Pleural Effusion Volume on CT

Estimate effusion volume from a single axial measurement on CT

Measurement

Formula

Greatest AP depth of effusion (mm)

Measure perpendicular to the chest wall on the slice with maximum effusion thickness, with the patient supine.

Side

Estimated volume

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Formulas

Author	Formula (mL)	Notes
Hazlinger 2014	V = (depth_mm × 16.8) − 153	Best linear correlation with reference; supine CT
Mergo 1999	V = depth_cm × 90	Earlier estimate; depth in cm
Moy 2013	V = depth_mm × 20	Approximation at hemidiaphragm dome

Clinical size categories (per side)

Category	Volume (mL)	Typical depth (mm)
Trace / minimal	< 100	< 15
Small	100–500	15–40
Moderate	500–1500	40–80
Large	> 1500	> 80

Sources

Hazlinger M et al. Quantification of pleural effusion on CT by simple measurement. Biomed Pap Med Fac Univ Palacky Olomouc 2014;158:107-111. doi:10.5507/bp.2012.042
Mergo PJ et al. New formula for quantification of pleural effusions from CT. J Thorac Imaging 1999;14:122–125.
Moy MP et al. A new, simple method for estimating pleural effusion size on CT. AJR 2013;200:761-766. doi:10.2214/AJR.12.9483

Estimates are approximations; loculated effusions, complex morphology and prone positioning reduce accuracy. Volumetric segmentation remains the reference standard.