How do you calculate functional residual capacity?

Predicted FRC is calculated using regression equations derived from large healthy-population studies. The most widely cited are the Quanjer 1993 ECCS reference values, which use standing height and age as inputs. However, a measured FRC cannot be obtained from spirometry — the spirometer cannot quantify the residual volume of air that remains trapped in the lungs after exhalation. Actual measurement requires body plethysmography (the gold standard), helium dilution, or nitrogen washout. This calculator outputs predicted FRC from demographics only.

What is the FRC formula?

The Quanjer 1993 ECCS prediction equations are: Males: FRC = 2.34H + 0.01A − 1.09. Females: FRC = 2.24H + 0.001A − 1.00. H is standing height in metres and A is age in years. An ethnicity correction factor is applied as a multiplier — typically 0.88 for Black/African populations and 0.94 for East/Southeast Asian populations — to account for systematic differences in lung volume relative to height that are not explained by demographics alone.

What is a normal FRC level, and what is a normal FVC level?

A normal predicted FRC is typically 2.5–3.5 L for adult men and 2.0–3.0 L for adult women, varying significantly with height. A measured FRC below 80% of predicted suggests restriction (e.g. pulmonary fibrosis, obesity), while above 120% suggests hyperinflation as seen in COPD and emphysema. For FVC, a measured value at or above 80% of predicted is considered within normal limits. Values of 70–79% are mildly reduced, 60–69% moderately reduced, and below 60% severely reduced.

How do you calculate FEF 25-75?

FEF 25–75 (Forced Expiratory Flow between 25% and 75% of the FVC exhalation) is not calculated — it is measured directly during a spirometry test from the flow–volume curve. The value reflects average flow through the middle portion of a forced breath out and is sensitive to small airway obstruction. Predicted FEF 25–75 is derived from regression equations using height and age. A measured value below 65% of predicted may indicate early small airway disease even when FEV1/FVC remains in the normal range.

How does Total Lung Capacity relate to FRC, and what are normal TLC values?

Total Lung Capacity (TLC) is the sum of all lung compartments. FRC sits at the natural resting equilibrium point of the respiratory system after a passive exhalation, and TLC equals FRC plus the Inspiratory Capacity (IC). Equivalently, TLC = Vital Capacity + Residual Volume. Normal predicted TLC is approximately 6–8 L for adult men and 5–6.5 L for adult women depending on height. A measured TLC below 80% of predicted confirms a restrictive defect; above 120% confirms hyperinflation and is the definitive criterion for obstructive lung over-inflation.

Health & Respiratory

Functional Residual Capacity Calculator

Estimate predicted Functional Residual Capacity (FRC) and a full set of spirometry reference values — FVC, FEV₁, FEF 25–75, TLC, and RV — using the Quanjer 1993 ECCS equations. Optionally enter measured spirometry values to see percentage of predicted for each parameter.

Sex

Age (years, 18–70)

Height (cm)

Ethnicity (applies correction factor to predicted volumes)

Measured spirometry values (optional)

Enter values from your spirometry report to calculate percentage of predicted alongside each figure and see a pattern interpretation.

FVC measured (litres)

FEV₁ measured (litres)

FEF 25–75 measured (L/s)

Predicted Functional Residual Capacity (FRC)

—

FVC—

FEV₁—

FEV₁/FVC ratio—

FEF 25–75—

Total Lung Capacity (TLC)—

Residual Volume (RV)—

Inspiratory Capacity (IC)—

Expiratory Reserve Volume (ERV)—

What is functional residual capacity?

Functional Residual Capacity is the volume of air that remains in the lungs after a normal, passive exhalation — the point at which the elastic recoil of the lungs and the outward spring of the chest wall reach equilibrium, with no active muscle effort. It is a foundational lung volume because it defines the starting point of every subsequent breath.

FRC is made up of two sub-volumes: the Expiratory Reserve Volume (ERV, the air you can forcibly push out below the resting level) and the Residual Volume (RV, the air that can never be expelled, even with a maximal exhalation). Because RV cannot be emptied, spirometry alone cannot measure FRC — the spirometer can only track air that moves in and out of the mouth. The lung volume compartments relate to each other as shown below:

TLC (Total Lung Capacity) ├── IC (Inspiratory Capacity) │ ├── IRV (Inspiratory Reserve Volume) │ └── TV (Tidal Volume ≈ 0.5 L at rest) └── FRC (Functional Residual Capacity) ← resting end-expiration ├── ERV (Expiratory Reserve Volume) └── RV (Residual Volume — cannot be exhaled) VC (Vital Capacity) = IRV + TV + ERV = TLC − RV FRC = ERV + RV = TLC − IC

Clinically, a low FRC (below ~80% of predicted) is associated with restrictive conditions — pulmonary fibrosis, obesity, pleural effusion, or neuromuscular weakness — where the lungs or chest wall are stiffer or smaller than normal. An elevated FRC (above ~120% of predicted) indicates hyperinflation, characteristic of emphysema and severe asthma, where loss of elastic recoil pushes the equilibrium outward.

How is FRC measured in clinical practice?

Three methods are used to measure FRC directly, each with different clinical implications:

Body plethysmography is the gold standard. The patient sits inside an airtight box and pants against a closed shutter; changes in box pressure allow calculation of the total thoracic gas volume, including poorly communicating areas of the lung. It is the preferred method in obstructive disease, because gas dilution techniques underestimate lung volume when airways are obstructed.

Helium dilution uses a closed-circuit system containing a known concentration of helium — an inert, insoluble tracer gas. The patient breathes in and out of the circuit at end-expiration; helium equilibrates with the FRC volume, and the dilution of the gas is used to back-calculate lung volume. This technique measures only communicating air spaces, so it underestimates FRC in emphysema.

