Bladder ultrasound looks trivial and is routinely done badly. The volume calculation is a single formula, but the inputs depend on getting three genuinely orthogonal planes, and a small error in each dimension compounds into a number that changes management. Post-void residual in particular drives catheterization decisions, so a wrong measurement has direct consequences.

Technique

Probe: Curvilinear, 2–5 MHz. Suprapubic, transverse and sagittal.

Filling matters. For assessment of the bladder itself, a comfortably full bladder is required — an empty bladder makes wall assessment impossible and hides small lesions. For post-void residual, scan immediately after voiding.

Position: Supine. Probe just above the pubic symphysis, angled caudally into the pelvis.

Volume Calculation

The bladder is modelled as an ellipsoid:

Volume (mL) = L × W × H × 0.52

Where L, W, and H are in centimetres.

How to get the three dimensions:

Transverse plane: measure width (side to side) and height/AP (anterior to posterior)
Sagittal plane: measure length (craniocaudal)

The 0.52 coefficient approximates π/6 and assumes an ellipsoid shape. Some departments use 0.7 for a fuller bladder or apply different coefficients depending on shape. Know what your protocol uses and apply it consistently — mixing coefficients across serial scans makes trends meaningless.

Where the Errors Come From

Oblique planes. The single biggest source of error. If your transverse image is oblique, you overestimate width. Square up to the bladder before freezing.

Measuring an incompletely visualized bladder. A very full bladder may extend beyond the field of view of a high-frequency setting. Drop frequency or increase depth so the entire bladder is on screen before measuring.

Bladder shape. The formula assumes an ellipsoid. Bladders distorted by a large prostate, a cystocele, prior surgery, or a pelvic mass depart from that assumption, and the calculated volume becomes an estimate rather than a measurement. Say so in the report when the shape is clearly non-ellipsoid.

Timing after voiding. Urine continues to enter the bladder from the ureters. A "post-void residual" measured ten minutes after voiding includes several minutes of new urine production. Scan within a few minutes of voiding and note the delay if it was longer.

Post-Void Residual

Widely used thresholds, though exact cutoffs vary between guidelines and clinical contexts:

PVR	General interpretation
< 50 mL	Normal
50–100 mL	Generally considered acceptable, particularly in older patients
100–200 mL	Abnormal — warrants correlation and often repeat measurement
> 200 mL	Significant retention
> 300–400 mL	Marked retention; catheterization frequently indicated

Interpretation caveats worth including in the report:

A single elevated PVR should generally be repeated before acting on it. Patients void poorly in unfamiliar settings, with a full schedule and a waiting room outside the door.
PVR rises with age; a modestly elevated value in an 85-year-old means something different than in a 30-year-old.
The patient must have voided normally, not partially or under instruction to hurry.

Bladder Wall

Normal wall thickness: Generally under about 3 mm when the bladder is well distended, and up to around 5 mm when empty or partially filled. Always state the degree of distension alongside the measurement — a 6 mm wall in an empty bladder is normal, and the same measurement in a fully distended bladder is not.

Wall thickening causes:

Bladder outlet obstruction — commonly benign prostatic enlargement in men, producing diffuse thickening with trabeculation
Chronic cystitis — diffuse thickening, sometimes with debris
Neurogenic bladder — thickened, trabeculated, often with diverticula
Malignancy — focal, irregular thickening

Trabeculation: Irregular, interdigitating internal wall contour from detrusor hypertrophy. A marker of chronic outlet obstruction.

Focal versus diffuse is the key distinction. Diffuse thickening is usually obstructive or inflammatory. Focal thickening or a polypoid lesion needs to be treated as a possible tumour until proven otherwise — apply colour Doppler, look for internal vascularity, and recommend cystoscopy. A non-mobile, vascular, focal lesion is a very different report from mobile echogenic debris.

Common Findings

Bladder Stones

Echogenic focus with posterior acoustic shadowing that is mobile with patient repositioning. Mobility is the discriminator — roll the patient into a decubitus position and rescan. A stone moves; a tumour does not.

Bladder Diverticula

Outpouching connected to the bladder lumen by a neck. Look for the neck to confirm — a fluid collection adjacent to the bladder without a demonstrable communication is something else. Diverticula are associated with chronic outlet obstruction, may retain urine after voiding, and can harbour stones or tumour. Note whether the diverticulum empties on voiding.

Debris, Clot, and Infection

Mobile low-level internal echoes that layer dependently and shift with position. Clot may be more organized and adherent. Apply Doppler: clot is avascular; tumour may show flow. An adherent, vascular mass needs cystoscopy.

Ureteric Jets

Colour Doppler over the trigone shows periodic jets of urine entering the bladder from each ureteric orifice. Present bilateral jets argue against complete ureteric obstruction on that side. Absent or markedly asymmetric jets support obstruction, though jets are intermittent — watch for a couple of minutes before calling one absent.

Catheters

A Foley balloon appears as a rounded anechoic structure with an echogenic rim within the bladder. Confirm correct position — a balloon inflated in the urethra or a catheter in a diverticulum is a genuine and reportable finding.

Related Prostate Assessment

While the bladder is full, the prostate can be assessed transabdominally through it.

Prostate volume uses the same ellipsoid formula: L × W × H × 0.52. Normal is roughly 25 mL in a young adult, increasing with age.

Intravesical prostatic protrusion (IPP): measured in the sagittal plane as the distance the prostate protrudes into the bladder lumen above the bladder base. Greater protrusion is associated with more significant outlet obstruction and is worth measuring in men with lower urinary tract symptoms.

Transabdominal assessment is a screening view — it does not replace transrectal ultrasound where detailed prostate evaluation is needed.

Key Images to Capture

Transverse bladder — width and AP measurements
Sagittal bladder — length measurement
Calculated volume displayed
Bladder wall — with distension state documented
Any focal wall lesion — two planes, with colour Doppler
Ureteric jets — bilateral, with colour Doppler
Post-void transverse and sagittal, with calculated PVR
Prostate measurements where relevant

Reporting Language

Retention with obstruction: "Pre-void bladder volume 620 mL. Diffuse bladder wall thickening measuring 7 mm with marked trabeculation. Two small diverticula arise from the left lateral wall, the larger measuring 2.3 cm, without internal stone or solid content. Prostate volume 78 mL with 1.4 cm intravesical protrusion. Post-void residual 340 mL. Findings consistent with chronic bladder outlet obstruction with significant retention."

Focal lesion: "A 1.8 cm focal polypoid lesion arises from the right posterolateral bladder wall. The lesion is immobile on repositioning and demonstrates internal vascularity on colour Doppler. Appearances are concerning for a bladder neoplasm; cystoscopic evaluation is recommended."

SonoBuddy includes a bladder and prostate volume calculator alongside its measurement reference — offline, in the scan room.

Bladder Ultrasound: Volume Calculation, Post-Void Residual, and Wall Assessment