What this calculator does
Spirometry results mean little in absolute terms, because a tall young person and a short older one should not have the same lung volume. Predicted values give the reference a measurement is compared against.
Age is the term that moves. At 175 cm, a predicted vital capacity falls from 4,443 mL at age 20 to 3,267 mL at age 80, a reduction of about a quarter that is entirely normal.
The formula
Height in centimetres is multiplied by a coefficient that declines with age, with separate constants for men and women. The result is the expected vital capacity in millilitres.
| Term | Meaning |
|---|---|
| Vital capacity | The maximum volume that can be exhaled after a full breath in. |
| Predicted value | The expected figure for someone of that height, age and sex. |
| Per cent predicted | A measured value expressed against the prediction, which is how results are reported. |
The inputs explained
| Field | What to enter |
|---|---|
| Height (cm) | Height in centimetres, the strongest predictor of lung volume. |
| Age (years) | Age in years. Vital capacity declines steadily through adult life. |
| Sex | Sex, which selects the coefficients. |
When to use it
Interpreting a spirometry result
A measured volume only means something against what would be expected.
Understanding normal decline
Lung volume falls with age in healthy people, which the prediction accounts for.
Comparing across people
Per cent predicted puts two very different bodies on a common scale.
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 much does age reduce the expectation?
The same height at four ages.
| Age | Predicted vital capacity | In litres |
|---|---|---|
| 20 years | 4,443 mL | 4.44 L |
| 40 years | 4,051 mL | 4.05 L |
| 60 years | 3,659 mL | 3.66 L |
| 80 years | 3,267 mL | 3.27 L |
Questions
Why does lung capacity decline with age?
Chest wall stiffness increases, respiratory muscles weaken and lung elastic recoil is lost. All three reduce the volume that can be moved, and the decline is gradual and universal.
Are these equations current?
No. The Baldwin equations date from 1948 and modern laboratories use newer reference sets such as GLI-2012, which draw on much larger and more diverse populations. Expect a laboratory report to differ from this figure.
What is per cent predicted?
A measured value divided by the predicted one. Around 80 per cent and above is generally treated as within normal limits, though modern practice increasingly uses a statistically defined lower limit instead.
Does this account for ethnicity?
Not at all, and that is a known limitation of older equations. Modern reference sets handle population differences explicitly, and how they should do so remains actively debated.
For airflow obstruction from the same test, see the FEV1/FVC ratio calculator. For air moved per minute, see the minute ventilation calculator.