Nitrogen washout uses the nitrogen already present in the lungs as the tracer. The patient breathes 100% oxygen and the cumulative nitrogen expired is measured until alveolar nitrogen reaches near zero. Like helium dilution, it measures communicating gas only.

The calculator on this page produces a predicted FRC from demographic inputs. This is used as the denominator for percentage-of-predicted comparisons and for clinical interpretation when a plethysmography result is available.

How the predicted values are calculated

All predicted values use the Quanjer 1993 ECCS (European Community for Steel and Coal) reference equations, which remain the standard for spirometry interpretation across much of Europe and are widely cited internationally. The GLI-2012 (Global Lung Function Initiative) equations provide improved accuracy across a wider age range and more ethnic groups and are preferred for clinical reporting — this calculator uses the simpler Quanjer linear approximations suitable for general reference.

Quanjer 1993 ECCS Reference Equations (H = height metres, A = age years) Males FRC = 2.34H + 0.01A − 1.09 RV = 1.31H + 0.022A − 1.23 FVC = 5.76H − 0.026A − 4.34 TLC = 7.99H − 7.08 FEV₁ = 4.30H − 0.029A − 2.49 FEV₁/FVC% = 87.2 − 0.18A FEF₂₅₋₇₅ = 4.46H − 0.045A − 2.21 Females FRC = 2.24H + 0.001A − 1.00 RV = 1.81H + 0.016A − 2.00 FVC = 4.43H − 0.026A − 2.89 TLC = 6.60H − 5.79 FEV₁ = 3.95H − 0.025A − 2.60 FEV₁/FVC% = 89.1 − 0.19A FEF₂₅₋₇₅ = 3.43H − 0.037A − 1.15 Ethnicity correction applied as multiplier to all volume-based measures. % predicted = (measured value ÷ predicted value) × 100

IC and ERV are derived values: IC = TLC − FRC; ERV = FRC − RV. The FEV₁/FVC predicted value represents the lower boundary of the normal range as a function of age; a measured ratio below 70% is conventionally considered obstructive regardless of individual predicted values.

Reading percentage of predicted and interpreting spirometry patterns

Percentage of predicted compares your measured value to the expected value for a healthy person of the same age, sex, height, and ethnicity. The standard severity grading for FVC and FEV₁ is:

≥80% — Normal. 70–79% — Mild reduction. 60–69% — Moderate. 50–59% — Moderately severe. 35–49% — Severe. <35% — Very severe.

For FEF 25–75, a value below 65% of predicted may suggest early small airway obstruction even when FEV₁/FVC is still in the normal range — it is sometimes the earliest spirometric sign of airway disease in smokers or patients with mild asthma.

Pattern interpretation from FEV₁ and FVC alone is a screening tool, not a diagnosis. A low FEV₁/FVC (<70%) with a normal or near-normal FVC suggests an obstructive pattern. A normal FEV₁/FVC with a reduced FVC (<80% predicted) suggests a restrictive pattern, which must be confirmed by measuring TLC. When both FEV₁/FVC and FVC are reduced, a mixed pattern is possible. Full interpretation requires clinical context, a bronchodilator response test, lung volume measurement, and DLCO in most cases.

Common questions

Predicted FRC is calculated using regression equations derived from large healthy-population studies. The most widely cited are the Quanjer 1993 ECCS reference values, which use standing height and age as inputs. However, a measured FRC cannot be obtained from spirometry — the spirometer cannot quantify the residual volume of air that remains trapped in the lungs after exhalation. Actual measurement requires body plethysmography (the gold standard), helium dilution, or nitrogen washout. This calculator outputs predicted FRC from demographics only.
The Quanjer 1993 ECCS prediction equations are: Males: FRC = 2.34H + 0.01A − 1.09. Females: FRC = 2.24H + 0.001A − 1.00. H is standing height in metres and A is age in years. An ethnicity correction factor is applied as a multiplier — typically 0.88 for Black/African populations and 0.94 for East/Southeast Asian populations — to account for systematic differences in lung volume relative to height that are not explained by demographics alone.
A normal predicted FRC is typically 2.5–3.5 L for adult men and 2.0–3.0 L for adult women, varying significantly with height. A measured FRC below 80% of predicted suggests restriction (e.g. pulmonary fibrosis, obesity), while above 120% suggests hyperinflation as seen in COPD and emphysema. For FVC, a measured value at or above 80% of predicted is considered within normal limits. Values of 70–79% are mildly reduced, 60–69% moderately reduced, and below 60% severely reduced.
FEF 25–75 (Forced Expiratory Flow between 25% and 75% of the FVC exhalation) is not calculated — it is measured directly during a spirometry test from the flow–volume curve. The value reflects average flow through the middle portion of a forced breath out and is sensitive to small airway obstruction. Predicted FEF 25–75 is derived from regression equations using height and age. A measured value below 65% of predicted may indicate early small airway disease even when FEV1/FVC remains in the normal range.
Total Lung Capacity (TLC) is the sum of all lung compartments. FRC sits at the natural resting equilibrium point of the respiratory system after a passive exhalation, and TLC equals FRC plus the Inspiratory Capacity (IC). Equivalently, TLC = Vital Capacity + Residual Volume. Normal predicted TLC is approximately 6–8 L for adult men and 5–6.5 L for adult women depending on height. A measured TLC below 80% of predicted confirms a restrictive defect; above 120% confirms hyperinflation and is the definitive criterion for obstructive lung over-inflation.