Educational tool — not a diagnosis. This applies a published formula to numbers you enter. Bladder volume and post-void residual are interpreted by clinicians in the context of symptoms, history and other findings.
Discuss any result with the healthcare professional who arranged your scan. Do not use this page to make decisions about tests or treatment.
Quick answer: volume = length × width × height × coefficient, with the diameters in centimetres. Use 0.52 for a true ellipsoid, or 0.7 if your scanner is calibrated to it. An 8 × 6 × 5 cm bladder is 125 mL at 0.52. Since 1 cm³ = 1 mL, the result is millilitres directly.
The two coefficients, and choosing between them
The geometric coefficient for an ellipsoid is π/6 ≈ 0.52. But a bladder — especially when only partly full — is flatter and more rounded than a smooth ellipsoid, so 0.52 tends to underestimate the true volume for many bladders. Research found that a coefficient nearer 0.7 matches measured volumes more closely across a range of filling states, and a number of automated bladder scanners are calibrated to it.
Which to use depends on your device and protocol. This calculator defaults to 0.52 so you can see the pure geometry, and lets you switch to 0.7 to match a scanner that uses it. The difference is large — about 35% — so it is worth knowing which your report is based on.
The 0.52 coefficient is the standard prolate-ellipsoid value (π/6). Adjusted coefficients around 0.7 derive from validation studies comparing ultrasound estimates against measured bladder volumes; the precise figure varies between studies and devices.
Capacity and residual reference figures
| Volume | Commonly described as |
|---|---|
| 150–250 mL | First urge to void usually felt |
| 300–500 mL | Typical adult functional capacity |
| Over ~500 mL | Full to uncomfortably full |
| PVR under 50 mL | Generally considered a normal residual |
| PVR 100–200 mL | May prompt further assessment |
These figures are general orientation, not diagnostic thresholds. Normal capacity varies with age, hydration and the individual, and post-void residual is always interpreted alongside symptoms and the reason the scan was requested.
Post-void residual
Post-void residual (PVR) is the urine left in the bladder straight after voiding. It is measured by scanning the bladder immediately after the patient urinates, using the same ellipsoid estimate. A low residual suggests the bladder empties well; a persistently high residual can indicate incomplete emptying, which is investigated in the context of the whole clinical picture.
To estimate PVR here, simply enter the three post-void diameters — the calculation is identical to a pre-void measurement, just performed on the smaller residual volume.
Frequently asked questions
How is bladder volume calculated?
With the ellipsoid formula: volume = length × width × height × a coefficient. The coefficient is 0.52 (π/6) for a true ellipsoid, though many ultrasound protocols use 0.7 to better match the bladder's actual shape when moderately full. A bladder measuring 8 × 6 × 5 cm gives 240 cm³ at 0.52 or 168 cm³ at 0.7 — check which your device uses.
What is a normal bladder capacity?
Adult bladder capacity is typically 300 to 500 mL, with the first urge to void usually felt around 150 to 250 mL. Capacity varies with age, hydration and individual differences, so these are general figures rather than fixed thresholds.
What is post-void residual (PVR)?
PVR is the volume of urine remaining in the bladder immediately after urinating, measured by scanning the bladder after voiding. A residual under about 50 mL is generally considered normal, while volumes above 100 to 200 mL may prompt further assessment. Interpretation depends on the clinical context.
Why do some scanners use 0.7 instead of 0.52?
0.52 (π/6) is the exact coefficient for a smooth ellipsoid, but a partially filled bladder is not a perfect ellipsoid — it is flatter and more rounded. Studies found that a coefficient around 0.7 gives volumes closer to the true measured value for many bladders, so some automated bladder scanners are calibrated to it. The right coefficient depends on the device and the degree of filling.
How accurate is ultrasound bladder volume?
It is an estimate. The ellipsoid method assumes a regular shape, and because three measurements are multiplied, small errors compound. Dedicated bladder scanners that build a 3-D image are more accurate than the three-diameter method, but both are approximations rather than exact volumes.
References
- Prolate ellipsoid volume formula — standard geometric derivation, V = (π/6) × d₁ × d₂ × d₃.
- Validation studies of ultrasound bladder volume estimation comparing ellipsoid coefficients against measured volumes support adjusted coefficients around 0.7 for partially filled bladders.
- General references on post-void residual thresholds in urological and continence assessment.