Predicted Lung Capacity (FEV1, FVC) Calculator

Understanding the predicted lung capacity calculator — concept and principle

Spirometry results are compared with values expected for a healthy person of your sex, age and height. This calculator estimates two of those reference values: forced vital capacity (FVC), the total air you can blow out after a full breath in, and FEV1, the amount blown out in the first second. If you enter your own measured FEV1, it also shows it as a percentage of predicted.

Of the 4.95 L predicted for the whole breath, 4.10 L should come out in the first second.

How it is calculated

The calculator uses simplified adult equations from Knudson et al. (1983), with height H in cm and age A in years; results are in litres:

Men: FVC = 0.0844H − 0.0298A − 8.7818; FEV1 = 0.0665H − 0.0292A − 6.5147
Women: FVC = 0.0444H − 0.0169A − 3.1947; FEV1 = 0.0332H − 0.0190A − 1.8210
% predicted = your FEV1 ÷ predicted FEV1 × 100

The principle

Lung size scales with body size, and standing height is the best simple proxy for chest size, so predicted volumes rise with height. After early adulthood, lung elasticity and chest-wall mobility decline gradually, so the equations subtract a small amount for each year of age. Men of the same height have larger lungs on average, hence separate equations. Knudson's equations were derived from healthy, mostly white non-smokers in the United States; modern lung-function laboratories use the Global Lung Function Initiative (GLI) equations, which cover a wider age range and several ethnic groups.

Worked example

Inputs: male, age 35, height 175 cm, no measured FEV1 entered.
FVC: 0.0844 × 175 = 14.770; minus 0.0298 × 35 = 1.043; minus 8.7818 → 4.95 L.
FEV1: 0.0665 × 175 = 11.6375; minus 0.0292 × 35 = 1.022; minus 6.5147 → 4.10 L.
% predicted: not shown (—) because FEV1 was set to 0.
If you had measured 3.5 L, the result would be 3.5 ÷ 4.10 × 100 ≈ 85%.

Each point is the calculator re-run with only “Height (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Height (cm)	Predicted FVC (L)
43.8	-6.13
87.5	-2.44
131	1.25
175	4.95
219	8.64
262	12.33
350	19.72

How to read the result

Traditionally, 80% predicted or more was called normal. Current guidance prefers the lower limit of normal (LLN), the 5th percentile of healthy people, because a fixed 80% cut-off misclassifies many older and younger adults. The ratio FEV1 ÷ FVC matters too: a low ratio suggests airflow obstruction such as asthma or COPD, while reduced volumes with a normal ratio may point to restriction.

Common mistakes

Entering a value from a home peak-flow meter, which measures peak flow, not FEV1.
Comparing your result with an equation different from the one your lab used.
Judging from a single blow; spirometry needs repeatable tests.

Keep in mind: These simplified equations are not those used by most laboratories and do not adjust for ethnicity or children. This is not a diagnosis; talk to a doctor about your results.

Frequently asked questions

What is a normal FEV1/FVC ratio?

In adults it is usually above 0.70; the GOLD COPD criteria use a post-bronchodilator ratio below 0.70.

Why does my report show a z-score?

GLI-based reports express how many standard deviations you are from the predicted value; below −1.645 is under the LLN.

Sources: Knudson RJ, Lebowitz MD, Holberg CJ, Burrows B. Changes in the normal maximal expiratory flow-volume curve with growth and aging. Am Rev Respir Dis 1983;127:725–734 · Quanjer PH, Stanojevic S, Cole TJ, et al. Multi-ethnic reference values for spirometry for the 3–95-yr age range: the global lung function 2012 equations. Eur Respir J 2012;40:1324–1343

Checked 8 Nov 2026. Knudson et al., Am Rev Respir Dis 1983.

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