Bladder Volume Calculator (Ultrasound)
Estimate bladder volume from three ultrasound dimensions using the standard ellipsoid formula (length x width x height x coefficient). Choose a bladder shape to apply the correct correction coefficient, enter measurements in centimetres or inches, and get an instant volume estimate with postvoid residual interpretation. Toggle between a pre-void and post-void mode to check how much urine remains after voiding.
How bladder volume is measured with ultrasound
Bladder volume is most commonly estimated using a transabdominal ultrasound (or a bladder scanner, which is a dedicated portable ultrasound device). The sonographer measures the bladder in three perpendicular planes: longitudinal length (superior to inferior), transverse width (left to right), and anteroposterior height (front to back). These three dimensions are then multiplied together and scaled by a shape-specific correction coefficient to account for the fact that a bladder is not a perfect rectangular box. The result is expressed in millilitres (mL), where 1 mL equals 1 cm^3. The standard ellipsoid formula achieves a mean measurement error of about 12 to 15 percent in clinical practice.
Shape correction coefficients explained
The correction coefficient compensates for how closely the bladder resembles a given geometric solid. Bih et al. (1998) classified bladders as cuboid, ellipsoid, or triangular prism and derived optimal coefficients by linear regression: 0.89 for cuboidal, 0.81 for ellipsoid, 0.66 for triangular prism, and 0.72 for the whole data set (used when shape is not assessed). Applying the shape-specific value cut mean estimation error from 16.9 percent to 12.7 percent in that study. The fifth option here, 0.52 for a spherical or prolate bladder, is not from Bih et al.: it is the pure geometric factor for an ellipsoid of revolution (pi/6 = 0.5236), which some sonographers prefer for a round, well-distended bladder. When the shape is uncertain, 0.72 is a practical and well-validated default. Using the wrong coefficient introduces systematic bias: applying the cuboid coefficient (0.89) to an ellipsoid bladder, for example, will consistently overestimate volume by about 10 percent.
Postvoid residual (PVR) and what it means
Postvoid residual (PVR) is the volume of urine left in the bladder immediately after voiding, measured by ultrasound or catheterisation. A PVR of less than 50 mL is widely considered normal and indicates efficient bladder emptying. Values between 50 and 99 mL are borderline and warrant a repeat measurement. Values from 100 to 299 mL suggest incomplete emptying, which may be caused by bladder outlet obstruction (such as benign prostatic hyperplasia), neurogenic bladder dysfunction, detrusor underactivity, or medication side effects. A PVR of 300 mL or more indicates significant urinary retention and usually requires prompt clinical evaluation. Because single PVR measurements can vary by up to 25 percent, most guidelines recommend confirming an elevated result on at least two separate occasions before acting.
Normal adult bladder capacity and clinical context
The average adult bladder holds 400 to 600 mL at maximum functional capacity, though most adults feel a comfortable urge to void at around 150 to 300 mL. A strong urge typically occurs around 300 to 400 mL. Maximum cystometric capacity (measured during formal urodynamic testing) can reach up to 800 mL in healthy adults but is generally lower in older people and those with conditions such as overactive bladder, interstitial cystitis, or bladder fibrosis. In children, a simple estimate of expected capacity is (age in years + 2) multiplied by 30 mL. Bladder capacity is influenced by hydration status, diuretic use, caffeine intake, neurological health, prior pelvic surgery, and inflammatory conditions. A single ultrasound measurement is a snapshot; clinical decisions should incorporate voiding diaries, symptom scores, and specialist assessment where indicated.
Postvoid residual (PVR) interpretation thresholds
| PVR volume | Classification | Clinical implication |
|---|---|---|
| Less than 50 mL | Normal | Adequate bladder emptying |
| 50-99 mL | Borderline | Repeat measurement; correlate with symptoms |
| 100-299 mL | Elevated | Likely incomplete emptying; further evaluation warranted |
| 300 mL or more | Significant retention | Prompt urological evaluation required |
Commonly used clinical thresholds. There is no single universally accepted cut point: the AUA BPH guideline states that no universally accepted definition of a clinically significant PVR exists and recommends following the trend over time, and published "normal" limits range from under 50 mL to under 100 mL. PVR should be measured on at least 2 occasions before clinical decisions are made.
Frequently asked questions
What is the standard formula for bladder volume from ultrasound?
The standard formula is: Volume (mL) = Length x Width x Height x Coefficient. The three dimensions are measured in centimetres from a transabdominal ultrasound, and the coefficient (typically 0.66-0.89 depending on bladder shape, or 0.72 when shape is unknown) corrects for the fact that a bladder is not a perfect rectangular box. Because 1 cm^3 equals 1 mL, the result is directly in millilitres.
What is a normal postvoid residual (PVR) volume?
A PVR of less than 50 mL is considered normal and indicates that the bladder is emptying adequately. Values between 50 and 99 mL are borderline and should be repeated. A PVR of 100 mL or more on two separate measurements usually warrants further urological investigation. A PVR of 300 mL or more indicates significant urinary retention.
How accurate is the ultrasound bladder volume estimate?
The three-dimensional ellipsoid formula has a mean measurement error of approximately 12 to 15 percent. Accuracy is affected by bladder shape, the skill of the operator, patient body habitus, and how well the bladder is visualised. The estimate is useful for clinical decision-making but should be interpreted alongside symptoms, voiding diaries, and other clinical findings rather than as an exact value.
Which correction coefficient should I use if I do not know the bladder shape?
Use 0.72, which is the most commonly applied default and is well-validated across mixed bladder shapes. If you can assess the shape on ultrasound, using the shape-specific coefficient (0.81 for ellipsoid, 0.89 for cuboid, 0.66 for triangular, 0.52 for spherical) will improve accuracy by 5 to 10 percent.
Can this calculator be used for children?
Yes, the formula is the same, but the interpretation of normal ranges differs. In children, expected bladder capacity in mL is approximately (age in years + 2) x 30. PVR thresholds also differ: more than 20 mL on repeated measurements is generally considered abnormal in children, compared to more than 50 to 100 mL in adults. Use paediatric-specific reference values when interpreting results for children.
What causes an elevated postvoid residual?
Common causes include bladder outlet obstruction (most often benign prostatic hyperplasia in men, or urethral stricture), neurogenic bladder from conditions such as diabetes, multiple sclerosis, spinal cord injury, or Parkinson disease, detrusor underactivity (weak bladder muscle), and medication side effects (anticholinergics, opioids, alpha-agonists). Less common causes include large cystocele, pelvic organ prolapse, and certain pelvic surgeries.
Sources
- Bih LI, Ho CC, Tsai SJ, Lai YC, Chow W. Bladder shape impact on the accuracy of ultrasonic estimation of bladder volume. Archives of Physical Medicine and Rehabilitation, 1998.
- Cleveland Clinic. Post-Void Residual Test: What It Is, Results, and Procedure. Cleveland Clinic Health Library.
- Sandhu JS et al. Management of Lower Urinary Tract Symptoms Attributed to Benign Prostatic Hyperplasia: AUA Guideline Part I. Journal of Urology, 2021 (PVR is an optional test; no universally accepted definition of a clinically significant PVR).