as obstruction, which has known limitations. |
The inputs explained
| Field | What to enter |
|---|---|
| FEV1 (L) | Forced expiratory volume in the first second, in litres. |
| FVC (L) | Forced vital capacity in litres, from the same manoeuvre. |
When to use it
Reading a spirometry report
The ratio is the first figure looked at when classifying a lung function result.
Distinguishing patterns
A low ratio points toward obstruction; a normal ratio with low volumes points elsewhere.
Following lung function over time
The ratio is tracked across repeated tests rather than read once.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does FEV1 move the ratio?
The same total capacity at three one-second volumes.
| FEV1 | FEV1/FVC ratio | Commonly used threshold |
|---|---|---|
| 2.0 L | 44.4% | below 70% is a common (fixed-ratio) marker of airway obstruction |
| 2.8 L | 62.2% | below 70% is a common (fixed-ratio) marker of airway obstruction |
| 3.5 L | 77.8% | below 70% is a common (fixed-ratio) marker of airway obstruction |
Questions
Why is the 70 per cent threshold criticised?
Because the ratio falls naturally with age, so a fixed cut-off over-diagnoses obstruction in older people and under-diagnoses it in younger ones. Using the lower limit of normal from reference equations avoids that, and is increasingly preferred.
What does a normal ratio with low volumes mean?
That air is leaving at a normal rate but there is less of it, which is a restrictive rather than obstructive pattern. Establishing that properly requires full lung volume measurement, not spirometry alone.
Does the test need to be done properly?
Very much so. Spirometry is effort-dependent and a poor manoeuvre produces misleading numbers. Results are normally accepted only when several attempts are reproducible and meet published quality criteria.
Can this diagnose asthma or COPD?
No. Both need clinical assessment, and distinguishing them typically involves testing the response to a bronchodilator. This calculator performs the arithmetic on two numbers and nothing more.
For total air moved per minute, see the minute ventilation calculator. For smoking exposure history, see the pack-years calculator.