Measuring bladder volume with ultrasound

Urinary retention is a quick bedside diagnosis: measure the bladder in two planes and multiply.

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How

Curvilinear probe just above the pubic symphysis. In transverse, measure width and depth (anteroposterior); rotate to longitudinal for length.

Formula

Volume ≈ length × width × height × 0.52 (ellipsoid). Some devices use coefficients up to ~0.7.

Interpretation

A post-void residual above roughly 100–200 mL is abnormal; several hundred millilitres in someone unable to void suggests retention. Check both kidneys for hydronephrosis.

Real scans

Full urinary bladder — A large, black, urine-filled bladder. The wall is irregular (trabeculated), a sign of chronic urinary retention. — **Full urinary bladder.** A large, black, urine-filled bladder. The wall is irregular (trabeculated), a sign of chronic urinary retention. Mikael Häggström · CC0 · source

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FAQ

Why 0.52?

It is π/6, the volume of an ellipsoid relative to the box enclosing it.

Related guides

Hydronephrosis on ultrasoundWhat hydronephrosis looks like on ultrasound, how it differs from renal cysts, and how to use the bladder to find the cause. Live simulator and real images.
Linear, curvilinear or phased array?The three ultrasound probes explained — frequencies, footprints, image shapes and which to use for vessels, lungs, abdomen and heart — plus the probe marker convention.
The FAST exam, explained by actually doing itHow to perform a FAST exam: RUQ, LUQ, pelvic and subxiphoid views, what a positive scan looks like, common pitfalls — with a live ultrasound simulator you can try.

Educational content for learners; follow local protocols and supervision. Simulated images are generated by Sonoduck’s physics model; real images are credited individually.