How this instrument works
Vital capacity (VC) is the volume of air a person can move out of the lungs in one slow, unhurried breath after inhaling as fully as possible. It is one of the oldest measurements in respiratory physiology, predating modern spirometry by decades, and it still appears on pulmonary function reports today because it captures something simple and useful: how much air the lungs and chest wall can hold and release when nothing is rushed.
The equation behind this page comes from a 1948 paper by Eleanor Baldwin, André Cournand and Dickinson Richards, published in the journal Medicine — one of the earliest attempts to predict a typical vital capacity from height, age and sex alone. It reflects the instruments, the sample and the statistical habits of a laboratory nearly eight decades gone: genuinely useful as a piece of pulmonary-physiology history and as an exercise anyone can work through by hand, but not the equation a pulmonology department reaches for today.
Later work replaced hand-fit regressions like this one for real clinical interpretation. Reference equations built from far larger and more representative populations — Crapo's 1981 tables, Knudson's 1983 update, and the Global Lung Function Initiative's 2012 (GLI-2012) equations, now endorsed by the major international respiratory societies — are what a modern lab actually uses. This page keeps the 1948 formula visible for what it is: an educational artifact, not a diagnostic instrument.
A slow VC and a forced vital capacity (FVC) are not identical tests, even though they measure similar territory: FVC is exhaled as hard and fast as possible, while VC is exhaled at a steady, comfortable pace, and the two numbers can diverge once airways start to narrow or close early during a forced blow — the reason standardized reference tables list separate columns for each. One more note on this page's own limits: the original 1948 paper sits behind a paywalled institutional archive and could not be pulled directly for this project; its two coefficients were instead cross-checked across two independently maintained calculator sites reporting identical values, and re-verified by hand against a worked example, rather than confirmed against the typeset original.
- Enter Height (cm) — standing height measured without shoes gives the most consistent figure to plug into the formula.
- Enter Age (years) as a whole number; each formula subtracts a fixed amount per year, so the prediction falls gradually as age rises.
- Set Sex is female to Yes or No. The calculator switches between two entirely separate regression lines rather than applying a shared correction factor.
- Read Predicted vital capacity (L) — the slow vital capacity that Baldwin, Cournand and Richards' 1948 equation predicts for someone of that height, age and sex.
- Compare the result against an actual spirometry reading if one exists, rather than treating it as a stand-alone figure.
Three heights, three ages, worked by hand
A 20-year-old woman standing 165 cm tall uses the women's line: VC = 165 × (21.78 − 0.101 × 20) / 1000. Work the bracket first: 0.101 × 20 = 2.02, and 21.78 − 2.02 = 19.76. Multiply by height: 165 × 19.76 = 3,260.4, then divide by 1000 for a predicted vital capacity of 3.2604 L — matching the calculator's output for these exact inputs.
A 30-year-old man, 175 cm tall, switches to the men's line instead: VC = 175 × (27.63 − 0.112 × 30) / 1000. Here 0.112 × 30 = 3.36, so 27.63 − 3.36 = 24.27; 175 × 24.27 = 4,247.25, divided by 1000 gives a predicted 4.24725 L — noticeably higher than the woman above, reflecting both the extra height and the men's larger coefficients.
Age pulls the number back down even as height goes up: a 65-year-old man, 180 cm tall, computes 0.112 × 65 = 7.28, so 27.63 − 7.28 = 20.35; 180 × 20.35 = 3,663, divided by 1000 lands at 3.663 L — lower than the 30-year-old above despite five extra centimetres of height, entirely because of the formula's age term.
Questions
What does vital capacity measure, and how is it different from FVC?
Vital capacity (VC) is the total air exhaled after a maximal inhalation, breathed out slowly and steadily. Forced vital capacity (FVC) asks for the same maximal exhalation but blown out as hard and fast as possible. In healthy young lungs the two land close together, but standardized reference tables keep them as separate columns, because airway narrowing or early closure during a forced blow can make FVC read lower than a slow VC. This calculator predicts slow VC, not FVC.
Why is this calculator based on a formula from 1948?
The equation comes from a landmark 1948 study by Baldwin, Cournand and Richards, one of the earliest attempts to predict a typical vital capacity from height, age and sex. It is included here because it is a genuine piece of pulmonary-physiology history with transparent, hand-checkable arithmetic — not because it is the equation a modern lab would actually use to interpret a patient's breathing test.
Is the Baldwin 1948 formula still used in clinical practice today?
No, not for real interpretive decisions. Spirometry technique, instrument calibration, and reference populations have all been overhauled since 1948. Modern pulmonology relies on newer, larger, more diverse reference equations — Crapo (1981), Knudson (1983), or the current international standard, the Global Lung Function Initiative's GLI-2012 equations. Treat this page's output as a historical and educational figure, not a clinical benchmark.
How confident can I be in the coefficients used on this page?
This project could not access Baldwin, Cournand and Richards' original 1948 paper directly — it sits behind a paywalled institutional archive. The two sets of coefficients used here were instead cross-checked across two independently maintained calculator sites that both report the same values, and re-verified by hand against a worked example, but they have not been confirmed against the original typeset table. Treat the formula as faithfully reproduced secondhand rather than independently verified against the source document.
Can this calculator replace an actual spirometry test?
No. This is arithmetic from a 78-year-old regression equation, not a measurement. A real vital capacity or FVC comes from breathing into a calibrated spirometer under a technician's supervision, following the acceptability and repeatability criteria current standards require. Use this tool to understand where an old formula comes from, not to assess your own lung function.
Why does the calculator use two separate formulas for men and women?
Baldwin, Cournand and Richards fit two independent regression lines from their 1948 sample rather than one shared line with a correction factor, because men in their dataset had measurably larger vital capacities on average than women of the same height and age — reflecting typical differences in chest and lung size between the sexes in that population.
Why does the predicted value fall as age goes up?
Each formula subtracts a fixed amount per year of age — 0.112 L per year for men, 0.101 L per year for women — from a height-based baseline. That reflects a pattern Baldwin and colleagues observed in 1948: vital capacity tends to decline gradually after early adulthood as chest wall compliance and respiratory muscle strength decrease, even in people who otherwise remain healthy.
What if my actual measured vital capacity differs a lot from this prediction?
A large gap between a real, spirometry-measured VC and this formula's prediction usually says more about the formula's age than about your lungs. Bring an actual reading to a clinician, who will judge it against modern reference equations built from far larger and more representative populations — the tools that determine whether a measured value falls inside or outside a normal range, not this historical calculator.
References
- Baldwin, Cournand & Richards, Medicine 1948 — original study (LWW)
- Laszlo, European standards for lung function testing: 1993 update (PMC)
- Quanjer et al., GLI-2012 multi-ethnic spirometry reference equations (PMC)
- Slow vital capacity vs. forced vital capacity in airflow limitation (PMC)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.